Study of Hillock and Zinc Whisker Evolution in Five Different Cable Tray Coatings

metals

Article Study of Hillock and Zinc Whisker Evolution in Five Different Cable Tray Coatings

Borja Arroyo 1,* , Marion Roth 1, José Alberto Álvarez 1, Ana Isabel Cimentada 1, Sergio Cicero 1 , Jaime Seco 2 and Guillermo Becedoniz 2

1 LADICIM, Departamento de Ciencia e Ingeniería del Terreno y de los Materiales, University of Cantabria, Avenida de los Castros, 44, 39005 Santander, Spain; [email protected] (M.R.); [email protected] (J.A.Á.); [email protected] (A.I.C.); [email protected] (S.C.) 2 VALDINOX Ltd., Villanueva 12, 39192 San Miguel de Meruelo, Spain; [email protected] (J.S.); [email protected] (G.B.) * Correspondence: [email protected]; Tel.: +34-42-20-18-37

Abstract: The main objective of this work is the study of the hillock and zinc whisker evolution of five different commercial zinc coatings applied on the same base steel wires of the patented EASY- CONNECT system cable trays manufactured by VALDINOX Ltd.: white zinc alkaline electrolyte, yellow zinc trivalent electrolyte, acid zinc electrolyte, hot dip galvanized, and zinc nickel coating. The limited literature on the subject is summarized, and then the coating thickness, chemical composition, hardness and surface rugosity are characterized. The hillock and whisker density evolution are evaluated over a period of 12 months, considering the presence of compression bending stresses. It is concluded that the white alkaline and yellow trivalent coatings are the most affected, while



the zinc-nickel shows the best behavior with no presence of whiskers; the acid zinc electrolyte also
 shows good results despite the delayed appearance of whiskers from the ninth month; the hot-dip

Citation: Arroyo, B.; Roth, M.; galvanized coating does not show any presence of zinc whiskers or hillocks. Álvarez, J.A.; Cimentada, A.I.; Cicero, S.; Seco, J.; Becedoniz, G. Study of Keywords: zinc whisker; hillock; coating; zinc; electroplating; white zinc alkaline electrolyte; yellow Hillock and Zinc Whisker Evolution zinc trivalent electrolyte; acid zinc electrolyte; hot dip galvanized; zinc nickel coating in Five Different Cable Tray Coatings. Metals 2021, 11, 325. https:// doi.org/10.3390/met11020325 1. Introduction Academic Editor: Sonia Mato Steel is widely used in different fields, such as for cable trays and other components within the electronics sector. However, this metal presents oxidation and corrosion prob- Received: 31 December 2020 lems when in contact with the environment, so it is common practice to cover it with Accepted: 9 February 2021 protective layers. The most commonly used coatings consist of paints of various types, or Published: 13 February 2021 the application of thin layers of other metals with superior resistance to environmental phenomena, such as chromium or zinc. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Zinc coatings, electrodeposited or hot-dip galvanized, are widely used; since the 1940s, published maps and institutional affil- the growth of zinc whiskers and hillocks has been detected in zinc coatings in different iations. industrial fields, but very few studies have been carried out so far. A zinc whisker is a filament emanating from the surface of a zinc-coated material, which has a hexagonal crystalline structure; there are three main types: nodular, rectilinear and with abrupt changes of direction. A hillock is a bump on the coating surface, and the place where whiskers can grow on. Hillocks consist of lumps from which whiskers may arise. The Copyright: © 2021 by the authors. main issue lies in the fact that these small fragments of material have relatively good Licensee MDPI, Basel, Switzerland. This article is an open access article conducting properties. distributed under the terms and These filaments lead to problems in facilities that have a high presence of electronic conditions of the Creative Commons equipment, the run for the miniaturization makes the weight of this phenomenon heavier Attribution (CC BY) license (https:// by an increase in the probability of causing short-circuits [2,3]. A simple cylindrical filament creativecommons.org/licenses/by/ of a few micrometer in diameter, able to reach 1mm long, can cause the dysfunction of an 4.0/). entire data center [4], as air flows can easily lead to its detachment and drag.

