Ni-P Coatings Electroplating — a Review Part I: Pure Ni-P Alloy

Ni-P coatings electroplating — A review Part I: Pure Ni-P alloy

Aleksandra LELEVIC´ H2020-MSCA-ITN-2016 SOLUTION Artia Nanoengineering & Consulting Athens, Greece [email protected] [email protected]

July 13, 2018

Abstract

In the electroplating industry Ni-P coatings are extensively employed owing to their excellent properties which enable substrate protection against corrosion and wear. Depending on their composition and structure, as-plated deposits demonstrate good mechanical, tribological and electrochemical features, catalytic activity but also beneﬁcial magnetic characteristics. With subsequent thermal treatment hardness of Ni-P metal-metalloid system can approach or be even higher than that of hard Cr coatings. The purpose of this paper is to provide a general survey of the research work dealing with the electrodeposition of Ni-P binary alloy coatings. Proposed phosphorus incorporation mechanisms, Ni-P alloy microstructure before and after thermal treatment, its mechanical, tribological, corrosion, catalytic and magnetic properties are considered, so are the key process variables inﬂuencing phosphorus content in the deposits and the roles of the main electrolytic bath constituents. Findings on the merits of employing pulse plating and fabrication of unconventional (layered and functionally graded) structures are succinctly explored.

1 Introduction It involves a process that takes place in the low cost electrolysis cell, in an aqueous solution, at normal In the modern world billions of dollars per year pressure and at relatively low temperature, which are lost due to machinery overhaul or complete makes it ideal for the industrial scaling up [7–9]. breakdown related to material structural failures Particular appeal of electroplating is in the possibil- induced by progressive corrosion, wear, fatigue ity of customizing appearance and properties of the and rupture. Surface degradation impacts a large coating by modifying the composition of the electro- number of industrial sectors and can account for plating bath and/or the electroplating conditions up to 3-4% of the Gross Domestic Product in de- and producing a wide range of metallic materials veloped economies [1,2]. One way to address and from metals and simple alloys to compositionally diminish issues related to material deterioration is modulated systems and composites. Deposition arXiv:1807.04693v1 [physics.app-ph] 24 Jun 2018 to impart on its surface a coating that will mitigate rates in the order of several tens of micrometers per the influence of the surrounding environment and hour can be routinely achieved [10]. of the working conditions and prolong the service In terms of types of coatings fabricated through life of the equipment. electroplating, in aerospace, automotive and gen- For the deposition of coatings a variety of tech- eral engineering industry functional Cr coatings niques exist that can be mechanical, physical, chem- are very widely employed owing to their remark- ical and electrochemical in nature [3]. Among them, able combination of properties which include: high simple electroplating offers many advantages [4–6]. hardness, good corrosion, wear and heat resistance and low friction coefficient [7]. However, the most for hydrogen evolution, thin film magnetic discs, common commercial Cr electrodeposition process microgalvanics, etc. [21]. employs an aqueous bath that contains hexavalent In order to attain the uttermost enhancement Cr. This ion is suspected to be a carcinogenic agent of the Ni-P deposit beneficial properties it is which in combination with its strong oxidizing na- paramount to control the coating’s composition and ture imposes serious human health and environ- its microstructure. This can be achieved through al- mental concerns [11]. As a consequence, the use tering electroplating bath composition, deposition of hexavalent Cr has been limited by the European conditions and by applying appropriate alloy post- Union Restriction of Hazardous Substances Direc- treatment. To go beyond the limits of the properties tive [12]. In addition, the typical wetting agent- that can be achieved for this binary alloy system, PFOS (perfluoro-octanesulfonic acid) employed in co-deposition of nano, sub-micron or micron size Cr electroplating to reduce the danger of the pro- particles and fabrication of composite coatings is cess through minimizing the misting generated the centre of attention of the modern electroplat- during plating is being prohibited due to its ex- ing research and innovation efforts. While classical ceptional stability which presents a problem for industrial demands on hard, homogeneous, wear the environment [7]. In the light of all mentioned, resistant coatings and corrosion protection are still attention is focused on finding alternatives to Cr valid, added capacities in terms of: chemical sta- while maintaining the request for excellent func- bility, bio-compatibility, microstructured surfaces tional properties of the fabricated deposits. and functional coatings (lubricant, magnetic) are It has been reported by many authors that cer- in high demand. This is where the production of tain binary, ternary or quaternary alloy coatings composites but also compositionally modulated may present a suitable replacement for hard Cr coatings, characterized by variable configuration and may possess an interesting combination of across their thickness, can offer new possibilities features that can culminate even in a possible im- and new combinations of properties that can be provement in terms of performance characteristics adapted to almost any need and satisfy the increas- [13–16]. Amongst binary alloys Ni-P has attracted ing demands on multi-functionality. a lot of attention. Vast number of studies have been conducted which prove that coatings based on Ni-P with careful tailoring of composition and 2 Electroplating benefits and structure can offer smart and adaptive solutions to a wide range of environmental and working prospects conditions [17, 18]. Compared to Cr coatings obtained from hexavalent Cr baths, effluents resulting Conventional electroplating possesses many advan- from Ni electroplating process have conventional, tages in terms of versatility, ease of use and cost simpler treatment, such as for example processes effectiveness. The technique has many assets when called Clean Technologies (based on electrodialy- compared to physical methods such as magnetron sis) [19]. Ni-P coatings exhibit many interesting sputtering and chemical methods such as chemi- features. They are, depending on the electroplating cal vapour deposition. Physical vapour deposition conditions and the applied post-treatment, charac- is a high cost line-of-sight process with possibly terized by: optimal mechanical properties, good poor throwing power, while in chemical vapour wear and corrosion resistance, electro-catalytic ac- deposition high operating temperatures may cause tivity and favourable tribological features. With substrate softening [3, 22]. Heat resistance of the proper customization of composition and structure substrate may very much limit the choice of the and subsequent thermal treatment hardness of Ni- deposition technique, in particular for metals such P electrodeposits can approach or surpass that of as Al, Cu, Mg for which maximum deposition tem- hard Cr coatings [20]. Ni-P alloy electrodeposits perature is about 100 ◦C. Disadvantages of elec- find applications as protective, functional and deco- troplating include mainly problems with current rative coatings primarily in automotive, aerospace efficiency, throwing power, substrate adhesion and and general engineering industries. Notable are coating uniformity [3]. the applications of Ni-P in the fabrication of dec- In his work Dini [23] performed a comparison orative coatings in automotive industry, high pre- of coatings fabricated by various deposition tech- cision parts, diffusion barriers, catalytic coatings niques (PVD, CVD, PS and EP) on the bases of a wide range of properties, namely: structure, poros-

2 ity, density, stress, corrosion, adhesion, tribology, demonstrate an expansion, predominantly in the fatigue, thermal conductivity, etc. His findings indi- Asia-Pacific excluding Japan region (APEJ). In the cated that no coating technology provides superior market report titled “Electroplating Market: Global results in terms of all considered parameters. The Industry Analysis and Opportunity Assessment, choice of coating technique must in fact be made 2016–2026” [27], Future Market Insights foresee by taking into consideration a number of factors, that the global electroplating market is expected such as: substrate compatibility, adhesion, porosity, to expand at a compound annual growth rate of possibility of repair or re-coating, inter-diffusion, 3.7% by the end of 2026 reaching worth of US$ effect of thermal cycling, resistance to wear and 21 Bn. APEJ will be according to them the fastest corrosion, whilst simultaneously establishing a sen- growing region and the Automotive and Electri- sible balance between the obtained benefits and the cal & Electronics segments are estimated to collec- incurred costs. tively hold about 65% of the total value share of Electroplating possesses advantages over tradi- the global electroplating market. On the basis of tional electroless plating also, in terms of providing metal type, owing to the growth of the Electrical & a higher deposition rate in a simpler electrolyte Electronics industry segments, copper and nickel and the added benefit of the possibility to control are expected to gain significant basis points. As deposit’s composition and microstructure by chang- major growth drivers Future Market Insights iden- ing the applied current waveform and electroplat- tify applications across a diverse set of industries ing conditions [4,5, 24]. Thick deposits are easily and expected eminent growth in the Asia Pacific achieved through electroplating. Amorphous Ni-P region, while the biggest challenges according to alloys can be electroplated up to mm thickness on them will be the rise of stern laws and stringent widely differing substrates [17]. However, it is still environmental regulations, decelerated economic necessary to work on improving the electroplated growth in mature markets or markets in a state of deposits’ uniformity and on maximizing the pro- equilibrium and the growing popularity of electro- cess efficiency when compared to electroless plat- less nickel plating. ing. By definition, electroless process must be metastable to ensure deposition. Plating in electroless mode is characterized by solution instability, slow 3 Ni-P alloy electroplating deposition rate (generally around 10 µm h−1, acidic electroless baths can plate at about 25 µm h−1), high 3.1 Phosphorus co-deposition mecha- ◦ temperatures of operation (85 C min for acceptable nism deposition rate), difficulties with barrel plating of small parts and high cost of the operation which Ni-P electroplating is predominantly performed in is approximately 5 to 10 times higher than for elec- an aqueous electrolytic bath that contains Ni+2 ions trodeposition [25]. Nevertheless, electroless plating as a source of nickel and a phosphorus oxyacid (or has excellent throwing power and the advantage its salt) which acts as a source of phosphorus. Elec- of producing deposits of uniform thickness partic- trodeposition process is being driven by the electric ularly on components with complex shapes [26]. current passing between the anode and the cathode In order to achieve the latter in case of electroplat- (plating substrate) [26]. Standard reduction poten- ing an intricate system of internal anodes and/or tials for Ni (-0.25 V) and for P (-0.28 V) are near shielding is necessary due to the non-uniform cur- to each other facilitating their easy co-deposition. rent distribution characterizing this process. Ni-P Earlier reviews that deal with the electrodeposition deposits fabricated by electroless deposition are of Ni-P alloy and its composites were devised by also reported to be harder and to possess better Berkh and Zahavi [18] and by Daly and Barry [17]. corrosion resistance compared to those obtained Two mechanisms are generally proposed in or- through electrodeposition [17]. der to elucidate the process of phosphorus incorpo- Electroplating is quite a mature technique and ration during Ni-P alloy electrodeposition, namely: although widely applied in industry since the end direct and indirect mechanism. of the 19th century, currently it is exhibiting stag- In the direct mechanism it is proposed that Ni+2 nancy in terms of growth owing to the lack of ions and phosphorus oxyacid are directly reduced technological innovations. However, according to to Ni and P atoms which then form a NixP solid several global market studies, electroplating is to solution. According to Brenner [28] who first suggested this mechanism, phosphorus oxyacid is re-

3 duced directly to phosphorus and phosphorus is Occurrence of indirect mechanism has been cor- co-deposited with nickel owing to the polarization roborated by authors who have reported to have involved in the deposition of nickel assisting the detected phosphine [33–35]. Harris and Dang deposition of phosphorus (induced co-deposition). [33] quantified by chemical analysis PH3 produced The main argument against the direct deposition during Ni-P electrodeposition. Formation of this mechanism is the claimed impossibility of obtain- species was also identified by Crousier et al. [29] ing phosphorus in aqueous solutions through elec- who performed a cyclic voltammetry study. Zeng trochemical methods in the absence of metal ions and Zhou [36] obtained Raman spectra during Ni-P [29]. electrodeposition that indicated the formation of According to the indirect mechanism, first pro- Ni(PH3)n intermediate species. According to the posed by Fedotev and Vyacheslavov [30] and sub- authors, this intermediate was then oxidized by sequently by Ratzker et al. [31], phosphorus oxy- Ni+2 with the consequent formation of the alloy. acid is initially reduced to phosphine (PH3) in the Saitou et al. [34] generated analytical solutions + presence of H ions, formed PH3 is then oxidized of kinetic equations for the reactions describing to P in the presence of Ni+2 ions, while Ni+2 is si- the phosphorus co-deposition according to both ad- multaneously reduced to Ni. Zeller and Landau vocated mechanisms. The results predicted depen- [32] supported the hypothesis of PH3 formation dence of the phosphorous content in Ni-P electrode- and suggested its subsequent reaction with Ni+2 posits on current density (Figure2). Phosphorous producing Ni-P, H+ and a series of phosphorous content, as measured by the electron probe micro- oxyacids. analyzer (EPMA), was found to decrease with the Figure1 gives an overview of the assumed half increase in current density which was in agreement reactions describing phosphorus incorporation ac- with the solution of kinetic equations correspond- cording to both proposed routes, direct and indi- ing to the indirect mechanism of phosphorus co- rect. deposition. Morikawa and colleagues [37] studied the electrodeposition of Ni-P from a citrate bath. Bind- ing energy data obtained from X-ray photoelectron spectroscopy (XPS) indicated the direct reduction of H2PHO3 to P, implying direct deposition of nickel- phosphide. Detection of phosphine by authors of previous studies Morikawa et al. explained with the possibility of partial reaction of phosphorus atoms on the electrode surface with hydrogen at low pH values, which in their case was negligible owing to high pH value of the employed citrate bath.

Figure 2: Schematic diagram of phosphorous concentration vs. Figure 1: Half reactions relevant to incorporation of phosphorus current density derived from rate equations for the indirect into Ni-P alloys. Reprinted with permission from [17]. Copy- and direct mechanism. Reprinted with permission from [34]. right (2003) Taylor & Francis. Copyright (2003) Electrochemical Society, Inc.

4 Ordine and coworkers [35] found kinetics of 3.2 Electrolytic bath composition phosphorus incorporation to be strongly affected by the electrolytic solution pH value. They employed The properties of the electrodeposited Ni-P coat- interfacial pH measurements, cathodic polariza- ings depend greatly on the composition of the em- tion curves and electrochemical impedance spec- ployed electrolytic bath. The majority of nickel troscopy and observed different mechanisms of Ni plating solutions are based on the ‘Watts’ formula- and Ni-P electrodeposition at different degrees of tion developed by Professor Oliver P. Watts in 1916 solution acidity in the presence of NaH2PO2·H2O [41] owing to its simplicity and low cost. Modi- in the electroplating bath. fied Watts electrolyte for Ni-P plating combines the Sotskaya and Dolgikh [38] conducted electro- traditional nickel sulfate, nickel chloride and boric chemical investigation of the kinetics regarding ca- acid with a phosphorus oxyacid which is a source thodic reduction of hypophosphite anion. Their re- of phosphorus. Modified Watts bath containing sults indicated that phosphorus formation proceeds sodium hypophosphite as a phosphorus source has via two parallel routes: electrochemical and chemi- also been reported in many works [35, 36, 42–44]. cal, whose realization depends on the nature of the Other than sulfate electrolyte, several other kinds employed metal catalyst. According to them atomic of Ni-P aqueous electroplating baths exist, contin- phosphorus is formed as a result of the cathodic re- gent on the nickel source nature mostly used are duction of hypophosphite ion. They observed also sulfamate and sulfonate ones. that the Ni-P alloy electro-synthesis takes place in Nickel source–Most of the studies dedicated to the region of potentials, which does not correspond nickel and nickel alloys electrodeposition have to the electro-reduction of hypophosphite anion, been restricted to simple sulfate containing or on thus they concluded that a chemical disproportion- Watts baths. A wide range of coatings can be de- ation reaction of hypophosphite occurs in this case posited from these versatile and stable electrolytes, [39]. thus they still remain a basis for electroplating Ni- Bredael et al. [40] proposed that two mecha- based coatings [45]. nisms are involved in the formation of Ni-P elec- Sulfamate baths are employed primarily for the trodeposits, one that is active at low current effi- purposes of high speed plating and electroform- ciency and leads to formation of amorphous de- ing. Nickel sulfamate possesses high solubility in posits and the other dominating at high current aqueous solutions, thus higher nickel concentra- efficiencies and leading to formation of crystalline tion can be achieved compared to other nickel elec- ones. Such a claim was corroborated by polariza- trolytes which facilitates higher plating rates [46, tion measurements in which a change of slope of 47]. Ni-P deposits obtained from sulfamate baths the polarization curve was observed, this at the exhibit lower internal stress, good ductility and average current density corresponding to the tran- enable higher current efficiency [4, 47, 48]. Stress sition from amorphous to crystalline structure. values of electrodeposits fabricated from sulfamate, One can infer that making an attempt to delin- sulfate and Watts baths are reported to be approxi- eate the phosphorus co-deposition process with Ni mately 30, 180 and 250 MPa, respectively [48]. Nev- constitutes a significant endeavour. Disagreements ertheless, several issues need to be addressed when exists on whether direct or indirect mechanism can a nickel sulfamate bath is used. Sulfamate ion is be assumed, with most of the authors supporting stable in neutral or slightly alkaline solutions, how- the latter. Additionally, indications exist that phos- ever because of the nickel hydroxide precipitation phorus incorporation is not completely straightfor- these solutions are not used at pH values greater ward and that several different mechanisms might than 5. Sulfamate hydrolysis reaction has been govern its co-deposition depending on the process found to proceed at an increased rate at lower pH conditions. In the origin of the incurred difficulties values and at higher temperatures, thus sulfamate lie the large number of factors directing and influ- baths need to be operated at lower temperatures encing the deposition process and many different and higher pH values than Watts-type nickel plat- approaches applied to define their nature and mag- ing solutions. pH values less than 3.0 and temper- nitude, all in various electroplating environments. atures above 70 ◦C ought to be avoided as nickel sulfamate can hydrolyze to its less soluble form- nickel ammonium sulfate [46, 47]. Incorporation of ammonium and sulfate ions in the deposit can lead to the internal tensile stress augmentation. Sulfa-