Metals 2021, 11, 325. https://doi.org/10.3390/met11020325 https://www.mdpi.com/journal/metals Metals 2021, 11, x FOR PEER REVIEW 2 of 22

by an increase in the probability of causing short-circuits [2] [3]. A simple cylindrical fila- ment of a few micrometer in diameter, able to reach 1mm long, can cause the dysfunction of an entire data center [4], as air flows can easily lead to its detachment and drag. There is a wide range of zinc coatings on the market, and a variety of electroplating or hot-dip galvanizing technologies are available, but no in-depth studies comparing their capacities with respect to the hillock or whisker phenomena have been carried out up to the present. In this work, a selection of five different zinc coatings are analyzed in detail. They are applied on the same base steel wires of the patented EASYCONNECT system cable trays manufactured by a leading company in the field of cable tray manufacturing, VALDINOX Ltd. [1], and make up a highly representative sample of the currently used coating technologies. The results are presented, comparing the behavior of the hillocks and behavior over 12 months in service, taking into account the presence of compression stresses, which is one of the most influential variables in this phenomenon. Metals 2021, 11, 325 2 of 21 2. State of the Art 2.1. Protection of Steel with Zinc Coatings There is a wide range of zinc coatings on the market, and a variety of electroplating or Zinc is used to protect steel from oxidation by playing the role of a sacrificial anode; hot-dip galvanizing technologies are available, but no in-depth studies comparing their it actually forms zinc oxide, which prevents further zinc corrosion [2]. The choice of the capacities with respect to the hillock or whisker phenomena have been carried out up to process for zinc deposition on a steel substrate has a great influence on the formation of the present. In this work, a selection of five different zinc coatings are analyzed in detail. whiskers. Zinc has a relatively low melting point, around 420°C. This intrinsic parameter They are applied on the same base steel wires of the patented EASYCONNECT system makes it highly prone to the growth of zinc whiskers [2]. Zinc coatings can present differ- cable trays manufactured by a leading company in the field of cable tray manufacturing, ent textures, such as alkaline electrolytes composed of trichite agglomerates, which makes VALDINOX Ltd. [1], and make up a highly representative sample of the currently used them relatively porous. The oriented growth direction with this element is (0001) whereas coating technologies. The results are presented, comparing the behavior of the hillocks acid electrolytes have a micro dendritic texture preferentially oriented (1122). Finally, and behavior over 12 months in service, taking into account the presence of compression there are coatings formed by hexagonal zinc structures characterized by their poor density stresses, which is one of the most influential variables in this phenomenon. of dislocations and stacking faults [2], which are sometimes called compact coatings. There2. State are oftwo the main Art processes for zinc coating application: hot dip galvanizing and electroplating.2.1. Protection Figure of 1 Steel shows with the Zinc external Coatings appearance of the two techniques in real com- ponents 10 months after fabrication. Zinc is used to protect steel from oxidation by playing the role of a sacrificial anode; In hot dip galvanizing, the metal substrate is immersed into a molten zinc bath; prior it actually forms zinc oxide, which prevents further zinc corrosion [2]. The choice of the to this,process the component for zinc depositionis submitted on to a a steel number substrate of pickling has a greatand rinsing influence steps. on The the formationresult- of ing coatingwhiskers. is actually Zinc has composed a relatively of several low melting metallurgical point, around bonded 420 layers.◦C. This The intrinsiccoatings parameter ob- tained makesby this it process highly proneare not to prone the growth to whisker of zinc growth, whiskers according [2]. Zinc to coatings World canEnvironment present different Servicestextures, [5] [2]. suchNevertheless, as alkaline whiskers electrolytes have composedbeen found of in trichite hot dip agglomerates, galvanized zinc which coat- makes ings [6]them [7]. relatively porous. The oriented growth direction with this element is (0001) whereas Electroplatingacid electrolytes consists have in a micro the application dendritic textureof an electrical preferentially current oriented to an electrolytic (1122). Finally, so- there lution arecoupled coatings to aformed zinc anode, by hexagonal while the zinc steel structures acts as cathode. characterized Zinc, the by metal their poorto be densityde- of posited,dislocations comes from and the stacking anode faults(soluble [2], anode) which areor from sometimes the reduction called compact of the electrolyte coatings. with an electricalThere arecurrent. two mainElectroplating processes has for been zinc coatingidentified application: to be more hot prone dip to galvanizing whisker and growthelectroplating. [2]; previous research Figure1 has shows shown the that external the chromium appearance passivation of the two layer techniques also has an in real impactcomponents on the whisker 10 months growth, after increasing fabrication. the incubation time but not preventing it [2].