5 mate ion additionally tends to decompose at the electrolyte systems for Ni-P electroplating: the one anode; at insoluble anodes such as platinum and at containing phosphorous acid and the other contain- passive nickel oxide electrodes, and its decomposi- ing hypophosphite. Generally, the quality of Ni- tion can result in intermediate species which may P deposits electroplated from electrolyte contain- affect the quality of the electrodeposited coatings ing hypophosphite is changeable because the de- [46, 47]. However, nickel sulfamate solutions are posits darken when electroplating time gets longer preferred in some cases over nickel sulfate solutions or current density becomes higher, while the Ni- because of the superior mechanical properties of P deposits electroplated from electrolyte contain- the fabricated coatings, high rates of deposition and ing phosphorous acid are smooth and bright [5]. lower influence of variations in pH and current den- However, as reported, phosphine is more readily sity on the deposit’s quality [49]. Figure3 shows produced from hypophosphite than from phospho- the comparison of phosphorus content, current effi- rous acid owing to the rate of reduction of H3PO3 ciency and stress in the obtained Ni-P deposits as being the limiting factor in the phosphorous co- a function of phosphorous acid concentration, for deposition in this case [18]. the sulfate and sulfamate electrolytes. Anode dissolution–If used as a contributing Another interesting electrolyte for Ni-P alloy source of Ni in the bath, nickel chloride has two ma- electroplating is the one containing methanesul- jor functions: it appreciably increases solution con- fonate or methanesulfonic acid. Sknar et al. [50] ductivity thereby reducing voltage requirements found the effect of reducing kinetic difficulties of and it is important in obtaining satisfactory dis- the Ni-P alloy electrodeposition to be more pro- solution of nickel anodes [53, 54]. Due to the in- nounced when using methanesulfonate than when crease in solution conductivity, plating thickness employing sulfate electrolyte. This is owing to and cathodic current efficiency are reported to in- weaker buffering properties and lower stability of crease with the increase of chloride concentration nickel acido-complexes of methanesulfonate which [44]. In the chloride electrolytes activity of Ni+2 contribute to the increase of concentration of the ions is higher than in sulfate electrolyte and metal present electroactive species. Alloys obtained from deposition potential is lower [55, 56]. However, de- sulfate baths possess higher phosphorus content posits obtained from chloride electrolytes with high which is explained by the elevated acidity in the chloride concentrations are reported to possess dif- near electrode layer due to stronger buffering prop- ferent texture and higher internal stress compared erties of the sulfate electrolyte [39]. Methanesul- to those obtained from sulfate ones [56]. It is possi- fonic acid has good electrolytic conductivity and ble to operate with zero chloride in the bath if sulfur is capable of dissolving many metals as well as activated nickel anodes are used under appropriate acting as a useful medium for dispersion of solids operating conditions, nonetheless a low concentra- prior to electrophoretic coating. A diverse range tion of nickel chloride (min 5 g L−1) is generally rec- of surface coatings and films are available from ommended so as to provide reasonable insurance methanesulfonic acid electrolytes [51]. Compared of satisfactory anode performance [57]. Alterna- with known nickel plating baths, such as Watts tively, nickel bromide that does not increase stress bath, methanesulfonate bath can be considered to as much as chloride may be used. enable higher current density and result in higher Buffering–As most authors claim, boric acid lim- throwing power. Maximum nickel deposition rates its the effects on the solution pH value resulting can be achieved with deposits having low poros- from the discharge of hydrogen ions and simpli- ity, low internal stress and high ductility [51]. This fies pH control [58]. However, many studies show organic acid may be considered as a ‘green’ elec- that the beneficial influence of boric acid on Ni electrolyte since it possesses few environmental, stor- trodeposition is somewhat complicated and that its age, transport or disposal problems being readily effect stems further from simple buffering, extend- biodegradable. Additionally, methanesulfonic acid ing to its impact on deposit’s crystallographic struc- exhibits high solubility for metal salts such as those ture, morphology, brightness, adhesion, etc. [56]. of Pb and Ag, which are soluble only in a limited Presence of boron in the bath waste is nonetheless number of acid electrolytes [52]. reported to be harmful for the environment. Since Phosphorus source–Source of phosphorus in the the enforcement of Water Pollution Control Act in Ni-P electroplating bath is typically a phosphorus Japan [59] the imposed national minimum effluent oxyacid or its salt. There exist mainly two kinds of standards have greatly influenced the electroplat-

6 (a) Phosphorus content vs C(H3PO3) (b) Current efﬁciency vs C(H3PO3) (c) Stress in the deposit vs C(H3PO3)

Figure 3: Comparison of stated parameters’ values versus H3PO3 concentration for the cases of Ni-P alloy electrodeposition from sulfamate and sulfate baths. Reprinted with permission from [48]. Copyright (2003) Electrochemical Society, Inc.

ing industry. Thus, it became necessary to find fied the requirement necessary to form nickel phos- more environmentally friendly solutions for the phide with a stoichiometric composition equivalent electrodeposition of nickel and its alloys. to Ni3P in a wide range of current densities. Phos- One of the possible alternatives is proposed in phorus content in the deposits was found to be the terms of substituting boric acid with citric acid. constant around 25 at.% in a broad range of plating This species can complex free nickel ions in the so- conditions. lution influencing the deposition rate but also acts When it comes to the environmental impact ex- as a buffering agent [60]. Doi et al. [61] found that erted by the electroplating process from either of the properties of Ni electrodeposits and resulting the two baths: the one containing boric or the other cathode current efficiency depend on the employed containing citric acid, findings are quite contradic- citrate bath pH. Electroplating resulted in high cath- tory. Takuma et al. [59] applied a life cycle as- ode current efficiency and hard deposits exhibiting sessment method to demarcate the extent of their crystal structure of nearly random orientation in influence in terms of human toxicity and ecotoxi- case of the bath pH value being between 4 and 6, city. Results indicated that the newly developed while when the pH value was 3.5 or less current ef- citrate plating bath exerts higher environmental im- ficiency was lower and hardness of the obtained de- pact compared to the traditional Watts electrolyte, posits decreased. Dadvand and colleagues [44] re- this owing to the release of nickel chelated with ported electrodeposition of Ni-P from a citrate bath citric acid whose harmful influence overshadows containing sodium chloride, citric acid, nickel sul- the benefit of reduced boron emissions achieved by fate, ammonia and sodium hypophosphite. They substituting boric acid in the electroplating bath. found cathode current efficiency in low current In the Brenner type sulfate bath [28, 62], pH density regions to be twice higher compared to buffering is achieved generally by employing phos- the one achieved when employing modified Watts phoric acid (H3PO4) whose slight excess can more- bath. Additionally, obtained deposits exhibited over aid in the H3PO3 oxidation prevention [63]. more uniform plating thickness. Even though com- Pillai and colleagues [63] investigated electrode- plexes form between nickel and citrate ions and position of Ni-P alloy from a Brenner type bath citrate ions as such can adsorb on the cathode sur- containing both phosphorous and phosphoric acid. face, block active sights for Ni+2 discharge process Their observation was that a 100% nickel coating and thus decrease the plating thickness, authors of without any phosphorous incorporation was ob- the mentioned work estimated that this effect was tained when the plating was carried out in the ab- −1 small for citric acid concentrations below 20 g L . sence of H3PO3 indicating that phosphorous acid Morikawa and colleagues [37] employed Ni-citrate is the only electrochemically active phosphorous bath and obtained uniform and bright Ni-P elec- species which acts as a phosphorous source in the trodeposits with high throwing power. H3PO3 was given solution. added in excess so as to generate a large amount Electroplating baths containing phosphorus of P atoms on the Ni electrode. Thus according to and/or phosphoric acid can be very acidic, hence them, the bath composition was such that it satis- typically partial neutralization is necessary in or-

7 der to increase the pH value of the solution up to Table1 gives an overview of the proposed com- an optimum value [62]. For this purpose different positions of the aqueous baths for Ni-P alloy elec- alkaline compounds can be employed, i.e.: potas- troplating. Applied deposition conditions and sium or sodium carbonate, potassium or sodium achieved phosphorus content where this informa- hydroxide, ammonia. Dadvand et al. [44] advo- tion was available are stated along with some basic cated the benefits of using ammonia for bath neu- observations. tralization, as carbonate salts decrease the solution Non-aqueous baths–All mentioned traditional Ni- conductivity and alkali metal hydroxides promote P electroplating baths are corrosive and their use the formation of nickel hydroxide complexes. possesses a drawback in terms of generating toxic Additives–Electrolyte additives are another cru- effluents. As an alternative to conventional elec- cial factor in the Ni-P electroplating process. They troplating solutions, You and colleagues [70] have can be incorporated in the electrolytic solution in proposed Ni-P electrodeposition from a bath con- order to influence grain growth and crystallites ori- taining choline chloride:ethylene glycol (1:2 molar entation, to suppress undesirable secondary reac- ratio) deep eutectic solvent (DES) and NiCl2·6H2O tions, to promote the co-deposition of alloys, dis- and NaH2PO2·H2O as nickel and phosphorous solution of metals, etc. Additives can be grouped sources, respectively. DESs are formed from mix- according to their main purpose as: carriers, surface tures of Lewis or Brønsted acids and bases and wetters, inhibitors or levellers, auxiliary brighten- can contain a variety of anionic and/or cationic ers, buffers, conductive electrolytes and chemical species. Employing ionic liquids in electroplating complexants [64]. It is important to keep in mind is beneficial owing to them being less corrosive, that these species are also incorporated in the coat- possessing a wider electrochemical window and ing during its deposition, hence they may produce exhibiting lower hydrogen evolution when com- various adverse effects on the deposit’s properties, pared to aqueous baths. As such they can present a for example decrease corrosion resistance and ex- suitable alternative for traditional plating solutions acerbate mechanical properties. As such, their use [71]. Disadvantages of DESs however include their should be carefully optimized. lower electrical conductivity, metal salts solubility Grain refiners/carriers are usually aromatic or- and higher viscosity in comparison with aqueous ganic compounds that contain sulfur whose co- electrolytes. deposition with nickel inhibits its grain growth. It has been proposed that environmental issues Most commonly employed grain refiner in Ni-P related to the use of aqueous baths in Nickel elec- electrodeposition is saccharin. This species is re- troplating can be mitigated also by performing elec- ported to effectively reduce the internal stress of trodeposition in a supercritical CO2 fluid [80, 81]. the fabricated deposits [65], however its use comes Chuang and colleagues [82] studied the properties with the concerns related to deterioration of the and the electrodeposition behaviour of Ni-P coat- corrosion resistance due to incorporation of sulfur ings in emulsified supercritical CO2 in the presence in the growing deposit [66]. Saccharin is reported of suitable surfactants. They observed improved to prompt the decrease in the phosphorus content hardness, wear resistance and surface quality of the in the coating and to bring an improvement in cur- obtained coatings compared to deposits fabricated rent efficiency [67, 68]. This is related to occupa- from conventional aqueous electroplating solutions. tion of cathode surface active centres by saccha- However, current efficiency from supercritical CO2 rin molecules and its conversion which consumes bath was lower. available hydrogen, hence restricts the phosphorous co-deposition [68]. 3.3 Current efficiency It is known that properties of Ni electrodeposits depend on their crystallographic structure. Ni (100) In accordance with both proposed phosphorus in- soft-mode texture is associated with deposits that corporation mechanisms, direct or indirect, concur- possess maximum ductility, minimum hardness rent reduction of protons at the surface of the grow- and internal stress [69]. Textural modifications of ing deposit layer is necessary for the co-deposition this matrix can be induced by the introduction of of phosphorus with nickel (Figure1). However, certain additives [68]. Mixed crystal orientation subsequent recombination of formed Hads limits can be achieved allowing to fabricate deposits with the amount of phosphorus that can be deposited. controlled properties. Thus, surface H+ concentration affects both the phosphorus content and the cathode current effi-

8 Table 1: Proposed bath compositions and deposition conditions for Ni-P alloy electroplating from sulfate, sulfamate and sulfonate baths according to various literature sources. Obtained phosphorus content is stated where applicable and some basic observations are noted. (jd-current density, t-temperature, ε-duty cycle, f-frequency, CE-cathode current efﬁciency) C, 1 1 ◦ 2 − − 25%) 50%) − 1 ∼ ∼ − , at 60 3 70%) 25%) C (CE C (CE ◦ ◦ PO ∼ ∼ 12 wt.% P 0.5M P levels off 3 ≥ > and pH 3.5 3 2 C (CE and 80 and 80 ◦ − PO 3 4 0.5M H 3 25%); 30 wt.%P for 12 g L 20 wt.%P for 125 g L ∼ ∼ ∼ PO PO ∼ 3 3 3 A dm Maximum P content from baths following conditions: H H carbonate (CE and 70 2-3 %P 12-15%P 2 2 2 2 2 2 − C I:35 wt.%P for 40 g L C up to 35 wt.%P II: 2 2 − − − − − ◦ ◦ − − C 0-25 at.%P for C IV:30 wt.%P for 5 A dm ◦ ◦ C C up to 20 wt.%P structure at C C III: ◦ ◦ ◦ ◦ 10 A dm 10 A dm 10 A dm < < < pH 1.7-3.0 pH 0.43-1.0 Transition to amorphous 63 pH 0.5-1.0 80 and 90 60 2-5 A dm 85 2-5 A dm 80 and 90 2-150 A dm RDE 750 rev/min 90 70-90 5-40 A dm 1 − 1 − 1 1 − 1 − 1 1 and: I,II,III and IV is achieved for − 1 − − 1 1 1 1 O 0-15 g L − − − − − 2 1 1 − − 1 1 1 1 − − − − ·4H 1 1 1 1 1 1 O 150 g L 2 − − − − − − O 330 g L 2 2 O 150 g L O 150 g L 50 g L 50 g L O 45 g L O 50 g L O 45 g L O 45 g L 0-200 g L 2 2 50 g L 4 4 2 2 2 2 ·6H 4 ·6H 4 4 0.49M 1-30 A dm 30 g L 0-2M For H 0.225-2.25 g L 0-40 g L 50 g L 40 g L 20 g L 42.5 g L 20 g L 3-70 g L 20 g L 4 0.38M pH 1.5-4.5 Maximum P content 25 at.% ·NiOH ·6H ·6H 0.13M 30-90 PO PO 3 3 3 3 3 4 3 3 4 3 3 3 3 ·6H ·6H ·6H ·6H PO 4 4 4 3 3 2 2 2 2 2 PO 3 3 PO BO PO BO PO PO PO PO PO PO PO PO 3 3 3 3 3 3 3 3 3 3 3 3 H Citric acid 0.5M (CE I: NiSO NiCl NiSO NiSO NiSO NiCl H Non-pitting agent 0.15 g L II: NiSO NiCl NiCl I: H H H H H H III: H H H NiCO IV: H H Reference Bath composition Deposition conditions Phosphorus content Additional information Narayan and II: H Durney, 1984 [ 72 ] Mungole, 1985 [ 73 ]H Bredael et al., 1993 [ 40 ] NiCl Morikawa et al., 1996 [ 37 ]H

9 Reference Bath composition Deposition conditions Phosphorus content Additional information −1 NiSO4·6H2O 330 g L pH 1.0 and 4.0 13-28 at.%P (pH 1.0 Composition control by −1 ◦ Hu and Bai, 2001 [42] NiCl2 45 g L 20 and 50 C , NaH2PO2·H2O 1M) simultaneous change −1 −2 Hu and Bai, 2003 [74]H3BO3 37 g L 5 and 25 A dm 4-12 at.%P (pH 4.0 of t and jd; Amorphous NaH2PO2·H2O 0.5 and 1M 200 and 400 rev/min , NaH2PO2·H2O 0.5M) structure for ≥12 at.%P −1 NiSO4·6H2O 150 g L pH 1.8 Primary nucleation of Ni −1 ◦ NiCl2·6H2O 45 g L 55 C 27 at.%P followed by Ni-P formation; −1 Kurowski et al., 2002 [75]H3PO4 50 g L -420 mV (SHE) Growth driven by −1 NiCO3 40 g L surface diffusion of −1 H3PO3 40 g L Ni and P species NiCl2·6H2O 0.2M pH 3.3 10 at.%P for 4M NH4Cl ◦ Li et al., 2003 [76] NaH2PO2·H2 0.1M 25 C 2-10 at.%P (CE∼60%) −2 NH4Cl 0.5-4M 5-50 A dm −1 NiSO4·6H2O 137 g L pH 1.5 Nanocrystalline deposit; −1 ◦ NiCO3 36.5 g L 70 C 0.5-2.5 wt.%P Maximum hardness (990HV) −1 −2 Jeong et al., 2003 [77]H3PO3 2-3 g L 1-3 A dm and wear resistance after −1 ◦ 10 Saccharin 5 g L Magnetic stirring heat treatment at 370 C SLS 0.1 g L−1

Ni(SO3NH2)2·4H2O pH 1.0-2.5 Maximum P content 14 wt.% (Ni+2 90 g L−1) 50 ◦C 2-12 wt.%P for ε=0.1 and f>100Hz −1 −2 Lin et al., 2006 [4] NiCl2·6H2O 3 g L jp 8 A dm (CE∼ 80%); −1 Chen et al., 2010 [65]H3BO3 40 g L 10-500 Hz Deposit still crystalline; −1 H3PO3 10 g L ε 0.1-0.5 Compressive internal stress SDS 0.4 g L−1 Air bubbling (-40 MPa) −1 NiSO4·6H2O 240 g L pH 0.8-1.8 Maximum P content −1 ◦ NiCl2·6H2O 28 g L 45 and 75 C up to 10 wt.%P for higher H3PO3 −1 −2 Yuan et al., 2007 [5]H3BO3 30 g L 4 and 10 A dm concentration and −1 H3PO3 2 and 8 g L Magnetic stirring lower pH values 150 and 550 rev/min

Ni(SO3NH2)2·4H2O pH 1.5-3.5 Maximum P content +2 −1 ◦ (Ni 96 g L ) 45 C for higher H3PO3 −1 −2 Chang et al., 2008 [78] NiCl2·6H2O 4 g L 2 A dm 1-4.5 wt.%P concentration and −1 H3BO3 32 g L Air bubbling lower pH values −1 H3PO3 0-4 g L (CE∼ 70%) Wetting agent 0.3 mL L−1 Reference Bath composition Deposition conditions Phosphorus content Additional information −1 NiSO4·6H2O 170 or 330 g L pH 0.5-3.0 −1 ◦ Schlesinger and NiCl2·6H2O 35-55 g L 60-95 C −1 −2 Paunovic, 2010 [7]H3BO3 0 or 40 g L 2-5 A dm −1 H3PO4 50 or 0 g L −1 H3PO3 2-40 g L −1 NiSO4·7H2O 150 g L pH∼1.5 P content decreases with −1 ◦ NiCl2·6H2O 45 g L 50-80 C 0-20 wt.%P H3PO3 concentration decrease −1 −2 H3PO4 0-40 g L 5-30 A dm and increase of jd and t −1 −1 Pillai et al.,2012 [63]H3PO3 0-20 g L (up to 15 g L H3PO3); SLS 0.25 g L−1 Deposit amorphous for ≥9 wt.%P; Hardness 8.57 GPa (4-7 wt.%P), after annealing at 400 ◦C maximal hardness (12 GPa)

NiSO4·6H2O 0.65M pH 1.5 Transformation to crystalline

11 ◦ NiCl2·6H2O 0.75M 10.6 at.%P at 500-600 C; −2 Nava et al., 2013 [20]H3BO3 0.15M 3 A dm Annealed deposit hardness NaCl 2M 990 HV H3PO3 0.1M NiSO4·6H2O 0.2M pH 3.0-4.0 at surface: Soft magnetic character, ◦ H3BO3 0.005M 70 C ∼8-12 at.%P plating conditions dependant; Alleg et al., 2016 [79] NaH2PO2·H2O 0.1M -1.3 to -1 V across depth: Deposits mixtures of Saccharin 0.005M ∼4-6 at.%P Ni(P) solid solutions and NaCl 0.7M amorphous or Ni2P phase I: Ni(CH3SO2)2 1M pH 3.0 Max P content for 0.12M ◦ −2 NaCl 0.3M 60 C NaH2PO2 and 2 A dm : −2 NaH2PO2 0.03-0.12M 2-7 A dm ∼8 (I) and ∼13 wt.%P (II); Sknar et al., 2017 [50] up to 13 wt.%P II: NiSO4 1M P content higher from NaCl 0.3M sulfate bath; H3BO3 0.7M Internal stress and hardness NaH2PO2 0.03-0.12M better from sulfamate ciency whose magnitude can never reach values As mentioned in the previous section, certain close to 100% as long as the phosphorus is co- additives such as saccharin are found to improve deposited with nickel. the current efficiency of Ni-P electrodeposition [68]. Cathode current efficiency of Ni-P electroplat- In Ni-P electroplating, anodes are usually made ing is primarily affected by the electrolytic bath of nickel and current efficiency of nickel dissolu- composition, plating regime, applied current den- tion in additive free solution is approximately 100% sity, temperature, bath pH value and agitation rate. (small percentage of the current is consumed on Ross et al. [83] found that the current efficiency the discharge of hydrogen ions from water). Hence decreases as the deposit’s phosphorus content in- there exists a difference in cathode and anode ef- creases and that it depends on the rotation speed ficiency which leads to nickel ions build up in the of the cathode. In the case of rotating-disc elec- solution and its pH value increase. This problem trode (RDE), maximum efficiency was attained for can be solved by solution drag-out [87] and by the low phosphorus plating solutions and low rotation use of soluble and inert anodes connected to dif- speeds. In general, electrodeposition of high phos- ferent power supplies, while the current to each phorus Ni-P coatings proceeds at a lower current ef- is controlled in order to compensate for the low ficiency than the electrodeposition of low phospho- cathode efficiency and incurred drag-out losses [7, rus ones [4, 40]. Naryan and Mungole [73] found 25]. that the current efficiency decreases with increasing H3PO3 concentration and decreasing temperature 3.4 Crystallographic structure of the sulfate electroplating bath. Similarly, Seo and colleagues [48] observed that with increasing Ni-P electrodeposits’ crystallographic structure is H3PO3 concentration in the sulfamate bath, phos- strongly influenced by their phosphorus content phorus content in the deposit increases, current and the employed deposition conditions. Incor- efficiency decreases and stress in the obtained de- poration of even small amounts of phosphorous posits augments. Morikawa [37] observed that in in the nickel lattice substantially refines the nickel citrate bath current efficiency exhibits a maximum grains. In the case of low-phosphorus Ni-P elec- at intermediate H3PO3 concentrations. trodeposits, obtained XRD patterns reveal a set of Cathode current efficiency in Ni-P electroplat- diffraction peaks corresponding to face-centred cu- ing is generally found to increase with the increase bic (f.c.c.) nickel, i.e.: (111), (200), (220), (311), (222) of current density [25, 84, 85], bath temperature [18] [88]. Presence of these textures is an indication and its pH value [18, 86]. Luke [25] found that the of crystallinity of the low-phosphorus coatings in cathode current efficiency does not vary apprecia- which case a material is a supersaturated solution bly with current density in the higher range (above of phosphorus in f.c.c. nickel [83]. As the phos- ∼20 A dm−2) but increases with the increase of cur- phorus content in Ni-P deposits increases, (111) rent density at its lower values. Similar trend was reflection becomes broader while others disappear, observed by Toth-Kadar et al. [85]. Li et al. [76] as it can be seen on Figure4. This is indicative found cathode current efficiency to be lower for a of nickel crystalline structure losing its long-range larger total current density. This is owing to the order due to the increased difficulty of more phos- discharge reaction of H+ ions which results in the phorus atoms accommodating into the nickel lat- evolution of hydrogen gas bubbles. This reaction tice. Co-deposition of phosphorus in octahedral shares a larger portion of the electrodeposition cur- interstial sites of f.c.c. nickel inhibits the surface rent at higher total current densities. diffusion of Ni atoms and the subsequent crystal In general, reactions involved in the conversion growth [17]. As the phosphorus content in the de- of phosphorus in the solution into phosphorus in posit increases the nucleation becomes dominant the deposit can be interpreted differently and thus over nucleus growth and when the critical phos- the discrepancies in elaborating cathode current phorus content is exceeded amorphous structure efficiency behaviour may arise. characterized by a short range order of only few Cathode current efficiency of Ni-P alloy elec- atomic distances is obtained. More colony-like mor- trodeposition can be as reported significantly im- phology is achieved where each colony consists of proved by employing pulse instead of direct cur- numerous grains with smaller grain size thereby rent plating [4]. Benefits of using pulse plating for making the coating brighter and smoother in ap- Ni-P deposits fabrication are explored in Section6. pearance [63]. Figure5 shows SEM photographs of Ni-P deposits with varying phosphorus contents.

12 phosphorus are entitled X-ray amorphous. Owing to the variations of the pH value at the electrode-solution interface inherent to the Ni-P electroplating process, phosphorus content varies across Ni-P deposit thickness, hence its composition and microstructure are not homogeneous (Sec- tion7). Alkaline solutions are more influenced by the pH variations compared to acidic ones, thus electrodeposits produced in this environment are often characterized by a lamellar structure [79]. Ni-P electrodeposits can be grouped in three cat- egories, alloys with: low (1-5 wt.%), medium (5-8 wt.%) and high (above 9 wt.%) phosphorus content [17]. Microstructure-wise, it is possible to fabricate polycrystalline, microcrystalline, nanocrystalline or fully amorphous Ni-P deposits through electroplating. Pillai et al. [63] found the structure of Ni-P alloy to undergo transition from crystalline to nanocrystalline and become amorphous at phosphorus contents above 9.14 wt.%. Bredael and coworkers [40] fabricated Ni-P electrodeposits with phosphorus contents ranging from 0 to 18.0 wt.% and a uniform composition profile across the samples. They Figure 4: XRD patterns of polycrystalline Ni and nanocrystalline observed that for phosphorus contents above 12 Ni-P coatings. Reprinted with permission from [77]. Copyright (2003) Elsevier. wt.% the as-plated Ni-P coatings were amorphous, whereas below this threshold value, which was in- At high-phosphorus content XRD pattern of a dependent of the plating parameters, crystalline Ni-P electrodeposit exhibits only a diffuse broad Ni-P coatings were obtained. In a subsequent work peak. Hence, an increase in phosphorus content Bredael and colleagues [89] conducted a study with converts the microstructure of Ni-P alloys from the goal to exactly identify the percentage of phos- crystalline to amorphous, which results in the de- phorus at which the transition from crystalline to creased ductility and the increased corrosion re- amorphous structure takes place. They found that sistance. It has been hypothesized that adsorbed the border between these two states can be found hydrogen blocks the surface and prevents regular at phosphorus contents between 11.6 and 13.1 wt.% crystal growth, hence playing a crucial factor in ob- P, with the structure being fully amorphous at P taining N-P deposits with amorphous structure, contents ≥13.1 wt.%. Although there is no full con- however amorphous deposits can be fabricated sensus many authors agree that the phase transition with high current efficiencies and low hydrogen of Ni-P happens over a wide range of wt.% of P evolution thus indicating that the adsorbed hydro- and not abruptly at a specific phosphorus content gen does not play a major role [17]. [63]. Lin et al. [4] found that not only phosphorus Bredael et al. [89] observed significant broaden- content influences the crystallographic structure ing of the (111) XRD peak for Ni-P deposits contain- of Ni-P electrodeposits. At the same phosphorus ing more than 9 wt.% of phosphorus. However they percentage substituting direct current deposition stated that the observed effect, other than being a by pulse current plating can result in preserving sign of the structure becoming amorphous, can also crystalline structure even at phosphorus contents originate from a crystalline material with very fine higher than a critical value. Contrary, phospho- grains (1nm) and may be due to non-uniform in- rus content at which transition from crystalline to ternal stresses and stacking faults. Hence, as they amorphous structure occurs has been reported to asserted, it is not possible to exactly distinguish decrease in the presence of certain additives, such between amorphous and microcrystalline structure as saccharin for example [17]. based only on XRD patterns. Nonetheless, conven- In general, discrepancies which exist in the lit- tionally Ni-P deposits with more than 9 wt.% of

13 erature related to the structural transitions of Ni- (Figure9) when compared to both coarse-grained P electrodeposits can be attributed to the lack of and amorphous ones [10, 64, 95, 96]. The main appropriate techniques that would allow to make feature, which makes nanocrystalline materials dis- a proper differentiation between different crystal- tinct from the other two mentioned groups, is the lographic structures. Additionally, Ni-P alloys existence of a larger number of atoms disposed at are frequently not straightforward amorphous or interfaces, as grain boundaries and triple junctions. crystalline but they represent a mixture of several These interfaces are considered to be involved in phases. This can be owing to the low solubility of the deformation mechanism occurring in the mate- phosphorus into the nickel lattice. Namely, often rial. It is postulated that for nanocrystalline materi- Ni-P deposits contain more of the alloying element als dislocations inside the grains hardly occur and (P) than what the f.c.c. Ni matrix can dissolve, thus that other plastic deformation mechanisms, such the surplus must separate out resulting in regions as grain boundary diffusion and sliding, grain ro- that have a different concentration of phosphorus, tation and a viscous flow of interfaces take place different lattice parameters and different crystal- [88]. lite sizes. Vafaei-Makhsoos et al. [90] found alloys with 3.8 and 6.7 wt.% of P to be solid solutions composed of micrometer-sized grains. However, 3.5 Variables influencing phosphorus amorphous alloys (11.7 and 13 wt.% P) were not ho- content mogeneous on a microscale and films contained microcrystalline regions. Ni-P electrodeposits display- Phosphorus content of Ni-P electrodeposits report- ing a structure that represents a mixture of solid edly depends on many factors, dominantly on the Ni(P) solutions and amorphous or nanocrystalline concentration of the phosphorus donor in the bath, phases have been reported [79]. temperature, pH, current density, waveform in the Microstructure of Ni-P electrodeposits under- case of pulse current deposition, agitation rate and goes a transformation if they are exposed to a sub- the presence of additives. Thus, phosphorus quan- sequent thermal treatment. This aspect is further tity can be controlled by altering the electroplat- elaborated in Section5. ing bath composition and the employed deposition Suitable structure of the electroplated Ni-P al- conditions. loy depends on the desired features of the fabri- In the 1950s, Brenner [28] found that the amount cated deposit. Crystalline materials are tradition- of phosphorus incorporated in the Ni-P alloy in- ally interesting on account of their exceptional me- creases with the increase of phosphorus acid con- chanical properties. Amorphous materials are very tent in the bath and with the decrease of the current appealing owing to the isotropy of their proper- density. However, too high H3PO3 concentration ties and them lacking disadvantages characteristic leads to deterioration of the current efficiency ow- for crystalline ones, namely the presence of crystal ing to more cathodic charges being spent on the boundaries, lattice defects, segregation, etc. [91]. reduction of protons. Additionally, proton reduc- Amorphous Ni-P electrodeposits are reported to tion results in the atomic and molecular forms of possess better corrosion resistance when compared hydrogen, both of which can be incorporated in to crystalline deposits. However, transition to amor- the deposit imposing inauspicious effects on its phous structure and consequent decrease of the properties and increasing its internal stress [97]. grain size causes deterioration in mechanical prop- Chang et al. [78] found that for the same content of erties which is termed an inverse Hall-Petch ef- H3PO3, phosphorous content can be increased by fect [92]. Its onset is related to the increase of the decreasing electrolytic bath pH value on account volume fraction of the triple junctions relative to of the consequent increase of the concentration of the volume fraction of the grain boundaries which non-dissociated phosphorous acid molecules in the exerts a detrimental effect on coating’s hardness electrolyte. This effect is however weakened at very [93]. Jeong et al. [94] produced Ni-P electrode- low phosphorus acid concentrations on account of posits with grain size of less than 10 nm, thereby diffusion becoming the rate-controlling process. achieving hardness values that fall into the range of Yuan and coworkers [5] conducted a study on the inverse Hall–Petch behaviour. Recently exten- the preparation of amorphous-nanocrystalline Ni-P sively reported are the results testifying even better electrodeposits from a Brenner type plating bath. mechanical properties of nanocrystalline materials Their goal was to interrogate the key electroplating factors and their influence on the deposit’s phos-

14 Figure 5: SEM photographs of Ni-P deposits with P contents of (a) 0, (b) 4, (c) 8, (d) 12, (e) 17, (f) 20, (g) 24 and (h) 28 at.%. Reprinted with permission from [74]. Copyright (2003) Elsevier. phorus content. Effects of: temperature, current current density of the electroplating with strong density, pH, H3PO3 concentration and agitation interactive effect between current density and pH. rate were investigated in the orthogonal experimen- Figure7 shows contour plots for the constant phos- tal design study. Findings indicated that only pH phorus content versus Ni-P electroplating tempera- value and H3PO3 concentration and their interac- ture and current density. tion are the key variables affecting phosphorus content in the deposit. Figure6 shows contour plots for the constant phosphorus content versus Ni-P electroplating pH and H3PO3 concentration. Hu and Bai [42] employed modiﬁed Watts Ni bath and similarly investigated the inﬂuence of the main electroplating variables, namely: temperature, current density, pH, NaH2PO2·H2O concentration and agitation rate on the phosphorus content in the fabricated Ni-P deposits. By using experimental strategies such as: fractional factorial design, path of steepest ascent and central composite design cou- pled with response surface methodology they came Figure 6: Contour plots for the constant phosphorus content (P to the conclusion that the predominant factors af- wt.%) of Ni-P deposits against the electroplating pH (XC) and fecting phosphorus content are temperature and H3PO3 concentration (XD). Reprinted with permission from [5]. Copyright (2007) Elsevier.