Figure 1.Figure Differences 1. Differences in ZW growth in ZW in growth zinc coated in zinc surf coatedaces after surfaces 10 months after 10in monthsservice. It in can service. be It can be observedobserved how white how zinc white alkaline zinc coating alkaline (left coating) developed (left) developed substantially substantially more ZW than more hot ZW dip than hot dip galvanized coating (right), where its presence is null. galvanized coating (right), where its presence is null.

In hot dip galvanizing, the metal substrate is immersed into a molten zinc bath; prior to this, the component is submitted to a number of pickling and rinsing steps. The re- sulting coating is actually composed of several metallurgical bonded layers. The coatings obtained by this process are not prone to whisker growth, according to World Environ- ment Services [2,5]. Nevertheless, whiskers have been found in hot dip galvanized zinc coatings [6,7]. Electroplating consists in the application of an electrical current to an electrolytic solution coupled to a zinc anode, while the steel acts as cathode. Zinc, the metal to be deposited, comes from the anode (soluble anode) or from the reduction of the electrolyte with an electrical current. Electroplating has been identified to be more prone to whisker growth [2]; previous research has shown that the chromium passivation layer also has an impact on the whisker growth, increasing the incubation time but not preventing it [2]. Metals 2021, 11, x FOR PEER REVIEW 3 of 22

2.2. Definition and Dimensions of Zinc Whiskers (ZW) and Hillocks A zinc whisker can be defined as a cylindrical or columnar filament emanating from Metals 2021, 11, 325the surface of a material. Bibliographic sources, although scarce, state that this cylinder is 3 of 21 made up of a single crystal [4]. Nevertheless, the possibility of polycrystalline material in Zn is described by [8], though it is concluded that this could not be proved due to the poor quality of the images. It should also be noted that ZW have a hexagonal crystalline struc- 2.2. Definition and Dimensions of Zinc Whiskers (ZW) and Hillocks ture. Three different kinds of whiskers have been detected so far (Figure 2): A zinc whisker can be defined as a cylindrical or columnar filament emanating from • Nodular whiskers: this shape usually corresponds to the beginning of the growth. the surface of a material. Bibliographic sources, although scarce, state that this cylinder • Rectilinear whiskers: the most constraining kind, which can reach greater lengths. is made up of a single crystal [4]. Nevertheless, the possibility of polycrystalline material The longer the ZW is, the easier it can be broken, moved away and cause electric in Zn is described by [8], though it is concluded that this could not be proved due to the damage. poor quality of the images. It should also be noted that ZW have a hexagonal crystalline • Whiskers structure.with abrupt Three changes different of direction. kinds of whiskers have been detected so far (Figure2): • Nodular whiskers: this shape usually corresponds to the beginning of the growth. It is necessary to describe in detail the concept of hillocks (Figure 2), as these are a part of • Rectilinear whiskers: the most constraining kind, which can reach greater lengths. The the phenomenon. A hillock can be characterized as a bump caused by the release of a longer the ZW is, the easier it can be broken, moved away and cause electric damage. residual stress on the coating surface. The main explanation for this phenomenon is the • Whiskers with abrupt changes of direction. mismatch of the thermal coefficient between the coating and the iron bulk [2].

Figure 2. (aFigure) Nodular 2. a) whiskers. Nodular whiskers. (b) Rectilinear b) Rectilinear whiskers whiskers and hillocks. and hillocks. (c) Whiskers c) Whiskers with with abrupt abrupt changes of direction. (d) regular shapechanges of hillocks.of direction. d) regular shape of hillocks.