15 rent density can be explained in terms of slow dif- fusivity of the large phosphorus acid ions compared with Ni+2 ions [40]. Pillai et al.[63] found that at higher H3PO3 concentration in the bath (≥ 15 g L−1) phosphorus content in the coating is in- dependent of current density. However, at low phosphorous acid amounts in the bath, the decreasing trend of phosphorus content in the coating with increasing current density was evident. Thus, they concluded that the observed scatter in the results presented by different authors, when it comes to the influence of current density on phosphorus content, can be mainly due to the different bath compositions used. In their study, an increase in the concentration of H3PO4 resulted in no significant effect on the phosphorus content in the coating although Figure 7: Contour plots for the constant phosphorus content (P at.%) of Ni-P deposits against the electroplating temperature the rate of deposition decreased continuously. An ◦ (xA) and current density (xB). Reprinted with permission from increase in the plating temperature (from 50 C to [42]. Copyright (2001) Elsevier. 80 ◦C) resulted in the decrease of the amount of phosphorous incorporated in the coating and in the increase of the deposition rate. Naryan and Mungole [73] observed contrary to the previously mentioned study a slight increase of the phosphorus amount in the coating up to H3PO4 concentrations of 125 g L−1 and a slight decrease with further increase in H3PO4 concentration, all at the bath tem- ◦ ◦ perature of 80 C. At 90 C an increase in the H3PO4 concentration generally produced a slight decrease in the phosphorus content in the coating. Naryan and Mungole additionally found that at low H3PO3 concentrations, more phosphorus was deposited ◦ ◦ at 90 C than at 80 C, but at H3PO3 contents in excess of 25 g L−1 more phosphorus was deposited at lower temperature. Sadeghi [21] employed hy- Figure 8: Literature results on phosphorus content vs current pophosphite as a phosphorous source and found density (up to 40 A dm−2). Reprinted with permission from [40]. Copyright (1993) Elsevier. that increasing its concentration and decreasing the current density (1-4 A dm−2) both caused higher Recurring trend of gradual decrease of phos- phosphorous contents in the fabricated Ni-P de- phorous content with the increase of the applied posits. However, the content of phosphorus in the current density is observed in the work of many deposits electroplated from the baths containing authors, however a large scatter between the data very low phosphorus source amounts did not vary points of different authors can be perceived (Figure appreciably with the current density. Deposits ob- 8)[40]. tained from the baths containing up to 10 g L−1 of Bredael and coworkers [40] observed, in their NaH2PO2 exhibited a decrease in the content of study of Ni-P electrodeposition on a rotating-disc phosphorus as the current density was raised. electrode, a steep transition from high to low phos- The increase in temperature has been observed phorus content with increasing local current den- to cause a decrease in phosphorus content in gen- sity, while the literature data according to them do eral. However, too low temperature leads to unsat- not show such a steep transition owing to the gen- isfactory current efficiency and plating rate, thus a eral approach where average current densities are compromise is needed. Temperatures from around used. In the bath where phosphorous acid is the 50 ◦C to 70 ◦C are generally adopted as acceptable only electrochemically active phosphorus species, for optimal feasibility of the Ni-P electroplating dependence of the phosphorus content on the cur-

16 process. higher phosphorus content. Pillai et al. [63] ob- Phosphorus content in the deposit has been re- served that Ni-P electrodeposits with phosphorus ported to be influenced by the presence of certain content in the range of 4-7 wt.% exhibit good micro- additives in the electroplating bath. Increase of the hardness (7.74–8.57 GPa) and the microhardness of saccharin concentration was found to induce the these alloys increases up to 12 GPa by annealing decrease of phosphorus content [17, 67, 68]. at 400 ◦C during 1 h. Yuan and colleagues [5] re- Influence of the pulse plating on Ni-P alloy ported that nanocrystalline composites with phos- structure and composition will be further elabo- phorus content 5-9 wt.% demonstrate better corro- rated in Section6. sion resistance than that of microcrystalline Ni-P deposits, and better abrasion resistance than that of amorphous Ni-P deposits. Therefore, under certain 4 Properties of the electroplated work conditions, when a compromise is needed in terms of obtaining both corrosion resistance and Ni-P alloys favourable mechanical properties, nanocrystalline deposits could present an optimal choice. 4.1 Mechanical, tribological and corro- Another way to ensure good mechanical and sion properties electrochemical properties is to design multilayered or graded coatings. For example, a duplex coating Alloying nickel with phosphorus via electroplating that consists of an inner layer having high phospho- effectuates many improvements of its properties: rus content and an outer layer with low phosphorus increased hardness (&500 HV as-plated Ni-P), wear content presents a structure that combines advan- and corrosion resistance, decreased coefficient of tageous features characteristic for each of the two friction (0.4-0.7 as-plated Ni-P), but also transition compositions [14, 15, 98]. Fabricating graded struc- towards paramagnetic features. Overall, proper- tures can also help to improve coating‘s adhesion ties and functional behaviour of Ni-P electrode- on the substrate owing to the gradual structure evo- posits depend on their composition and microstruc- lution across layer thickness and the lack of abrupt ture. Ni-P coatings with low phosphorus content interfaces (Section7). demonstrate high hardness and good wear resis- Adhesion of the coating on the substrate’s sur- tance, while coatings with higher phosphorus con- face is critical for all possible functional applica- tent exhibit good corrosion resistance but poor me- tions. Ni-P electrodeposits exhibit good adhesion chanical properties owing to the transition towards on a range of widely differing substrates (brass, soft amorphous structure. Amorphous Ni-P deposits steel, copper, etc.). However, stainless steel for ex- are generally brittle and possess low ductility. At ample can be problematic owing to the formation the same phosphorus content, microstructure of of a passive oxide layer which prevents Ni-P coat- the deposit can be modified by altering the depo- ing from sticking to its surface. Conventional way sition conditions. Applying pulse plating instead of addressing this issue is to apply on the substrate of direct current plating can help to preserve Ni-P surface a Wood’s or another nickel strike before crystalline structure even at higher incorporated Ni-P electroplating [99]. Being highly acidic it dis- phosphorus quantities, thus good mechanical prop- solves the oxide and concurrently forms a thin layer erties characteristic for low-phosphorus deposits of nickel on the stainless steel surface. can be maintained [4]. Published data offer diverse information regard- Initially, microhardness of the alloy increases ing corrosion characteristics of Ni-P alloy, particu- with phosphorus content augmentation at very low larly about the nature of its anodic dissolution, abil- phosphorous amounts, while further enhancement ity to passivate and pitting susceptibility. Corrosion of phosphorous quantity leads to hardness deteri- resistance of Ni-P coatings is much better than in oration [18]. Figure9 shows the evolution of the the case of pure Ni [18]. In chloride containing and material’s hardness with the change in its grain slightly acidic environments Ni-P electrodeposits size. Nava et al. [20] found that the wear volume demonstrate lower corrosion resistance when com- of Ni-P electrodeposits is inversely proportional pared to neutral or slightly alkaline settings [100]. to the microhardness of the deposits. They perceived also that nanocrystalline Ni-P coatings, with phosphorus content of 2-8 %, exhibit significantly higher wear resistance compared to coatings with

17 them, the choice of the electroplating bath composition can very much aid in obtaining deposits with low values of this quantity. Baudrand [47] fabricated electrodeposits from a sulfamate bath without the presence of additives with stress value of approximately 30 MPa, while the minimal value of stress for deposits obtained from a Watts bath was 180 MPa. The amount of phosphorus, hence crystallographic structure, of the Ni-P electrodeposit influences its stress value. Lin et al. [97] studied Figure 9: Hardness of a material as a function of its grain size [64]. Ni-P electrodeposition from a sulfamate bath containing no additives. They observed that amor- Amorphous and crystalline deposits have been phous Ni-P deposits exhibit lower internal stresses found to exhibit different corrosion behaviour [17]. compared to crystalline ones. The internal stress Generally the weight loss of Ni-P deposits in corro- and the amount of adsorbed hydrogen exhibited sive solutions decreases with the increase of their a maximum at intermediate phosphorus contents phosphorus content [101]. Splinter and cowork- suggesting that hydrogen incorporation and subse- ers [102] reported the preferential dissolution of Ni quent escape play an important role when it comes on the surface of the Ni-P coating and the surface to the internal stress of the Ni-P electrodeposits. with the enriched P content after corrosion. They Nonetheless, high phosphorus content character- asserted that the formation of Ni3(PO4)2 film on istic for amorphous Ni-P alloys comes with lower the surface impedes the speed of corrosion. Addi- cathode current efficiency, thus conditions must be tionally, their study demonstrated that the corro- carefully optimized in order to secure optimal fea- sion behaviour of nanocrystalline Ni-P alloys with sibility of the process along with desired properties lower phosphorus contents (1.4 wt.% and 1.9 wt.%) of the obtained deposits. in 0.1M H2SO4 approaches that of amorphous Ni-P Certain organic additives are known to induce a electrodeposits (6.2 wt.% P). X-ray photoelectron reduction of the internal stress of the Ni-P deposits. spectroscopy revealed that neither the nanocrys- A grain refiner saccharin is typically employed for talline (1.9 wt.% P, grain size 8.4 nm) nor the amor- this purpose, inducing compressive internal stress phous (6.2 wt.% P) Ni-P alloys formed a passive [10, 17, 46, 105]. Nevertheless, additives contain- layer in this environment. Diegle et al. [103] re- ing sulfur can cause deterioration in the corrosion ported that the addition of phosphorus improves resistance of the fabricated coatings hence their ap- the corrosion resistance of nickel owing to the re- plication must be carefully optimized [66]. action with water in which hypophosphite ions Another possibility to reduce stress of a Ni-P are formed. These ions prevent further dissolu- electrodeposit is to apply a low temperature heat tion of nickel through chemical passivation. Kro- treatment that would help to desorb hydrogen likovski and Butkiewicz [104] determined that the present in the coating [17]. behaviour of Ni-P alloys is similar in neutral solu- Employing pulse current instead of direct cur- tions under open circuit potential. However, under rent deposition is reported to be beneficial for stress conditions of anodic polarization amorphous al- reduction. Chen et al. [65] fabricated Ni-P deposits loys exhibit dissolution suppression while in case with high phosphorus contents and low internal of crystalline alloys intensive dissolution occurs. stresses, ranging from tensile to compressive, with high current efficiency by using pulse current in a 4.2 Internal stress nickel sulfamate bath without the addition of any stress reducers. Well controlled internal stress is a very important criteria for successful fabrication and use of protec- 4.3 Magnetic properties tive Ni-P coatings. Compressive stresses are advan- tageous in mechanical structures under load, since Nickel is a typical ferromagnetic material, while they tend to inhibit crack formation and growth, magnetic properties of the Ni-P alloy present a func- they rise strength and hardness of the coating [21]. tion of its composition and preparation technique Factors influencing stress are many [47]. Among [106]. Magnetic moment and Curie temperature are

18 Figure 10: SEM images of wear track of Ni-P coatings, heat treated at different temperatures after sliding against AISI 8620 ball in air: a) without heat treatment, b) 200, c) 300, d) 400, e) 500, f) 600 ◦C. Reprinted with permission from [20]. Copyright (2013) ESG.

found to decrease with the introduction of phospho- surements and differential scanning calorimetry rus into Ni matrix [107]. According to Weiss theory, analysis of the Ni-P alloys obtained via electrode- nickel exhibits ferromagnetic properties owing to position. Their results indicated that the untreated quantum-mechanical exchange forces which cause alloys with phosphorus contents exceeding 12 at.% the spins of vicinal Ni atoms to be parallel [108]. were paramagnetic, owing to the lack of exchange The introduction of phosphorus into nickel matrix interactions due to fluctuations in chemical com- enlarges the separation between Ni atoms and with position and the formation of a network of phos- the increase of interatomic distance exchange forces phorus rich paramagnetic domains. Amorphous decrease rapidly. Stated effect renders the transi- Ni-P alloys that were originally paramagnetic were tion from ferromagnetic to paramagnetic properties rendered ferromagnetic through thermal treatment [109]. that also led to their devitrification. Dhanapal et Bakonyi et al. [106] performed a study of the al. [109] studied the influence of phosphorus con- magnetic properties of Ni100-xPx alloys fabricated tent, employed duty cycle and current density on by employing various techniques. For electrode- the magnetic properties of Ni-P alloy fabricated by posited alloys the composition range studied was pulse current deposition. They observed satura- 11&x.23. Throughout the whole concentration tion magnetization dependence on the phosphorus range magnetic inhomogeneities were observed. content. At low phosphorus amounts saturation Alloys were found to exhibit paramagnetism at magnetization value of the obtained deposits was x&17. Hu and Bai [74] observed that the ferromag- high. Increasing the duty cycle resulted in the in- netic property of Ni was transformed into param- crease of soft ferromagnetic nature of the Ni-P alloy agnetism at phosphorus content of 17 at.% . They and decrease of the coercivity and retentivity val- corroborated that the magnetic properties of Ni-P ues. alloys are a function of their phosphorus content. In a subsequent study [110], they demonstrated that the paramagnetic Ni-P deposits become fer- 4.4 Catalytic activity romagnetic after thermal treatment at 400 ◦C ow- Ni-P alloy is known to exhibit catalytic activity fore- ing to Ni and Ni3P phases separation. Knyazev and colleagues [6] performed magnetization mea- most in water splitting reaction. This presents a very important feature especially in the light of to-

19 day’s rapidly increasing energy demands and pro- should exhibit optimal activity for the whole HER. gressive exhaustion of fossil fuels resources. Cat- Paseka [114] found that the alloy deposited at 65 ◦C alysts (platinum and ruthenium) conventionally demonstrates worse catalytic activity than alloys employed to reduce the large overpotentials for hy- deposited at temperatures ≤ 53 ◦C. He asserted drogen and oxygen evolution reactions (HER and that this is owing to the presence of larger amounts OER respectively) are fairly scarce and their use not of adsorbed hydrogen in alloys fabricated at lower feasible when it comes to incurred costs, thus suit- temperatures, which contributes to their higher in- able alternatives are in high demand. Nickel is a ternal stresses. According to Paseka, catalytic ac- very popular electrode material owing to its reason- tivity also depends on the deposit’s thickness and ably high hydrogen generation activity, availability exhibits an increase as deposit grows. Thick de- as well as its low cost [111]. Although Ni as a cata- posits possess great catalytic activity owing to their lyst does not perform as well as steel from the view- high internal stress, however their mechanical in- point of its electro-catalytic activity, it possess an stability presents a problem and thickness increase excellent resistance to corrosion in hot concentrated will be beneficial only up to a critical width of coat- alkaline solutions [112]. Issues encountered when ing at which it will suffer a final failure. In the study employing this metal as cathode material are its low by Li et al. [76] it was determined that at low bath catalytic activity or low resistance to intermittent temperatures high NH4Cl content in combination electrolysis, however alloying nickel with other ele- with low NaH2PO2 concentration can contribute to ments (P, Mo, etc.) can aid in mitigating these issues creating thick films (6-8 at.% of phosphorus) with [112, 113]. According to Paseka [114], the reason for optimum catalytic activity even at high current den- the improvement of nickel activity in HER achieved sities. However, ammonia concentration must be through alloying it with phosphorus is the augmen- chosen carefully because exceeding the ammonia tation of the amount of amorphous phase surround- concentration above the certain limit can negatively ing the Ni crystals. Alloys containing amorphous affect the cathode current efficiency, hence decrease phase which is able to dissolve large amounts of the plating rate [44]. Bai and Hu [110] observed that hydrogen possess high internal stress which con- for Ni-P electrodeposits with P content from 0 to 28 tributes to their good electro-catalytic activity. at.% the ability of catalysing hydrogen evolution Hu and Bai [115] conducted a study in order decreases with increasing annealing temperature, to interrogate the effects of key electroplating vari- concluding that annealed deposits are not suitable ables on the hydrogen evolution activity of Ni-P for HER. deposits. By employing fractional factorial design, Elias et al. [117] studied the efficiency of elec- path of steepest ascent and central composite de- trodeposited Ni-P for both HER and OER in alka- sign, they found that the key factors influencing line media. They found that the alloy thin films catalytic activity of Ni-P electrodeposits include: with 9.0 wt.% of P and 4.2 wt.% of P are the temperature, pH and NaH2PO2·H2O concentra- best electrode materials for HER and OER. Re- tion and their interactions. The models they de- cently Tang and coworkers [118] reported a room- veloped indicated that the alloy containing 7 at.% temperature electrodeposition of Ni-P nanoparticle of P should exhibit maximal electro-catalytic activ- film on Ni foam which acts as a bifunctonal water- ity. In the subsequent study [74], they experimen- splitting catalyst in strongly alkaline media with tally found deposit with 8 at.% of P to be the best very small overpotentials for HER and OER: 80 mV electrode material for HER. They attributed this and 309 mV, respectively. deposit’s highest specific activity to its largest true As an alternative to alkaline media in which wa- surface area and hence its maximum roughness. ter electrolysis systems are simple but demonstrate Wei et al. [116] investigated catalytic activity of low efficiency and high energy consumption, Lu Ni-P in HER both experimentally and theoretically. et al. [119] interrogated electrocatalytic activity of Alloys that contain 10,8 at.% P were found to per- Ni based alloys in acidic media. They observed a form the best. In order to elaborate the influence decrease of catalytic activity with the increase of of phosphorus content in Ni-P amorphous alloys phosphorus content which they attributed to mov- on their catalytic activity they employed density- ing away from the optimal electronic configuration functional theory and front molecular orbital theory. with phosphorus incorporation but additionally to Obtained results indicated that alloys with phos- the decrease of the number of grain boundaries phorus content anywhere from 9.1 at.% to 14.3 at.% which present active sights for HER. Contrary, au-

20 thors of [120] claimed that increasing the phospho- tion for Ni-P deposits containing ≤ 24 at.% of P to rus content, even past contents that can be achieved occur at 400 ◦C, while deposits with 28 at.% demon- through conventional electrodeposition would im- strated a phase transformation at 200 ◦C. Addition- part exceptional catalytic properties to these sys- ally, phosphorus content in the deposits decreased tems for applications in acidic media and hence with increasing the annealing temperature owing very high phosphorus contents ought to be benefi- to the replacement of phosphorus by the oxygen cial. from the air. Habazaki et al. [124] conducted a study in order to interrogate the effect of annealing on the 5 Thermal treatment of electro- microstructure and the corrosion behaviour of the electrodeposited amorphous Ni-P alloys. Obtained plated Ni-P alloy results indicated that Ni-P alloys with 19.2 at.% phosphorus crystallize directly to f.c.c. Ni and Ni3P With subsequent heat treatment the hardness of phases. High phosphorus alloys (≥ 24.6 at.% P) Ni-P electrodeposits increases substantially. This were first crystallized to a metastable single phase phenomenon is caused by the precipitation of the and then decomposed to Ni3P. Crystallization of crystalline phases. Most studies report detecting a alloys with the intermediate P content (19.2 at.% mixture of Ni3P and f.c.c. nickel as a final product < P < 24.6 at.%) resulted first in a mixture of f.c.c. of the thermal treatment [20]. Ni, Ni3P and the metastable phase. The precipita- The annealing temperature under which crys- tion of f.c.c. Ni in the amorphous phase occurred tallization occurs depends on the phosphorous con- for Ni-16.7P alloy before complete crystallization. tent in the deposit as well as on the heating rate Annealing decreased corrosion resistance for alloys [121]. Phase transformation temperatures of the containing ≤22.7 at.% P owing to the formation deposits increase with increasing heating rate and of phosphorus deficient f.c.c. Ni phase. Keong et decreasing phosphorus content [110, 121]. Contin- al. [121] similarly reported that amorphous coat- ued heating at temperatures higher than the tran- ings with high phosphorus content follow a se- sition temperature leads to a decrease of hardness quence of transformations during annealing and owing to the subsequent recrystallization and grain form metastable phases, such as Ni2P and Ni12P5 coarsening [18, 122, 123]. The degree of loss of hard- before forming stable Ni3P and f.c.c. Ni phases. ness is higher for deposits having lower phospho- Zoikis et al. [125] observed crystallization of the rus content on account of more pronounced grain amorphous Ni-P electrodeposit into Ni and Ni3P growth and coarsening of Ni phase compared to phases after annealing at ∼ 400 ◦C. The presence ◦ Ni3P phase [122]. Transition from amorphous to of Ni2P phase after thermal treatment above 330 C crystalline state is followed by a thermal contrac- was detected. Ni2P phase formation was also re- tion phenomena due to higher density of crystalline ported in [43]. structure when compared to amorphous one, the Jeong et al. [77] reported the increase of hard- amplitude of contraction being higher at higher ness and abrasive wear resistance after heat treat- phosphorus contents [105]. ment for nanocrystalline Ni-P coatings, with Taber Nava et al. [20] fabricated Ni-P electrodeposits abrasive wear resistance being linearly propor- containing 10.6 at.% of P which exhibited maxi- tional to the hardness of the coatings. Heat treat- mum hardness (990 HV) after thermal treatment at ment causes also the elastic modulus of the deposit 500 ◦C (comparable to the hardness of hard Cr coat- to increase significantly according to the authors of ings ∼1000 HV). Obtained deposits demonstrated [63]. When maximum hardness is achieved upon also the lowest wear rate as indicated by the SEM annealing fracture toughness exhibits the lowest images of their wear track patterns that exhibited value [126]. the narrowest width and shallowest plough lines Chang et al. [127] argued that significant (Figure 10). A linear relationship was detected be- strengthening by annealing for electrodeposited tween the hardness and the wear resistance of the Ni-P alloys with low phosphorus content and heat treated Ni-P alloy coatings. Corrosion resis- nanocrystalline grains is not induced by the pre- tance of the deposits deteriorated upon annealing cipitation of Ni3P phase. The increase in hardness owing to the formation of cracked structure in the upon annealing is according to them a result of thermally treated coatings, which promoted local- grain boundaries relaxation, phosphorus segrega- ized corrosion. Bai and Hu [110] found crystalliza-

21 tion, reduction of interior defects with the possible verse on high frequency pulse, modified sine-wave contribution from the increase of density owing to pulses and square-wave pulses [129]. degassing of hydrogen. During electroplating, a negatively charged Opting for subsequent Ni-P alloy thermal treat- layer is formed around the cathode as the process ment and accordingly choosing the suitable heat advances. When performing DC deposition, this treatment temperature, finally depend very much layer charges to a definite thickness and prevents on the employed substrate nature and the intended ions from reaching the substrate. In a simple form process complexity. of PC electrodeposition, where cathodic pulse is alternatively switched on and off, when the output is turned off this layer discharges which allows eas- 6 Application of pulse plating ier transport of ions from the bulk of the solution for Ni-P alloy fabrication [129]. Additionally, during plating high current density areas in the bath become more depleted of Constant direct current is the most commonly ap- ions compared to low current density areas. Dur- plied regime in which metallic coatings are elec- ing time when the current is off ions migrate to the trodeposited. However, in recent years the use depleted areas and when subsequent current pulse of pulsed, alternating currents is exhibiting an in- occurs more evenly distributed ions are available crease. This is owing to number of findings that for deposition onto the part [129]. In the absence demonstrate that pulse current (PC) exerts a benefi- of current, small grains are recrystallized owing to cial influence on the structure and properties of the their higher surface energy which makes them less fabricated deposits. Among others, comprehensive thermodynamically stable than large grains, hydro- reviews on pulse and pulse reversed plating are gen is also desorbed decreasing the internal stress composed by Devaraj and Seshadri [128] and by of the obtained deposits. In general, PC plating Chandrasekar and Pushpavanam [129]. results in finer grain deposits exhibiting improved Development of modern electronics has granted properties, including hardness, roughness, poros- a great flexibility in programming of the applied ity, wear resistance, etc. Pulse plating can reduce modulated current waveforms, with trains of additive requirements substantially [128, 129]. pulses that can be programmed to have very com- Pulsed current results in metal deposition at the plex sequences and forms. same rate as direct current provided that the average pulse current is equal to the mean direct current of DC electrodeposition [64, 129]. In PC plating, choice of the applied waveform is critical and care must be taken to appropriately optimize all parameters (peak current, duty cycle, frequency, pulse shape, etc.). In his work Pearson [130] explores the benefits and the limits of the PC electrodeposition technique. He asserts that very low duty cycles are not feasible, because in order to produce the same average deposition rate as for DC, as duty cycle is reduced the pulse peak current needs to be increased. In practical applications, Figure 11: Typical pulse-current waveform. too high peak current densities are seldom viable due to limitations of rectifiers capacities. On the Rectanglular waves are the easiest waveforms other hand, as duty cycle is increased the process to produce [130]. They have been demonstrated to begins to approach direct current deposition, thus produce a higher nucleation rate of the grains com- a compromise must be made. When it comes to pared to triangular waveform [131]. Typical wave- the frequency, practical maximum frequency which forms employed in pulse plating include: cathodic can be applied is limited by the capacitance of the pulse followed by a period without current (or an double layer at the interface between the plating anodic pulse), direct current (DC) with superim- electrolyte and the article being plated. If the fre- posed modulations, duplex pulse, pulse-on-pulse, quency is very high, the double layer does not have cathodic pulses followed by anodic pulses (pulse enough time to fully charge during the pulse and reverse current-PRC), superimposing periodic re-

22 the process begins to resemble DC deposition [130]. value variations. Additionally, unlike DC-plated Maximum useful frequency is around 500 Hz for Ni-P deposits that might become amorphous when most applications. However, higher frequencies their phosphorus content exceeds a critical value, can be used where very high peak current densities pulse-plated deposit with 14 wt.% phosphorus still are employed because the double layer charge and consisted of equiaxed crystalline grains. This is in discharge times become shorter as the peak current agreement with the study done by Chen et al. [65] density is increased. who concluded that a reduction in duty cycle from In pulse reversed current technique (PRC) an- 0.5 to 0.1 simultaneously increases phosphorus con- odic pulse is introduced into the plating cycle. PRC tent and grain size of the deposits. They found that has the same effect of replenishing the diffusion Ni-P deposits with high P content can be achieved layer as PC does. It results in dissolution of the with high current efficiencies by employing pulse protrusions on the metal surface ensuring a more plating and that the obtained deposits exhibit lower uniform deposition through elimination of the dis- internal stress than the DC-plated coatings. Pulse- crepancies between high and low current density plated deposits were also consistently harder than areas and increases coating thickness uniformity the DC-plated ones. [129]. PRC plated amorphous Ni-P deposits are reported to exhibit better ductility owing to the absence of voids and to consist of layers with different amorphous structures [132]. It has been reported that the application of PC deposition results in the increase in limiting current density [129]. However according to Pearson [130] total thickness of the diffusion layer is equivalent to that obtained when plating in DC regime, owing to this the use of PC has very little effect on the limiting current density. In Ni electrodeposition, pulse plating is extensively employed and studied [133–135]. Pavlatou et al. [69] investigated the use of pulse current in order to interrupt the columnar growth of the nickel grains and to produce more compact and thus more Figure 12: Deposit phosphorus content as a function of the peak corrosion resistant coatings. current density, pulse frequency, and duty cycle. Reprinted with In Ni-P electroplating, pulse plating also pos- permission from [4]. Copyright (2006) Electrochemical Society, sesses several advantages over DC plating. Lin and Inc. coworkers [4] studied Ni-P alloy deposition in a PC regime from a sulfamate bath. They established that compared with DC plating, current efficiency associated with high phosphorus content deposits can be improved by applying pulse current having low duty cycle, high frequency, and proper peak current density. It was demonstrated that after applying the same total charge associated with DC and PC waveforms of different duty cycles, a more uniform concentration profile is maintained for PC than for DC deposition, particularly with the PC having small duty cycles. By employing 0.1 duty cycle with frequencies exceeding 100 Hz deposits with 14 wt.% of P were plated with an efficiency of around 80%. Low duty cycles and high frequencies are beneficial in terms of maintaining more stable surface proton concentration distribution inhibit- Figure 13: Dependence of current efficiency on the deposit phos- ing alloy composition modulations induced by pH phorus content. Reprinted with permission from [4]. Copyright (2006) Electrochemical Society, Inc.

23 7 Compositionally modulated a cohort of hydrogen bubbles from the cathode resultant enhanced convection increases the interfa- Ni-P electrodeposits cial proton concentration. Interfacial pH variations cause the variations in the deposit phosphorus con- In DC plating ways of improving deposit properties tent as electroplating continues [97]. Crousier et al. include microstructure manipulation by varying de- [29, 143] confirmed that Ni-P electrodeposits con- position conditions or alloying in which case a two- sist alternately of layers having varying phospho- component deposit is formulated by employing an rus contents. Sadeghi [21] observed that the forma- appropriate bath formulation. However, techno- tion of layers with different phosphorus amounts logical development imposes the demand for coat- occurs more readily for alloys with low or medium ings that combine favourable properties of different phosphorus content and at higher current densi- metals, a task unachievable solely by employing in- ties. A homogeneous deposit can be fabricated and dividual components neither their resultant alloys. stratification avoided by enhancing mass transport In this case compositionally modulated (function- to the cathode surface or by employing pulse plat- ally graded or multi-layered) coatings present them ing [97]. Additionally, oscillations in pH value are selves as a remarkable opportunity owing to the damped out as coating grows in thickness and elec- possibility of engineering coatings fitting a wide troplating continues [17]. However variations in range of requirements in terms of functional prop- phosphorus content can be sometimes intentionally erties. induced in order to produce composition modula- In nickel electroplating, production of multi- tion in fabricated Ni-P electrodeposits. With careful layer coatings consisting of films of bright and semi optimization this approach can result in a signifi- bright nickel (60-75% of the total nickel thickness) cant improvement of features when compared to is traditionally employed to improve deposits’ cor- conventional homogeneous Ni-P electrodeposits. rosion resistance. Preferential corrosion in the elec- Layered crystalline/amorphous Ni-P alloys are for trochemically more active upper bright nickel layer example reported to exhibit higher tensile strength, and propagation of the corrosion in the lamellar moduli of elasticity and improved corrosion resis- direction protects the columnar lower layer of semi- tance [143]. bright nickel through retarded pitting attack pene- It is known that Ni-P deposits with low phos- tration [57]. phorus content exhibit high hardness and good In the number of applications including the use wear resistance but poor corrosion resistance, con- of coatings for thermal, wear or corrosion protec- versely high phosphorus coatings exhibit good cor- tion and microelectronics, the mismatch in prop- rosion resistance but poor mechanical properties. erties at the interface between the coating and the Developing multilayer coatings can be an effective substrate can cause stress concentration that could way to obtain deposits characterized by both op- result in the failure of the interface. Employing timal mechanical and electrochemical properties. functionally graded materials (FGMs) [136–138] For example, a duplex coating with an outer layer which are characterized by a position-dependent having a low phosphorus content and an inner chemical composition, microstructure or atomic or- layer with high phosphorus content is a good way der [15] and by a subtle gradient of their proper- to ensure corrosion stability in the contact with ties along the coating thickness is reported to help surrounding environment and in the same time in reducing the stress usually incurred due to the favourable mechanical properties [98]. abrupt composition change when going from sub- Techniques in order to produce layered Ni-P strate toward deposit and decreasing the danger of electrodeposits include either using two baths with its delamination. different phosphorus source contents or a single A non-uniform, graded [14] or layered distribu- electroplating bath in which case electrochemical tion [139, 140] relative to the phosphorus content methods are employed in order to achieve compo- along the coating thickness or a grain size gradi- sition modulation (changing cathodic current or ent [141, 142] are deemed to be beneficial also for potential). imparting favourable properties to Ni-P deposits. In the dual-bath electrodeposition technique, Phosphorus content in Ni-P electrodeposits in- the item to be electroplated is moved between two herently varies with layer thickness. pH at the plating baths of arbitrary composition and a layer electrode-solution interface rises concomitantly is plated from each electrolyte in cycles. First multi- with the discharge of hydrogen. After escape of

24 Figure 14: Diagram showing the mechanism of increased corrosion protection in the case of multilayer Ni-P coating (left), compared to monolayer Ni-P coating (right). It demonstrates that the time required for the corroding medium to reach the substrate by penetrating through the multilayer coating (T1) is much greater than that for the monolayer coating (T2). Reprinted with permission from [147]. Copyright (2016) Royal Society of Chemistry.

layer deposition by employing dual bath technique taken, including washing and drying the substrate was reported by Blum for Ni(24 µm)/Cu(24 µm) with N2, in order to mitigate problems related to as early as 1921 [144]. This approach is mechani- cross-contamination between electrolytes. Multi- cally complex compared to the single-bath method layered films of Ni/NiPx, NiPx/NiPy, Cu/Ni, and and carries a risk of contamination during the sub- Co/NiPx were fabricated with a range of repeated strate transfer [83]. Historically, first realization lengths. Ni/NiPx and NiPx/NiPy multilayers ex- of single-bath electrodeposition was reported by hibited the highest quality, with repeat lengths as Brenner and Pommer in 1948 who produced mul- low as 19 Å and up to three orders of reflection tilayered coatings CuBi(∼µm)/BiCu(∼µm) by al- in low angle X-ray scans. NiPx/NiPy multilayers ternately switching the deposition current between were fabricated over a wide range of compositions low and high value [145]. Single bath method is and with crystalline or amorphous structure. Prob- efficient, versatile and technologically simple. A lems were encountered with other compositions, review of both dual-bath and single-bath electrode- in case of Cu/Ni and Cu/Co systems due to gal- position methods is compiled by Ross [146]. vanic coupling as well as contamination and in case When employing single bath deposition of Co/Ni-P owing to non-uniformity of Co nucle- method pulsed current is a useful tool to achieve ation. the desired composition modulation. Early attempt Wang et al. [14] described electrodeposition and with two-pulse plating was reported by Girard investigated properties of Ni-P deposits having a [148] for the electrodeposition of permalloy films varying phosphorus content in the direction of the with a composition as close as possible to the zero- coating thickness. Single bath method was applied magnetostriction (Ni81Fe19) ensuring the smallest and composition gradient was achieved through magnetic coercive force. varying current density (5-30 A dm−2). The wear re- In the realm of compositionally modulated Ni- sistance of the fabricated Ni-P electrodeposits was P electrodeposits, Goldman and coworkers [86] approximately two times grater than that of the demonstrated a method to fabricate Ni/Ni-P films, ungraded Ni-P deposits. Beneficial wear proper- with wavelengths between 2.1 and 4.0 nm and an ties of the obtained deposits were attributed to the average phosphorus content around 12 at.%, by inhibition of formation and propagation of through- alternating electrodeposition in two baths of dif- thickness cracks during the wear process owing to ferent composition. Ross et al. [83] revealed a their graded structure. Heat treated coatings ex- dual bath electrodeposition technique for the prohibited low friction coefficient and hardness that duction of thin-film metal multilayers in which was close to the one of hard Cr coatings. In an- substrate was suspended above nozzles of elec- other study, Wang and colleagues [15] examined trolyte and rotated by a motor. Specific steps were corrosion resistance of the developed Ni-P gradi-