It is necessary Itto is describe necessary in to detail describe the inconcept detail theof hillocks concept (Figure of hillocks 2), (Figureas these2), are as thesea are a part part of the phenomenon,of the phenomenon. and the place A hillock wher cane whiskers be characterized can grow as on. a bump A hillock caused can by be the release of a characterized asresidual a bump stress caused on theby the coating release surface. of a residual The main stress explanation on the coating for this surface. phenomenon is the The main explanationmismatch for of this the phenomenon thermal coefficient is the mismatch between the of coatingthe thermal and thecoefficient iron bulk be- [2]. tween the coating andIt is the necessary iron bulk to [2]. describe in detail the concept of hillocks (Figure2), as these are a By X-ray diffractionpart of the analysis, phenomenon, Compton and et the al. place[9] and where Takemura whiskers et al. can [10] grow determined on. A hillock can be that zinc whiskerscharacterized are not compounds, as a bump but caused metall byic the filaments release in of the a residual form of single stress oncrystals. the coating surface. Zinc has a hexagonalThe main crystalline explanation structure. for this The phenomenon orthogonal axis is theis parallel mismatch to the of filament. the thermal coefficient between the coating and the iron bulk [2]. By X-ray diffraction analysis, Compton et al. [9] and Takemura et al. [10] determined that zinc whiskers are not compounds, but metallic filaments in the form of single crystals.

Zinc has a hexagonal crystalline structure. The orthogonal axis is parallel to the filament. Several authors [11,12] have observed recrystallized zinc in the zinc electroplate, with columnar grains extending in a direction perpendicular to the surface. Others [13,14] found Metals 2021, 11, x FOR PEER REVIEW 4 of 22

Metals 2021, 11, 325 4 of 21 Several authors [11,12] have observed recrystallized zinc in the zinc electroplate, with columnar grains extending in a direction perpendicular to the surface. Others [13,14] found that a zinc coating has a structure of small grains between the electroplated zinc that a zinc coating has a structure of small grains between the electroplated zinc and the and the root of a rectilinear whisker. No thinning of the coating was observed, even root of a rectilinear whisker. No thinning of the coating was observed, even around the whiskeraround root.the whisker root. ElectroplatedElectroplated zinc zinc coatings coatings on on a steela steel substrate, substrate, with with low low process process temperatures, temperatures, do not do shownot show intermetallic intermetallic compounds compounds (IMC) (IMC) at the at scale the scale of SEM of observations;SEM observations; the solubility the solubility limit oflimit iron of in iron zinc in is veryzinc lowis very (<0.03% low (<0.03% at 450 ◦C at according 450 °C according to the Fe-Zn to the phase Fe-Zn diagram phase presented diagram inpresented Figure3)[ in2 Figure]. Nevertheless, 3) [2]. Nevertheless, IMC does formIMC rapidlydoes form in hotrapidly dip galvanizingin hot dip galvanizing processes whenprocesses the temperaturewhen the temperature is high [13 is]. high The assessment[13]. The assessment of the behavior of the behavior of Zn electrodes of Zn elec- in alkalinetrodes in electrolyte alkaline electrolyte reported inreported the literature in the lit byerature Minakshi by Minakshi et al. provides et al. provides a useful a insight useful intoinsight the into behavior the behavior of Zn in of different Zn in different applications applications [15–17]. [15–17].