25 ent deposits and their tribological behaviour un- Improved corrosion properties of multilayers der the oil-lubricated conditions. Deposits heat of Ni-P/Zn-Ni, when compared to pure Ni-P and treated at 400 ◦C exhibited two orders of magni- Zn-Ni alloys, was detected by Liu and colleagues tude better corrosion resistance than hard Cr coat- [149]. According to them, this was due to the cor- ings (Figure 15). The best corrosion resistance was rosion of the sacrificial sublayers of Zn-Ni which found for deposits heat-treated at 200 ◦C, which extends in the direction parallel to the substrate was attributed to the preserved amorphous struc- surface. Continued corrosion generation through ture and the stress relaxation at this temperature. the subsequent sublayers overall slows down the Heat-treated coatings exhibited relatively higher corrosion reaching the substrate and an eventual wear rate and friction coefficients than hard Cr de- material brake down. Bahadormanesh and Ghor- posits under oil-lubricated wear conditions. bani [150] recently devised a single bath deposition method for electrodeposition of Ni-P/Zn-Ni compositionally modulated multilayer coatings. At low current densities Ni–P was deposited, while at higher current densities Zn–Ni alloy containing 3.2 wt.% P was obtained. It was observed that the Ni–P/Zn–Ni compositionally modulated multilayer coatings were sacrificial to the steel substrate. Bozzini and colleagues [100] fabricated Ni-P and Sn multilayer amorphous deposits (layer thickness 0.1 µm to 0.5 µm) by employing a dual bath electrodeposition technique in order to explore the possibility of improving the Ni-P deposits passivation behaviour. They investigated anodic behaviour of obtained coatings in acidic chloride so- Figure 15: Potentiodynamic polarization curves obtained for lution. Current densities at the passivation plateau, Ni-P graded deposits and hard Cr coatings, measured in 10 wt.% were in the range of 2 µA cm−2 to 5 µA cm−2 for Ni- HCl solution. Reprinted with permission from [15]. Copyright P/Sn multi layers, in comparison to a passivation (2006) Elsevier. current density of 45 µA cm−2 for Ni-P coatings Elias et al. [147] fabricated multilayer Ni-P al- with similar phosphorus content. They found that loy coatings for better corrosion protection of mild the present interfaces improve considerably the pas- steel by employing cyclic modulation of the cath- sivation behaviour provided that the incorporated ode current density. Achieved improvement in the Sn layers are thin enough. corrosion protection of fabricated multilayer Ni-P alloy coatings was attributed to the large number of interfaces between layers of alloys having different 8 Conclusion composition and phase structures, at which corrosion propagates laterally until the interface breaks Ni-P protective coatings can be electrodeposited down (Figure 14). This mechanism systematically with a wide range of crystallographic structures. slows down corrosion and improves the coatings Deposits extending from fully crystalline to amor- stability. Corrosion protection efficiency of multi- phous ones can be easily fabricated. Properties of layer coatings was found to increase with the num- the obtained coatings very much depend on the ber of layers, however only up to a certain point deposition conditions and their phosphorous con- (300 layers). At very high layer numbers corrosion tent. Applied post treatment, such as alloy heating, resistance deteriorated owing to the lack of distinct can bring significant improvement of its overall me- interfaces between individual films (approaching a chanical, tribological and electrochemical proper- monolayer structure). However, not only the num- ties. Designing unconventional structures, such as ber of layers but also their composition determined graded or multilayered ones, can also with proper the performance of fabricated deposits. optimization give rise to substantial deposits’ char- Multilayers of Ni-P with other metals or alloys acteristics amelioration. can be engineered also in order to ensure opti- However, even though technology of Ni-P elec- mal functional performance in demanding envi- troplating is quite mature, there are still many un- ronments. knowns and numerous issues still remain to be

26 addressed. Phosphorus incorporation mechanism [6] A. V. KNYAZEV, L. A. FISHGOIT, P. A. CHERNAVSKII, is still a matter of great disagreement. Factors inﬂu- V. A. SAFONOV, and S. E. FILIPPOVA. “Magnetic properties of electrodeposited amorphous encing phosphorus content of the deposit are many nickel–phosphorus alloys”. In: Russian Journal of and with the plethora of variables characterizing Electrochemistry 53.3 (2017), pp. 270–274. DOI: Ni-P electroplating process and poor deﬁnition of 10.1134/S1023193517030090 (cit. on pp.1, 19).

certain process parameters it is not easy to estab- [7]M.S CHLESINGER and M. PAUNOVIC, eds. Modern lish clearly the key inﬂuencers, manner and the Electroplating: Fifth Edition. John Wiley & Sons, Inc., 2011. extent of their impact. Up-scaling of the electroplat- DOI: 10.1002/9780470602638 (cit. on pp.1,2, 11, 12). ing procedure induces new process variables and [8]N.K ANANI. Electroplating. Elsevier, 2004. DOI: more attention in research needs to be bestowed 10.1016/B978-1-85617-451-0.X5000-3 (cit. on p.1).

on feasibility study of method transferral to larger [9] F. NASIRPOURI. Electrodeposition of Nanostructured scales and its robustness. Current density distri- Materials. Springer, 2017, p. 325. DOI: bution is a parameter that is often poorly deﬁned. 10.1007/978-3-319-44920-3 (cit. on p.1). Ageing of the electrolytic baths is not addressed in [10] L. P. BICELLI, B. BOZZINI, C. MELE, and L. D’URZO. “A many of the research works. Additionally, distin- review of nanostructural aspects of metal guishing between different Ni-P microstructures electrodeposition”. In: International Journal of Electrochemical Science 3.4 (2008), pp. 356–408. DOI: and establishing a point of transition from one state 10.1.1.655.5201 (cit. on pp.1, 14, 18). to another is still a matter of some difﬁculty. [11]M.L IPPMANN. Environmental Toxicants: Human Exposures and Their Health Effects: Third Edition. Ed. by M. LIPPMANN. John Wiley and Sons Ltd, 2009, p. 1168. Acknowledgements DOI: 10.1002/9780470442890 (cit. on p.2). This work was supported by the European Union’s [12] The RoHS Directive. URL: http://ec.europa.eu/envir onment/waste/rohs_eee/index_en.htm (cit. on p.2). Horizon2020 research and innovation programme SOLUTION, under grant agreement No. 721642. [13]C.M A, S. C. WANG, L. P. WANG, F. C. WALSH, and R. J. WOOD. “The electrodeposition and characterisation of low-friction and wear-resistant Co-Ni-P coatings”. In: Surface and Coatings Technology 235 (2013), pp. 495–505. DOI: 10.1016/j.surfcoat.2013.08.009 (cit. on p.2).

[14] L. WANG, Y. GAO, Q. XUE, H. LIU, and T. XU. “A novel electrodeposited Ni-P gradient deposit for replacement of conventional hard chromium”. In: Surface and References Coatings Technology 200.12-13 (2006), pp. 3719–3726. DOI: 10.1016/j.surfcoat.2004.10.016 (cit. on pp.2, 17, [1] G. E. TOTTEN and H. LIANG, eds. Surface modification 24, 25). and mechanisms: friction, stress, and reaction engineering. CRC Press, 2004, p. 800 (cit. on p.1). [15]L.W ANG, Y. GAO, T. XU, and Q. XUE. “Corrosion resistance and lubricated sliding wear behaviour of [2] Y. QIN, ed. Micromanufacturing Engineering and novel Ni-P graded alloys as an alternative to hard Cr Technology, Second Edition. William Andrew Publishing, deposits”. In: Applied Surface Science 252.20 (2006), 2015, p. 858 (cit. on p.1). pp. 7361–7372. DOI: 10.1016/j.apsusc.2005.08.040 (cit. on pp.2, 17, 24–26). [3] P. SIDKY and M. HOCKING. “Review of inorganic coatings and coating processes for reducing wear and [16]X.Z HANG, J. QIN, M. K. DAS, R. HAO, H. ZHONG, corrosion”. In: British Corrosion Journal 34.3 (1999), A. THUEPLOY, S. LIMPANART, pp. 171–183. DOI: 10.1179/000705999101500815 Y. BOONYONGMANEERAT, M. MA, and R. LIU. (cit. on pp.1,2). “Co-electrodeposition of hard Ni-W/diamond nanocomposite coatings”. In: Scientific Reports IN EE HEN HIEN IN [4] C. S. L , C. Y. L , F. J. C , C. T. C , P. L. L , 6.November 2014 (2016), pp. 1–11. DOI: and W. C. CHUNG. “Electrodeposition of 10.1038/srep22285 (cit. on p.2). Nickel-Phosphorus Alloy from Sulfamate Baths with Improved Current Efficiency”. In: Journal of The [17] B. P. DALY and F. J. BARRY. “Electrochemical nickel – Electrochemical Society 153.6 (2006), pp. C387–C392. DOI: phosphorus alloy formation”. In: International Materials 10.1149/1.2186798 (cit. on pp.1,3,5, 10, 12, 13, 17, Reviews 48.5 (2003), pp. 326–338. DOI: 23). 10.1179/095066003225008482 (cit. on pp.2–4, 12, 13, 17, 18, 24). [5]X.Y UAN, D. SUN, H. YU, H. MENG, Z. FAN, and X. WANG. “Preparation of amorphous-nanocrystalline [18]O.B ERKH and J. ZAHAVI. “Electrodeposition and composite structured Ni-P electrodeposits”. In: Surface properties of Ni-P alloys and their composites-A and Coatings Technology 202.2 (2007), pp. 294–300. DOI: literature survey”. In: Corrosion Reviews 14.3-4 (1996), 10.1016/j.surfcoat.2007.05.040 (cit. on pp.1,3,6, pp. 323–342. DOI: 10.1515/CORRREV.1996.14.3-4.323 10, 14, 15, 17). (cit. on pp.2,3,6, 12, 17, 21).

27 [19] R. F. DALLA COSTA, C. W. KLEIN, A. M. BERNARDES, [33] T. M. HARRIS and Q. D. DANG. “the Mechanism of J. Z. FERREIRA, and J. ZOPPAS FERREIRA. “Evaluation Phosphorus Incorporation During the Electrodeposition of the electrodialysis process for the treatment of metal of Nickel-Phosphorus Alloys”. In: Journal of the ﬁnishing wastewater”. In: Journal of the Brazilian Electrochemical Society 140.1 (1993), pp. 81–83. DOI: Chemical Society 13.4 (2002), pp. 540–547. DOI: 10.1149/1.2056114 (cit. on p.4). 10.1590/S0103-50532002000400021 (cit. on p.2). [34]M.S AITOU, Y. OKUDAIRA, and W. OSHIKAWA. [20]D.N AVA, C. E. DÁVALOS, A. MARTÍNEZ-HERNÁNDEZ, “Amorphous Structures and Kinetics of Phosphorous F. MANRÍQUEZ, Y. MEAS, R. ORTEGA-BORGES, Incorporation in Electrodeposited Ni-P Thin Films”. In: J. J. PÉREZ-BUENO, and G. TREJO. “Effects of heat Journal of The Electrochemical Society 150.3 (2003), treatment on the tribological and corrosion properties pp. C140–C143. DOI: 10.1149/1.1545459 (cit. on p.4). of electrodeposited Ni-P alloys”. In: International Journal [35] A. P. ORDINE, S. L. DÍAZ, I. C. P. MARGARIT, of Electrochemical Science 8.2 (2013), pp. 2670–2681 O. E. BARCIA, and O. R. MATTOS. “Electrochemical (cit. on pp.2, 11, 17, 19, 21). study on Ni-P electrodeposition”. In: Electrochimica Acta [21]A.S ADEGHI. “Microstructure evolution and 51.8-9 (2006), pp. 1480–1486. DOI: strengthening mechanism in Ni-based composite 10.1016/j.electacta.2005.02.129 (cit. on pp.4,5). coatings”. PhD thesis. Technische Universität Chemnitz, [36] Y. ZENG and S. ZHOU. “In situ surface Raman study of 2016, p. 128 (cit. on pp.2, 16, 18, 24). the phosphorus incorporation mechanism during electrodeposition of Ni-P alloys”. In: Journal of [22] T. W. SCHARF and S. V. PRASAD. “Solid lubricants: A Electroanalytical Chemistry 469.1 (1999), pp. 79–83. DOI: review”. In: Journal of Materials Science 48.2 (2013), 10.1016/S0022-0728(99)00179-5 (cit. on pp.4,5). pp. 511–531. DOI: 10.1007/s10853-012-7038-2 (cit. on p.2). [37] T. MORIKAWA, T. NAKADE, M. YOKOI, Y. FUKUMOTO, and C. IWAKURA. “Electrodeposition of Ni-P alloys [23] J. W. DINI. “Properties of coatings: Comparisons of from Ni-citrate bath”. In: Electrochimica Acta 42.1 (1997), electroplated, physical vapor deposited, chemical vapor pp. 115–118. DOI: 10.1016/0013-4686(96)00173-9 deposited, and plasma sprayed coatings”. In: Materials (cit. on pp.4,7,9, 12). and Manufacturing Processes 12.3 (1997), pp. 437–472. DOI: 10.1080/10426919708935157 (cit. on p.2). [38] N. V. SOTSKAYA and O. V. DOLGIKH. “Kinetics of cathodic reduction of hypophosphite anions in aqueous [24]L.S HI, C. SUN, and W. LIU. “Electrodeposited solutions”. In: Russian Journal of Electrochemistry 41.12 nickel-cobalt composite coating containing MoS2”. In: (2005), pp. 1336–1340. DOI: Applied Surface Science 254.21 (2008), pp. 6880–6885. DOI: 10.1007/s11175-005-0223-8 (cit. on p.5). 10.1016/j.apsusc.2008.04.089 (cit. on p.3). [39] O. SAVCHUK, Y. SKNAR, I. SKNAR, A. CHEREMYSINOVA, [25] D. A. LUKE. “Nickel-phosphorus electrodeposits”. In: and Y. KOZLOV. “Examining the effect of Transactions of the IMF 64.1 (1986), pp. 99–104. DOI: electrosynthesis conditions on the Ni-P alloy 10.1080/00202967.1986.11870744 (cit. on pp.3, 12). composition”. In: Eastern-European Journal of Enterprise Technologies 4.6 (88) (2017), pp. 41–46. DOI: ANANI Electroplating-Basic Principles, Processes and [26]K .N. 10.15587/1729-4061.2017.106900 (cit. on pp.5,6). Practice. Elsevier Science, 2005, p. 353 (cit. on p.3). [40]E.B REDAEL, J. CELIS, and J. ROOS. “NiP [27] Electroplating Market:Global Industry Analysis and electrodeposition on a rotating-disc electrode and in a Opportunity Assessment, 2016-2026. URL: jet cell: relationship between plating parameters and https://www.futuremarketinsights.com/reports structural characteristics”. In: Surface and Coatings /electroplating-market (cit. on p.3). Technology 58.1 (1993), pp. 63–71. DOI: 10.1016/0257-8972(93)90175-N [28]A.B RENNER. Electrodeposition of Alloys: Principles and (cit. on pp.5,9, 12, Practice. Vol. 1. Academic Press, 1963, p. 734. DOI: 13, 16). 10.1016/B978-1-4831-9807-1.50001-5 (cit. on pp.3, [41] O. P. WATTS. In: Trans. Am. Electrochem. Soc. 29 (1916), 7, 14). pp. 395–403 (cit. on p.5).

[29]J.C ROUSIER, Z. HANANE, and J.-P. CROUSIER. [42] C. C. HU and A. BAI. “Composition control of “Electrodeposition of NiP amorphous alloys. A electroplated nickel-phosphorus deposits”. In: Surface multilayer structure”. In: Thin Solid Films 248.1 (1994), and Coatings Technology 137.2-3 (2001), pp. 181–187. DOI: pp. 51–56. DOI: 10.1016/0040-6090(94)90210-0 10.1016/S0257-8972(00)01117-8 (cit. on pp.5, 10, 15, (cit. on pp.4, 24). 16).

[30]N.F EDOTEV and P. VYACHESLAVOV. In: Tr. Leningr. [43]N.S PYRELLIS, E. A. PAVLATOU, S. SPANOU, and Tekhnol. Inst. im. Lensoveta 53 (1959), p. 30 (cit. on p.4). A. ZOIKIS-KARATHANASIS. “Nickel and nickel-phosphorous matrix composite electrocoatings”. [31]M.R ATZKER, D. S. LASHMORE, and K. W. PRATT. In: Transactions of Nonferrous Metals Society of China 19.4 “Electrodeposition and corrosion performance of (2009), pp. 800–804. DOI: nickel-phosphorus amorphous alloys”. In: Plating And 10.1016/S1003-6326(08)60353-2 (cit. on pp.5, 21). Surface Finishing 73.9 (1986), pp. 74–82 (cit. on p.4). [44]M.D ADVAND, N. DADVAND, and G. J. KIPOUROS. [32] R. L. ZELLER and U. LANDAU. “Electrodeposition of “Electrodeposition of nickel-phosphorus from a highly Ni-P amorphous alloys - Observations supporting the conductive citrate bath for wire bonding applications”. indirect mechanism of phosphorus incorporation”. In: In: Canadian Metallurgical Quarterly 55.1 (2016), pp. 1–12. Journal of The Electrochemical Society 139.12 (1992), DOI: 10.1179/1879139515Y.0000000029 (cit. on pp. 3464–3469. DOI: 10.1149/1.2069100 (cit. on p.4). pp.5–8, 20).

28 [45]S.W ANG, N. ZHOU, and F. C. WALSH. “Diverse [58]J.J I, W. C. COOPER, D. B. DREISINGER, and E. PETERS. electrodeposits from modiﬁed acid sulphate (Watts “Surface pH measurements during nickel nickel) baths”. In: Transactions of the IMF 94.5 (2016), electrodeposition”. In: Journal of Applied Electrochemistry pp. 274–282. DOI: 10.1080/00202967.2016.1208383 25.7 (1995), pp. 642–650. DOI: 10.1007/BF00241925 (cit. on p.5). (cit. on p.6).

[46] G. A. DI BARI. “Electrodeposition of Nickel”. In: Modern [59] Y. TAKUMA, H. SUGIMORI, E. ANDO, K. MIZUMOTO, Electroplating. John Wiley & Sons, Inc., 2011, pp. 79–114. and K. TAHARA. “Comparison of the environmental DOI: 10.1002/9780470602638.ch3 (cit. on pp.5,6, 18). impact of the conventional nickel electroplating and the new nickel electroplating”. In: International Journal of [47] D. BAUDRAND. “Nickel sulfamate plating, its mystique Life Cycle Assessment (2017), pp. 1–15. DOI: and practicality”. In: Metal Finishing 94.7 (1996), 10.1007/s11367-017-1375-y (cit. on pp.6,7). pp. 15–18. DOI: 10.1016/0026-0576(96)81353-5 [60] T. DOI, K. MIZUMOTO, M. KAYASHIMA, and (cit. on pp.5,6, 18). S.-i. TANAKA. “Nickel Electroplating Bath Using Citric [48] M. H. SEO, J. S. KIM, W. S. HWANG, D. J. KIM, Acid.” In: Journal of the Surface Finishing Society of Japan S. S. HWANG, and B. S. CHUN. “Characteristics of Ni-P 52.6 (2001), pp. 462–466. DOI: 10.4139/sfj.52.462 alloy electrodeposited from a sulfamate bath”. In: (cit. on p.7). Surface and Coatings Technology 176.2 (2004), pp. 135–140. [61] T. DOI, K. MIZUMOTO, S. TANAKA, and T. YAMASHITA. DOI: 10.1016/S0257-8972(03)00661-3 (cit. on pp.5, “Effect of bath pH on Nickel Citrate electroplating bath”. 7, 12). In: Metal Finishing 102.6 (2004), pp. 104–111. DOI: 10.1016/S0026-0576(04)82610-2 (cit. on p.7). [49]N.M ARTYAK. Electroplating of low-stress nickel. CA Patent 2236933 A1. Dec. 1998 (cit. on p.6). [62]A.B RENNER, D. COUCH, and E. WILLIAMS. “Electrodeposition of Alloys of Phosphorous with [50] Y. E. SKNAR, O. SAVCHUK, and I. SKNAR. Nickel or Cobalt”. In: Journal of Research of the National “Characteristics of electrodeposition of Ni and Ni-P Bureau of Standards 44 (1950), pp. 109–122 (cit. on pp.7, alloys from methanesulfonate electrolytes”. In: Applied 8). Surface Science 423 (2017), pp. 340–348. DOI: 10.1016/j.apsusc.2017.06.146 (cit. on pp.6, 11). [63] A. M. PILLAI, A. RAJENDRA, and A. K. SHARMA. “Electrodeposited nickel-phosphorous (Ni-P) alloy [51] F. C. WALSH and C. PONCEDE LEÓN. “Versatile coating: an in-depth study of its preparation, properties, electrochemical coatings and surface layers from and structural transitions”. In: Journal of Coatings aqueous methanesulfonic acid”. In: Surface and Coatings Technology and Research 9.6 (2012), pp. 785–797. DOI: Technology 259.PC (2014), pp. 676–697. DOI: 10.1007/s11998-012-9411-0 (cit. on pp.7, 11–13, 16, 10.1016/j.surfcoat.2014.10.010 (cit. on p.6). 17, 21).

[52] M. D. GERNON, M. WU, T. BUSZTA, and P. JANNEY. [64]C.L OW and F. WALSH. “Multifunctional “Environmental benefits of methanesulfonic acid”. In: nanostructured metallic coatings by electrodeposition”. Green Chemistry 1.3 (1999), pp. 127–140. DOI: In: Multifunctional Materials for Tribological Applications. 10.1039/a900157c (cit. on p.6). Ed. by R. J. K. WOOD. Vol. 4. Pan Stanford, 2015. Chap. 7, pp. 227–258. DOI: 10.1201/b18311-8 (cit. on [53]B.M ACDOUGALL. “Effect of Chloride Ion on the pp.8, 14, 18, 22). Localized Breakdown of Nickel Oxide Films”. In: [65] F. J. CHEN, Y. N. PAN, C. Y. LEE, and C. S. LIN. “Internal Journal of The Electrochemical Society 126.6 (1979), Stress Control of Nickel-Phosphorus Electrodeposits pp. 919–925. DOI: 10.1149/1.2129194 (cit. on p.6). Using Pulse Currents”. In: Journal of The Electrochemical [54]E.A BD EL AAL, W. ZAKRIA, A. DIAB, and S. ABD EL Society 157.3 (2010), pp. D154–D158. DOI: HALEEM. “Anodic Dissolution of Nickel in Acidic 10.1149/1.3285108 (cit. on pp.8, 10, 18, 23). Chloride Solutions”. In: Journal of Materials Engineering [66] T. OSAKA, M. TAKAI, Y. SOGAWA, T. MOMMA, and and Performance 12.2 (2003), pp. 172–178. DOI: K. OHASHI. “Influence of Crystalline Structure and 10.1361/105994903770343312 (cit. on p.6). Sulfur Inclusion on Corrosion Properties of Electrodeposited CoNiFe Soft Magnetic Films”. In: [55]D.M OCKUTE, R. BUTKIENE, and O. NIVINSKIENE. Journal of The Electrochemical Society 146.6 (1999), “Effect of chloride ions on the behavior of saccharin, pp. 2092–2096. DOI: 10.1149/1.1391896 (cit. on pp.8, N-methylsaccharin, and 2-butyne-1,4-diol during 18). electrodeposition of nickel from acid electrolytes”. In: Russian Journal of Electrochemistry 37.4 (2001), [67] W. E. HANSAL, G. SANDULACHE, R. MANN, and pp. 376–381. DOI: 10.1023/A:1016673922524 (cit. on P. LEISNER. “Pulse-electrodeposited NiP-SiC composite p.6). coatings”. In: Electrochimica Acta 114 (2013), pp. 851–858. DOI: 10.1016/j.electacta.2013.08.182 (cit. on [56]E.D ÁVALOS, R. LÓPEZ, H. RUIZ, A. MÉNDEZ, pp.8, 17). R. ANTAÑO-LÓPEZ, and G. TREJO. “Study of the role of boric acid during the electrochemical deposition of Ni [68]A.Z OIKIS-KARATHANASIS, in a sulfamate bath”. In: International Journal of T. MILICKOVIC-KOSANOVIC, and I. DELIGKIOZI. Electrochemical Science 8.7 (2013), pp. 9785–9800 (cit. on “Effect of organic additives in the structure and p.6). functional properties of Ni-P composite coatings reinforced by nano-SiC and MWCNT”. In: 10th [57]I.R OSE and C. WHITTINGTON. Nickel plating handbook. Pan-Hellenic Scientific Congress of Chemical Engineering, Tech. rep. Nickel Institute, 2014 (cit. on pp.6, 24). Patra. 2015 (cit. on pp.8, 12, 17).

29 [69] E. A. PAVLATOU, M. STROUMBOULI, P. GYFTOU, and [81]H.Y OSHIDA, M. SONE, A. MIZUSHIMA, H. YAN, N. SPYRELLIS. “Hardening effect induced by H. WAKABAYASHI, K. ABE, X. T. TAO, S. ICHIHARA, and incorporation of SiC particles in nickel electrodeposits”. S. MIYATA. “Application of emulsion of dense carbon In: Journal of Applied Electrochemistry 36.4 (2006), dioxide in electroplating solution with nonionic pp. 385–394. DOI: 10.1007/s10800-005-9082-y surfactants for nickel electroplating”. In: Surface and (cit. on pp.8, 23). Coatings Technology 173.2-3 (2003), pp. 285–292. DOI: 10.1016/S0257-8972(03)00734-5 (cit. on p.8). [70] Y. YOU, C. GU, X. WANG, and J. TU. “Electrochemical Synthesis and Characterization of Ni-P Alloy Coatings [82] Y.-C. CHUANG, S.-T. CHUNG, S.-Y. CHIU, C.-Y. LI, and from Eutectic-Based Ionic Liquid”. In: Journal of the W.-T. TSAI. “Effect of surfactant on the electrodeposition Electrochemical Society 159.11 (2012), pp. D642–D648. of Ni-P coating in emulsified supercritical CO2 baths”. DOI: 10.1149/2.012211jes (cit. on p.8). In: Thin Solid Films 529 (2013), pp. 322–326. DOI: 10.1016/j.tsf.2012.05.039 (cit. on p.8). [71]K.N EUROHR, L. POGANY, B. G. TOTH, A. REVESZ, I. BAKONYI, and L. PETER. “Electrodeposition of Ni [83] C. A. ROSS, L. M. GOLDMAN, and F. SPAEPEN. “An from Various Non-Aqueous Media: The Case of Electrodeposition Technique for Producing Multilayers Alcoholic Solutions”. In: Journal of the Electrochemical of Nickel-Phosphorus and Other Alloys”. In: Journal of Society 162.7 (2015), pp. D256–D264. DOI: the Electrochemical Society 140.1 (1993), pp. 91–98. DOI: 10.1149/2.0381507jes (cit. on p.8). 10.1149/1.2056116 (cit. on pp. 12, 25). [72]L.D URNEY. Electroplating Engineering Handbook. 1st ed. [84] C. LOW, R. WILLS, and F. WALSH. “Electrodeposition of Springer US, 1984, pp. 441–447 (cit. on p.9). composite coatings containing nanoparticles in a metal deposit”. In: Surface and Coatings Technology 201.1-2 [73]R.N ARAYAN and M. MUNGOLE. “Electrodeposition of Ni-P alloy coatings”. In: Surface Technology 24.3 (1985), (2006), pp. 371–383. DOI: 10.1016/j.surfcoat.2005.11.123 (cit. on p. 12). pp. 233–239. DOI: 10.1016/0376-4583(85)90073-1 (cit. on pp.9, 12, 16). [85]E.T OTH-KADAR, I. BAKONYI, A. SOLYOM, J. HERING, G. KONCZOS, and F. PAVLYAK. “Preparation and [74] C.-c. HU and A. BAI. “Influences of the phosphorus characterization of electrodeposited amorphous Ni-P content on physicochemical properties of alloys”. In: Surface and Coatings Technology 31.1 (1987), nickel-phosphorus deposits”. In: Materials Chemistry and pp. 31–43. DOI: 10.1016/0257-8972(87)90151-4 Physics 77.1 (2003), pp. 215–225. DOI: (cit. on p. 12). 10.1016/S0254-0584(01)00592-2 (cit. on pp. 10, 15, 19, 20). [86] L. M. GOLDMAN, B. BLANPAIN, and F. SPAEPEN. “Short wavelength compositionally modulated Ni/Ni-P films [75]A.K UROWSKI, J. SCHULTZE, and G. STAIKOV. “Initial prepared by electrodeposition”. In: Journal of Applied stages of Ni-P electrodeposition: growth morphology Physics DOI and composition of deposits”. In: Electrochemistry 60.4 (1986), pp. 1374–1376. : 10.1063/1.337313 (cit. on pp. 12, 25). Communications 4.7 (2002), pp. 565–569. DOI: 10.1016/S1388-2481(02)00372-7 (cit. on p. 10). [87] G. A. DI BARI. “Nickel plating”. In: Metal Finishing 99 (2001), pp. 270–288. DOI: [76]L.L I, Y. ZHANG, S. DENG, and Y. CHEN. “Effect of 10.1016/S0026-0576(01)85285-5 ammonium on low-temperature electrodeposition of (cit. on p. 12). Ni-P alloys”. In: Materials Letters 57.22-23 (2003), [88]I.A PACHITEI and J. DUSZCZYK. “Autocatalytic nickel pp. 3444–3448. DOI: 10.1016/S0167-577X(03)00097-1 coatings on aluminium with improved abrasive wear (cit. on pp. 10, 12, 20). resistance”. In: Surface and Coatings Technology 132.1 (2000), pp. 89–98. DOI: [77] D. H. JEONG, U. ERB, K. T. AUST, and G. PALUMBO. 10.1016/S0257-8972(00)00864-1 “The relationship between hardness and abrasive wear (cit. on pp. 12, 14). resistance of electrodeposited nanocrystalline Ni-P [89]E.B REDAEL, B. BLANPAIN, J. P. CELIS, and J. R. ROOS. coatings”. In: Scripta Materialia 48.8 (2003), “On the Amorphous and Crystalline State of pp. 1067–1072. DOI: 10.1016/S1359-6462(02)00633-4 Electrodeposited Nickel-Phosphorus Coatings”. In: (cit. on pp. 10, 13, 21). Journal of The Electrochemical Society 141.1 (1994), pp. 294–299. DOI: 10.1149/1.2054703 (cit. on p. 13). [78]L.C HANG, C.-H. CHEN, and H. FANG. “Electrodeposition of Ni-P Alloys From a Sulfamate [90]E.V AFAEI-MAKHSOOS, E. L. THOMAS, and L. E. TOTH. Electrolyte Relationship Between Bath pH and “Electron microscopy of crystalline and amorphous Structural Characteristics”. In: Journal of The Ni-P electrodeposited films: In-situ crystallization of an Electrochemical Society 155.1 (2008), pp. D57–D61. DOI: amorphous solid”. In: Metallurgical Transactions A 9.10 10.1149/1.2803516 (cit. on pp. 10, 14). (1978), pp. 1449–1460. DOI: 10.1007/BF02661817 (cit. on p. 14). [79]S.A LLEG, A. BOUSSAHA, W. TEBIB, M. ZERGOUG, and J. J. SUNOLˇ . “Microstructure and Magnetic Properties of [91] F. ZHAOHENG. “Electrodeposition of amorphous Ni-P NiP Alloys”. In: Journal of Superconductivity and Novel alloy coatings”. In: Transactions of Nonferrous Metals Magnetism 29.4 (2016), pp. 1001–1011. DOI: Society of China (English Edition) 7.3 (1997), pp. 148–151 10.1007/s10948-016-3397-2 (cit. on pp. 11, 13, 14). (cit. on p. 14). [80]H.Y OSHIDA, M. SONE, A. MIZUSHIMA, K. ABE, [92]J.S CHIØTZ, T. VEGGE, F. D. DI TOLLA, and X. T. TAO, S. ICHIHARA, and S. MIYATA. “Electroplating K. W. JACOBSEN. “Atomic-scale simulations of the of Nanostructured Nickel in Emulsion of Supercritical mechanical deformation of nanocrystalline metals”. In: Carbon Dioxide in Electrolyte Solution”. In: Chemistry Physical Review B 60.17 (1999), pp. 11971–11983. DOI: Letters 31.11 (2002), pp. 1086–1087. DOI: 10.1103/PhysRevB.60.11971. arXiv: 9902165 10.1246/cl.2002.1086 (cit. on p.8). [cond-mat] (cit. on p. 14).

30 [93]G.P ALUMBO, U. ERB, and K. AUST. “Triple line [104]A.K RÓLIKOWSKI and P. BUTKIEWICZ. “Anodic disclination effects on the mechanical behaviour of behavior of NiP alloys studied by impedance materials”. In: Scripta Metallurgica et Materialia 24.12 spectroscopy”. In: Electrochimica Acta 38.14 (1993), (1990), pp. 2347–2350. DOI: pp. 1979–1983. DOI: 10.1016/0013-4686(93)80327-V 10.1016/0956-716X(90)90091-T (cit. on p. 14). (cit. on p. 18).