FigureFigure 3.3.Fe-Zn Fe-Zn phasephase diagram.diagram. Baated et al. [18] stated that zinc oxides and Fe–Zn intermetallic compounds are a Baated et al. [18] stated that zinc oxides and Fe–Zn intermetallic compounds are a key source of compressive stress in zinc coatings. Zinc reacts with the iron diffused from key source of compressive stress in zinc coatings. Zinc reacts with the iron diffused from the substrate and with the oxygen diffused from the environment, producing an IMC and the substrate and with the oxygen diffused from the environment, producing an IMC and oxide layer, respectively. Both the oxide and the IMC can be a source of compressive stress oxide layer, respectively. Both the oxide and the IMC can be a source of compressive stress on zinc grains, which actually diffuse to the surface of the coating forming the whiskers. on zinc grains, which actually diffuse to the surface of the coating forming the whiskers. This compressive stress then becomes the driving force of the zinc whisker growth [2]. This compressive stress then becomes the driving force of the zinc whisker growth [2]. 2.3. Zinc Whisker Growth Phenomenon 2.3. Zinc Whisker Growth Phenomenon The growth of whiskers can be described as a two-stage phenomenon. It begins with a periodThe of growth incubation, of whiskers known can as “incubationbe described time”, as a two-stage which can phenomenon. last from a few It begins months with to severala period years. of incubation, It has been known shown as that “incubation the incubation time”, timewhich is inverselycan last from proportional a few months to the to stress,several and years. that It there has been is a dependence shown that ofthe the incu growthbation rate time on is theinversely whiskers’ proportional length [2]. to The the secondstress, and stage that corresponds there is a dependence to the growth of ofthe the growth whisker rate and on can,the whiskers’ in certain length cases, reach[2]. The a ratesecond of 1 stage mm/year. corresponds The first to importantthe growth discovery of the whisker and validated and can, hypothesisin certain cases, was the reach fact a thatrate whiskersof 1 mm/year. grow fromThe first their important base and notdisc fromovery their and edges validated [2,19 ].hypothesis was the fact that Severalwhiskers models grow from have their been base elaborated and not in from order their to beedges able [2,19]. to accurately describe the phenomenon.Several models Frank [have19] proposed been elaborated a model in based order on to thebe rotationable to accurately of a right-angle describe edge- the screwphenomenon. dislocation Frank that [19] adds proposed an additional a model plane based to the on whisker the rotation base of after a right-angle each revolution. edge- Eshelbyscrew dislocation [20] states that that adds surface an additional tractions tendplane to to pull the whisker out a whisker, base after while each providing revolution. a constrainingEshelby [20] collarstates atthat the surface root which tractions keeps te thend to diameter pull out constant. a whisker, Lindborg while providing concluded a thatconstraining for each metal collar atom at the leaving root which the electroplate, keeps the therediameter should constant. be a corresponding Lindborg concluded vacancy formedthat for somewhereeach metal atom below leaving the root the of electrop the whiskerlate, there [21]. should As holes be havea corresponding never been vacancy seen in electroplating,formed somewhere diffusion below must the take root place of the over whisker long distances[21]. As holes compared have never to the been diameter seenof in the whisker. Therefore, he proposed a model for zinc whisker growth [22], consisting of a Metals 2021, 11, 325 5 of 21

long-distance diffusion stage (where a climbing dislocation loop acts as a vacancy-emitting source) and a glide stage (where dislocations glide to the surface). Mechanisms proposed by Eshelby, Franks and Lindborg are based on creep diffusion at the grain boundaries. A total of two creep mechanisms are considered: bulk or lattice diffusion (NabarroHerring creep) and grain boundary diffusion (Coble creep), as well as a combined diffusion (bulk diffusion plus grain boundary diffusion) [2,23]. According to [2], a new half plan of atom would stand out each time a loop is emitted. The surface is subjected to oxidation, so the interface energy has to be negative; there is tensile stress. This stress tends to pull out the whisker. In order for the growth to be possible, a stress threshold needs to be crossed. At values close to the threshold, growth is driven by the glide and there is a significant increase in slip rate, creating a linear dependence on stress. Spontaneous whisker growth has been observed mainly in electroplated coatings of low melting point metals, where the recrystallization temperature is under room temperature, such as cadmium, zinc and tin [2]. Based on this, Ellis et al. [24] concluded that dislocations could not explain their experimental observations, so they suggested recrystallization as a mechanism of whisker growth, a hypothesis that was later supported by Glazunova and Kudryavtsev [25], Kakeshita et al. [26], Le-bret et al. [27] and Boguslavsky and Bush [28], among other authors. The recrystallization mechanism is widely accepted today by the scientific commu- nity [2]. However there are different approaches or models. The two main recrystallization mechanisms [2], proposed by Smetana in 2007 [29] and Vianco and Rejent in 2009 [30], have been elaborated for tin whiskers but they can also be correlated to zinc whiskers. According to this theory, the ZW base is subjected to compressive stress. Close to the surface we can find recrystallized grains. These new grains are separated from each other by two different types of grains boundaries: columnar and oblique grain boundaries; the latter are submitted to lower stresses. The atomic density in the grain boundaries is lower than anywhere else in the material, which allows easier movements. As all the material is submitted to the same force, a difference in the atom density tends to create a stress gradient. Smetana [29] proposed that the atoms at the grain boundary in the whisker base are averagely at a lower energy (compressive stress) level than the surrounding areas, which allows the movement of atoms. Grain boundaries are actually sites with a lower degree of order and atom packing density. This theory considers compressive stress in the coating as the driving force for the whisker growth. As shown in [29], this mechanism is based on the columnar grain boundaries observed at the electroplate, where the grain boundary interfaces are subjected to biaxial compressive stress (σ) at the plane of the coating, resulting from a force (F). After recrystallization of new grains, oblique grain boundaries are formed in the coating. Compressive stress in oblique grain boundaries is lower than in vertical grain boundaries, although the applied force is equal, which leads to stress gradients in the grain boundaries, necessary for diffusion to the base of the whisker growth. Additionally, there is a lower atomic packaging density in the grain boundaries, leading to a stress gradient between the grain boundary and the bulk tin grain. The components of the stress applied to the oblique grain boundaries result in shear stress, parallel and perpendicular to the grain boundary. Vianco and Rejent [30] stated that compressive stress does not explicitly cause whisker growth through the bulk movement of the material. Rather, compressive stress generates inelastic deformation and, thus, an increase in strain energy that initiates DRX, which is actually the source of the whisker growth from the surface. Dynamic recrystallization (DRX) is an enhancement of the static recrystallization caused by the simultaneous occurrence of deformation. In contrast, static recrystallization occurs when the strain energy of defective structures is reduced without additional deformation occurring at the same time. The slower the strain rate, the more likely it is for the deformation (strain energy build-up) and recrystallization (strain energy loss) processes to overlap (DRX). According to the authors, the DRX model requires two processes: first, the deformation mechanism that initiates DRX and second, the mass transport mechanism that sustains grain (whisker) growth. Metals 2021, 11, 325 6 of 21