[94]D.J EONG, U. ERB, G. PALUMBO, and K. AUST. “Effect [105] J.-P. BONINO, S. BRUET-HOTELLAZ, C. BORIES, of Heat Treatment on the Wear Properties of P. POUDEROUX, and A. ROUSSET. “Thermal stability of Electrodeposited Nanocrystalline Ni-P Coatings”. In: electrodeposited Ni-P alloys”. In: Journal of Applied Journal of Metastable and Nanocrystalline Materials 15-16 Electrochemistry 27.10 (1997), pp. 1193–1197. DOI: (2003), pp. 635–642. DOI: 10.1023/A:1018423701791 (cit. on pp. 18, 21). 10.4028/www.scientific.net/JMNM.15-16.635 [106]I.B AKONYI, A. BURGSTALLER, W. SOCHER, (cit. on p. 14). J. VOITLÄNDER, E. TÓTH-KÁDÁR, A. LOVAS, H. EBERT, [95]I.G URRAPPA and L. BINDER. “Electrodeposition of E. WACHTEL, N. WILLMANN, and H. H. LIEBERMANN. nanostructured coatings and their characterization-A “Magnetic properties of electrodeposited, review”. In: Science and Technology of Advanced Materials melt-quenched, and liquid Ni-P alloys”. In: Physical 9.4 (2008), p. 043001. DOI: Review B 47.22 (1993), pp. 14961–14976. DOI: 10.1088/1468-6996/9/4/043001 (cit. on p. 14). 10.1103/PhysRevB.47.14961 (cit. on pp. 18, 19).

[96] S. ACHANTA, D. DREES, and J. P. CELIS. “Nanocoatings [107] P. A. ALBERT, Z. KOVAC, H. R. LILIENTHAL, for tribological applications”. In: Nanocoatings and T. R. MCGUIRE, and Y. NAKAMURA. “Effect of Ultra-Thin Films: Technologies and Applications. Phosphorus on the Magnetization of Nickel”. In: Journal Woodhead Publishing Limited, 2011. Chap. 12, of Applied Physics 38.3 (1967), pp. 1258–1259. DOI: pp. 355–396. DOI: 10.1063/1.1709568 (cit. on p. 19). 10.1016/B978-1-84569-812-6.50012-9 (cit. on [108] B. D. CULLITY and C. D. GRAHAM. Introduction to p. 14). magnetic materials. Vol. 12. 3. IEEE/Wiley, 2009, p. 544. [97] C. S. LIN, C. Y. LEE, F. J. CHEN, and W. C. LI. arXiv: arXiv:1011.1669v3 (cit. on p. 19). “Structural Evolution and Internal Stress of [109]K.D HANAPAL, V. NARAYANAN, and A. STEPHEN. Nickel-Phosphorus Electrodeposits”. In: Journal of The “Effect of phosphorus on magnetic property of Ni-P Electrochemical Society 152.6 (2005), p. C370. DOI: alloy synthesized using pulsed electrodeposition”. In: 10.1149/1.1901064 (cit. on pp. 14, 18, 24). Materials Chemistry and Physics 166 (2015), pp. 153–159. [98]H.L UO, X. WANG, S. GAO, C. DONG, and X. LI. DOI: 10.1016/j.matchemphys.2015.09.039 (cit. on “Synthesis of a duplex Ni-P-YSZ/Ni-P nanocomposite p. 19). coating and investigation of its performance”. In: [110]A.B AI and C.-C. HU. “Effects of annealing Surface and Coatings Technology 311 (2017), pp. 70–79. temperatures on the physicochemical properties of DOI: 10.1016/j.surfcoat.2016.12.075 (cit. on pp. 17, nickel-phosphorus deposits”. In: Materials Chemistry and 24). Physics 79.1 (2003), pp. 49–57. DOI: [99] J. DINI and H. JOHNSON. Plating on some difﬁcult-to-plate 10.1016/S0254-0584(02)00455-8 (cit. on pp. 19–21). metals and alloys. Tech. rep. Albuquerque, NM, and [111]K.Z ENG and D. ZHANG. “Recent progress in alkaline Livermore, CA (United States): Sandia National water electrolysis for hydrogen production and Laboratories (SNL), 1980, p. 28. DOI: 10.2172/5472921 applications”. In: Progress in Energy and Combustion (cit. on p. 17). Science 36.3 (2010), pp. 307–326. DOI: 10.1016/j.pecs.2009.11.002 [100]B.B OZZINI, P. L. CAVALLOTTI, J.-P. CELIS, and (cit. on p. 20). A. FANIGLIULO. “Anodic behaviour of amorphous [112] F. SAFIZADEH, E. GHALI, and G. HOULACHI. NiP/tin multilayers in an acidic chloride solution”. In: “Electrocatalysis developments for hydrogen evolution Corrosion Science 45.6 (2003), pp. 1161–1171. DOI: reaction in alkaline solutions - A Review”. In: 10.1016/S0010-938X(02)00221-4 (cit. on pp. 17, 26). International Journal of Hydrogen Energy 40.1 (2015), pp. 256–274. DOI: 10.1016/j.ijhydene.2014.10.109 [101]A.B AI, P.-Y. CHUANG, and C.-C. HU. “The corrosion behavior of Ni-P deposits with high phosphorous (cit. on p. 20). contents in brine media”. In: Materials Chemistry and [113]H.E ZAKI, M. MORINAGA, and S. WATANABE. Physics 82.1 (2003), pp. 93–100. DOI: “Hydrogen overpotential for transition metals and 10.1016/S0254-0584(03)00193-7 (cit. on p. 18). alloys, and its interpretation using an electronic model”. In: Electrochimica Acta 38.4 (1993), pp. 557–564. DOI: [102] S. J. SPLINTER, R. ROFAGHA, N. S. MCINTYRE, and 10.1016/0013-4686(93)85012-N (cit. on p. 20). U. ERB. “XPS Characterization of the Corrosion Films Formed on Nanocrystalline Ni-P Alloys in Sulphuric [114] I. PASEKA. “Hydrogen evolution reaction on Ni-P alloys: Acid”. In: Surface and Interface Analysis 24.3 (1996), The internal stress and the activities of electrodes”. In: pp. 181–186. DOI: 10.1002/(SICI)1096-9918(199603 Electrochimica Acta 53.13 (2008), pp. 4537–4543. DOI: )24:3<181::AID-SIA92>3.0.CO;2-N (cit. on p. 18). 10.1016/j.electacta.2008.01.045 (cit. on p. 20).

[103] R. B. DIEGLE, N. R. SORENSEN, C. R. CLAYTON, [115] C.-C. HU and A. BAI. “Optimization of hydrogen M. A. HELFAND, and Y. C. YU. “An XPS Investigation evolving activity on nickel-phosphorus deposits using into the Passivity of an Amorphous Ni-20P Alloy”. In: experimental strategies”. In: Journal of Applied Journal of the Electrochemical Society 135.5 (1988), Electrochemistry 31.5 (2001), pp. 565–572. DOI: pp. 1085–1092. DOI: 10.1149/1.2095880 (cit. on p. 18). 10.1023/A:1017525507665 (cit. on p. 20).

31 [116]Z.W EI, A. YAN, Y. FENG, L. LI, C. SUN, Z. SHAO, and [127] L. CHANG, P. W. KAO, and C. H. CHEN. “Strengthening P. SHEN. “Study of hydrogen evolution reaction on Ni-P mechanisms in electrodeposited Ni-P alloys with amorphous alloy in the light of experimental and nanocrystalline grains”. In: Scripta Materialia 56.8 (2007), quantum chemistry”. In: Electrochemistry pp. 713–716. DOI: Communications 9.11 (2007), pp. 2709–2715. DOI: 10.1016/j.scriptamat.2006.12.036 (cit. on p. 21). 10.1016/j.elecom.2007.09.006 (cit. on p. 20). [128]G.D EVARAJ, S. GURUVIAH, and S. SESHADRI. “Pulse [117]L.E LIAS, V. H. DAMLE, and A. C. HEGDE. plating”. In: Materials Chemistry and Physics 25.5 (1990), “Electrodeposited Ni-P alloy thin films for alkaline pp. 439–461. DOI: 10.1016/0254-0584(90)90111-M water splitting reaction”. In: IOP Conference Series: (cit. on p. 22). Materials Science and Engineering 149.1 (2016), p. 012179. [129] M. S. CHANDRASEKAR and M. PUSHPAVANAM. “Pulse DOI: 10.1088/1757-899X/149/1/012179 (cit. on p. 20). and pulse reverse plating-Conceptual, advantages and [118]C.T ANG, A. M. ASIRI, Y. LUO, and X. SUN. applications”. In: Electrochimica Acta 53.8 (2008), “Electrodeposited Ni-P Alloy Nanoparticle Films for pp. 3313–3322. DOI: Efficiently Catalyzing Hydrogen- and 10.1016/j.electacta.2007.11.054 (cit. on pp. 22, Oxygen-Evolution Reactions”. In: ChemNanoMat 1.8 23). (2015), pp. 558–561. DOI: 10.1002/cnma.201500163 (cit. on p. 20). [130] T. PEARSON and J. K. DENNIS. “Facts and fiction about pulse plating”. In: Transactions of the Institute of Metal [119]G.L U, P. EVANS, and G. ZANGARI. “Electrocatalytic Finishing 69.3 (1991), pp. 75–79. DOI: Properties of Ni-Based Alloys Toward Hydrogen 10.1080/00202967.1991.11870897 (cit. on pp. 22, 23). Evolution Reaction in Acid Media”. In: Journal of The U HAN Electrochemical Society 150.5 (2003), A551–A557. DOI: [131] F. H and K. C. C . “Deposition behaviour and 10.1149/1.1561629 (cit. on p. 20). morphology of Ni-SiC electro-composites under triangular waveform”. In: Applied Surface Science 243.1-4 [120] A. R. J. KUCERNAK and V. N. NARANAMMALPURAM (2005), pp. 251–258. DOI: SUNDARAM. “Nickel phosphide: the effect of 10.1016/j.apsusc.2004.09.068 (cit. on p. 22). phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium”. In: J. Mater. [132] R. L. ZELLER and U. LANDAU. “Electrodeposition of Chem. A 2.41 (2014), pp. 17435–17445. DOI: Ductile Ni-P Amorphous Alloys Using 10.1039/C4TA03468F (cit. on p. 21). Periodic-Reverse Current”. In: Journal of The Electrochemical Society 138.4 (1991), pp. 1010–1017. DOI: [121]K.K EONG, W. SHA, and S. MALINOV. “Crystallisation 10.1149/1.2085707 (cit. on p. 23). kinetics and phase transformation behaviour of electroless nickel-phosphorus deposits with high [133]C.K OLLIA, N. SPYRELLIS, J. AMBLARD, M. FROMENT, phosphorus content”. In: Journal of Alloys and and G. MAURIN. “Nickel plating by pulse electrolysis: Compounds 334.1-2 (2002), pp. 192–199. DOI: textural and microstructural modifications due to 10.1016/S0925-8388(01)01798-4 (cit. on p. 21). adsorption/desorption phenomena”. In: Journal of Applied Electrochemistry 20.6 (1990), pp. 1025–1032. DOI: [122]K.K EONG, W. SHA, and S. MALINOV. “Hardness 10.1007/BF01019584 (cit. on p. 23). evolution of electroless nickel-phosphorus deposits with thermal processing”. In: Surface and Coatings [134]C.K OLLIA and N. SPYRELLIS. “Microhardness and Technology 168.2-3 (2003), pp. 263–274. DOI: roughness in nickel electrodeposition under pulse 10.1016/S0257-8972(03)00209-3 (cit. on p. 21). reversed current conditions”. In: Surface and Coatings Technology 58.2 (1993), pp. 101–105. DOI: [123]I.A PACHITEI, F. TICHELAAR, J. DUSZCZYK, and 10.1016/0257-8972(93)90180-V (cit. on p. 23). L. KATGERMAN. “The effect of heat treatment on the structure and abrasive wear resistance of autocatalytic [135] C. KOLLIA, Z. LOIZOS, and N. SPYRELLIS. “Influence of NiP and NiP-SiC coatings”. In: Surface and Coatings pulse reversed current technique on the crystalline Technology 149.2-3 (2002), pp. 263–278. DOI: orientation and surface morphology of nickel 10.1016/S0257-8972(01)01492-X (cit. on p. 21). electrodeposits”. In: Surface and Coatings Technology 45.1-3 (1991), pp. 155–160. DOI: [124]H.H ABAZAKI, Y.-P. LU, A. KAWASHIMA, K. ASAMI, 10.1016/0257-8972(91)90218-L (cit. on p. 23). and K. HASHIMOTO. “The effects of structural relaxation and crystallization on the corrosion behavior [136]N.Z HAO, P. Y. QIU, and L. L. CAO. “Development and of electrodeposited amorphous Ni-P alloys”. In: Application of Functionally Graded Material”. In: Corrosion Science 32.11 (1991), pp. 1227–1235. DOI: Advanced Materials Research 562-564 (2012), pp. 371–375. 10.1016/0010-938X(91)90134-B (cit. on p. 21). DOI: 10.4028/www.scientific.net/AMR.562-564.371 [125]A.Z OIKIS-KARATHANASIS, E. A. PAVLATOU, and (cit. on p. 24). N. SPYRELLIS. “Pulse electrodeposition of Ni-P matrix composite coatings reinforced by SiC particles”. In: [137]G.U DUPA, S. S. RAO, and K. GANGADHARAN. Journal of Alloys and Compounds 494.1-2 (2010), “Functionally Graded Composite Materials: An pp. 396–403. DOI: 10.1016/j.jallcom.2010.01.057 Overview”. In: Procedia Materials Science 5 (2014), (cit. on p. 21). pp. 1291–1299. DOI: 10.1016/j.mspro.2014.07.442 (cit. on p. 24). [126]A.Y ONEZU, M. NIWA, J. YE, and X. CHEN. “Contact fracture mechanism of electroplated Ni-P coating upon [138]M.N IINO and S. MAEDA. “Recent development status stainless steel substrate”. In: Materials Science and of functionally gradient materials.” In: ISIJ International Engineering A 563 (2013), pp. 184–192. DOI: 30.9 (1990), pp. 699–703. DOI: 10.1016/j.msea.2012.11.054 (cit. on p. 21). 10.2355/isijinternational.30.699 (cit. on p. 24).

32 [139] Y. D. GAMBURG. “Electrodeposition of Alloys with [145]A.B RENNER and A. M. POMMER. “Production of a Composition Modulated over Their Thickness: A diffraction grating by depositing copper-bismuth alloy”. Review”. In: Russian Journal of Electrochemistry 37.6 In: National Bureau of Standards (1948) (cit. on p. 25). (2001), pp. 585–590. DOI: 10.1023/A:1016662316553 [146] C. A. ROSS. “Electrodeposited Multilayer Thin Films”. (cit. on p. 24). In: Annual Review of Materials Science 24.1 (1994), [140] S. ROY. “Electrochemical Fabrication of Nanostructured, pp. 159–188. DOI: Compositionally Modulated Metal Multilayers 10.1146/annurev.ms.24.080194.001111 (cit. on (CMMMs)”. In: Electrochemistry at the Nanoscale. p. 25). Springer Science & Business Media, 2009, pp. 349–376. [147] L. ELIAS, K. U. BHAT, and A. C. HEGDE. “Development DOI: 10.1007/978-0-387-73582-5_9 (cit. on p. 24). of nanolaminated multilayer Ni–P alloy coatings for better corrosion protection”. In: RSC Adv. 6.40 (2016), [141] L. QIN, J. XU, J. LIAN, Z. JIANG, and Q. JIANG. “A novel 10.1039/C6RA01547F electrodeposited nanostructured Ni coatingwith grain pp. 34005–34013. DOI: (cit. on size gradient distribution”. In: Surface and Coatings pp. 25, 26). Technology 203.1-2 (2008), pp. 142–147. DOI: [148]R.G IRARD. “The Electrodeposition of Thin Magnetic 10.1016/j.surfcoat.2008.08.044 (cit. on p. 24). Permalloy Films”. In: Journal of Applied Physics 38.3 (1967), pp. 1423–1430. DOI: 10.1063/1.1709651 (cit. on [142] C. GU, J. LIAN, and Q. JIANG. “Layered nanostructured p. 25). Ni with modulated hardness fabricated by surfactant-assistant electrodeposition”. In: Scripta [149] J. H. LIU, J. L. CHEN, Z. LIU, M. YU, and S. M. LI. Materialia 57.3 (2007), pp. 233–236. DOI: “Fabrication of Zn-Ni/Ni-P compositionally modulated 10.1016/j.scriptamat.2007.04.005 (cit. on p. 24). multilayer coatings”. In: Materials and Corrosion 64.4 (2013), pp. 335–340. DOI: 10.1002/maco.201106140 [143] J. CROUSIER, Z. HANANE, and J.-P. CROUSIER. “A cyclic (cit. on p. 26). voltammetry study of the NiP electrodeposition”. In: [150]B.B AHADORMANESH and M. GHORBANI. Electrochimica Acta 38.2-3 (1993), pp. 261–266. DOI: 10.1016/0013-4686(93)85137-N (cit. on p. 24). “Electrodeposition of Zn-Ni-P compositionally modulated multilayer coatings: An attempt to deposit [144] W. BLUM. “The Structure and Properties of Alternately Ni-P and Zn-Ni alloys from a single bath”. In: Deposited Metals”. In: Trans. Am. Electrochem. Soc. 40 Electrochemistry Communications 81 (2017), pp. 93–96. (1921), pp. 307–320 (cit. on p. 25). DOI: 10.1016/j.elecom.2017.06.002 (cit. on p. 26).