The steps of the DRX model for whisker growth, illustrated in [30], are as follows: (a) the compressive stress leads to the creation of dislocations, which pile up at pre-existing grain boundaries; (b) the resulting strain energy increases to the point where new grains are initiated as the DRX grain refinement step described above (whisker does not grow from a pre-existing grain; therefore the new grain, and thus the whisker grain orientation, does not need to correlate exactly to the texture of the nearby grains); (c) the new grain grows by migration of grain boundaries, although the size of the new grain is limited to the size of the already existent grains, which corresponds to the thickness of the electrodeposited film in the case of coatings; (d) the strain continues under the action of the compressive stress, producing a driving force in order to reduce the energy of the system: at this point, the whisker growth starts, and since no further growth is possible in the coating, the grain has to grow outside, from the surface with a whisker shape. While these recrystallization mechanisms are proposed for tin whiskers, recrystallized grains have been also observed in zinc electroplates in recent experiments. Etienne et al. [31] analyzed a whisker root on a zinc electroplated steel by EBSD; the zinc coating was found to be formed by columnar grains, while the grains at the root of the whisker were recrystallized.

2.4. Influencing Parameters in the Phenomenon There are many parameters than can affect the ZW growth phenomenon, both intrinsic and environmental; the difficulty lies in the fact that these parameters are somehow already correlated. The most important influencing and non-influencing parameters, according to the information collected until 2014 by J. Cabrera-Anaya [2], are summarized in Table1. Table 1. Summary of influence of parameters on whiskers growth.

Influencing Parameters Parameter Influence The thinner coating favors whisker growth [9] by influencing the coating Coating thickness texture [21] and the residual stress in thin (<5 µm) coatings [32]. The thinner substrate favors whisker growth by influencing the coating Substrate thickness texture [21]. Cyanide electrolytes produce less residual stress [11], and therefore a lower Electroplating electrolyte whisker growth rate. Favors whisker growth [9] by increasing compressive localized stress in Organic contaminants (including brighteners) electroplated metals [32]. Chrome Inhibits the onset of whiskers but does not avoid them [32]. Elongated grains tend to have larger whisker growth rates than specimens with Microstructure of the Zn coating equiaxed columnar grains [31]. Texture Planes {11-20} and {10-1-1} favor whisker growth [10,21]. Temperature Favors whisker length and density [9] and growth rate [21]. Tends to aggress the chromium passive layer, that protects the zinc coating, which Corrosion eases the whiskers to pop out [6,7]. Residual stress in the coatings Favors whisker growth rate [11,21] and decreases incubation time [33]. Applied external Compressive applied stress favors growth more than tensile applied external compressive stress stress [14]. Stress relaxation Might possibly be the driving force of zinc whisker growth [1,23]. Non-Influencing Parameters Parameter Comments Grain size No clear influence observed [21]. Hardness No influence observed [21]. Microstrain No influence observed [21]. Light and electromagnetic field No influence observed [6,7,9]. Humidity No influence observed [9]. Metals 2021, 11, 325 7 of 21

3. Materials and Methods This section is divided by subheadings and aims to provide a concise and precise description of the experimental results and their interpretation, as well as the experimental conclusions that can be drawn.

3.1. Coatings under Study and Base Metal In this work, the following five different coatings are analyzed. All of them are applied on Φ4 mm carbon steel base wires from the same manufacturer. The storage time between their fabrication and the study was of around two months. • White alkaline electrolyte coating (re-named coating 1 for simplicity): The sample was covered with a zinc coating electrolytic method. Prior to this, some stages of chemical preparation had been applied: Protel FR21 chemical degreasing, Uni- clear H2425 electrolytic degreasing, HCl scouring, Protolux AZURE applied during 35–40 minutes, NO3H 1% neutralization and passivation and drying at temperature below 60 ◦C. • Yellow trivalent zinc electrolyte coating (re-named coating 2 for simplicity): the steps were identical to those of the white alkaline electrolyte coating, except for the passivation stage, which corresponds to the deposition of the chromium layer, which uses a different solution in this case. • Acid zinc electrolyte coating (re-named coating 3 for simplicity): In this case no information was supplied by the manufacturing company, VALDINOX [4]. • Hot dip galvanizing zinc coating (re-named coating 4 for simplicity): the resulting coating is composed of several metallurgical bonded layers. Firstly, several surface preparation steps need to be carried out: degreasing with caustic solutions, rinsing, pickling with hydrochloric acid, second rinsing, oxidation protection layer (called the flux stage) and drying. Then, the galvanization step consists of immersing the studied elements in a crucible with melted zinc, where a chemical reaction takes place between the steel and the zinc that allows the creation of the coating layer; the bath composition is essentially zinc, the presence up to 1.5% of other elements being authorized. A final passivation layer is added in order to delay the formation of zinc oxide. • Zinc–nickel coating (re-named coating 5 for simplicity): this coating also uses an electrolyte method, but in this case a zinc–nickel one was employed according to UN UNE-EN 12,329 standard [34]. Prior to this, some stages of chemical preparation had been applied: 20% HCl scouring, water continually renewed rinsing, Uniclean 298 chemical and electrolytic degreasing, 2nd water continually renewed rinsing. Then, the Zn-Ni bath is applied and after this, two steps of water continually renewed rinsing, plus a 1% Nitric acid pre-passivation and a Cr blue passivation, to finally undergo a water continually renewed rinsing and drying in oven. The microstructure of the base material (steel wire) was analyzed. For this purpose, sub-samples of the cross and longitudinal sections were obtained, encapsulated in Bakelite cylinders (Figure4), polished and finally a 5% NITAL chemical attack was used (5% of nitric acid dissolved in ethanol) for 7 s on each sample. It was observed that all the coatings had been applied on the same base material, showing the ferritic-pearlitic structure presented in Figure4; for all the samples, an anisotropy in the shape of the grains can be observed due to the cold drawn of the steel. The approximate size of the grain is between 30 and 70 µm in length. Metals 2021, 11, x FOR PEER REVIEW 8 of 22

coatings had been applied on the same base material, showing the ferritic-pearlitic struc-

Metals 2021, 11ture, 325 presented in Figure 4; for all the samples, an anisotropy in the shape of the grains can 8 of 21 be observed due to the cold drawn of the steel. The approximate size of the grain is be- tween 30 and 70 µm in length.

FigureFigure 4. (a) Wire 4. ( longitudinala) Wire longitudinal and cross section and cross encapsulated section inencapsulated Bakelite and polished.in Bakelite (b) and Microstructure polished. and(b) grain size of the samplesMicrostructure studied. Remark: and grainDecimal size separatorof the samples in these studied. images isRemark: “,” instead Decimal of “.”. separator in these im- ages is "," instead of ".". 3.2. Coating Characterization 3.2. Coating CharacterizationSome of the most relevant parameters of the coatings were analyzed, using the afore- Some of the mentionedmost relevant sub-samples parameters of theof the cross coatings and longitudinal were analyzed, sections using encapsulated the afore- in Bakelite mentioned sub-samplescylinders of (Figure the cross4). and longitudinal sections encapsulated in Bakelite cylinders (Figure 4). Firstly, the coating thickness was measured by means of an optical microscope. Then, the hardness of the coating was obtained by means of a microhardness testing equipment. Firstly, the coating thickness was measured by means of an optical microscope. Then, For this purpose, the tests were carried out under a 5 mN load and unload model in order the hardness of the coating was obtained by means of a microhardness testing equipment. not to puncture out of the layer (due to their reduced thickness); the Vickers Hardness (HV) For this purpose,scale the tests was employedwere carried and out a total under of 5 a indentations 5 mN load and were unload performed model in eachin order sample, taking not to puncture outthe averageof the layer as a representative (due to their result. reduced thickness); the Vickers Hardness (HV) scale was employedThen, and the rugositya total of of 5 the indentations five samples were was performed measured; ain portable each sample, roughness meter taking the averagewas as employeda representative for a total result. of5 measurements in each sample, taking the average as a Then, the rugosityrepresentative of the five result. samples Finally, was the measured; composition a portable of each coating roughness was analyzedmeter was by means of employed for a totalsemi-quantitative of 5 measurements SEM microscopy. in each Asample, pattern linetaking was the drawn, average covering as a the repre- entire thickness sentative result. Finally,of the coating, the composition and an average of each composition coating was calculatedanalyzed fromby means the spectrum of semi- obtained by quantitative SEMthe microscopy. apparatus employed.A pattern line was drawn, covering the entire thickness of the coating, and 3.3.an average Whisker andcomposition Hillock Quantification was calculated from the spectrum obtained by the apparatus employed. The main aim of this report is to study the evolution of zinc whisker growth in the different coatings throughout in-service time, with the aim of quantifying the density 3.3. Whisker and Hillockof whiskers Quantification and hillocks. In this regard, the bibliography [2,3] points out the positive The main aiminfluence of this onreport ZW is development to study the in evolution compressed of areaszinc whisker when electroplated growth in the or galvanized different coatingscomponents throughout are in-service subjected time to bending,, with the a common aim of workquantifying situation the for density loaded trays. of In order whiskers and hillocks.to study In thisthis effect, regard, two the loading bibliography conditions [2,3] have points been out considered: the positive samples influ- without any ence on ZW developmentsolicitation, in samplescompressed subjected areas to when compression electroplated stresses or caused galvanized by a flexural compo- solicitation, designed respectively as “R” (as Received) and “F” (Flexural solicitation) for simplicity. nents are subjected to bending, a common work situation for loaded trays. In order to An experimental program was designed consisting of a monthly inspection by means study this effect, oftwo scanning loading electron conditions microscopy have been (SEM) considered: of the surface samples of the sampleswithout in any the so- situations “R” licitation, samples(without subjected efforts) to compression and “F” (subjected stresses to bending). caused by For a aflexural SEM observation, solicitation, all de- of the samples signed respectivelyhave as a“R” size (as restriction Received) in orderand “F” to fit (Flexural into the solicitation) examination platform;for simplicity. so, from each of the An experimentalreceived program meshes, was two designed samples wereconsisting extracted, of a asmonthly shown ininspection Figure5, one by means for the stress-free of scanning electronstate microscopy (R) and another (SEM) that of was the subjectedsurface of to the compression samples in bending the situations using an “R” experimental (without efforts) tooland built“F” (subjected for this purpose to bendin (F). g). For a SEM observation, all of the samples have a size restriction in order to fit into the examination platform; so, from each of the received meshes, two samples were extracted, as shown in Figure 5, one for the stress-free

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Metals 2021, 11, 325 9 of 21 state (R) and another that was subjected to compression bending using an experimental tool built for this purpose (F).

FigureFigure 5. Example 5. Example of some of some of the of samples the samples prepared prepar for introductioned for introduction into the SEM.into the (a) RSEM. samples; (a) R (samples;b) F samples. (b) F samples. In order to reproduce the environmental conditions present in data centers, or facil- In order to reproduceities where the cable environmental trays are usually condit installed,ions present the samples in data were centers, stored or in facili- the microscopy ◦ ◦ ties where cable laboratory,trays are usually where the installed, temperature the samples conditions were are kept stored constant in the between microscopy 20 C and 22 C and the presence of electronic equipment, wiring and the magnetic fields generated by laboratory, where the temperature conditions are kept constant between 20 °C and 22 °C them, are similar to those of these facilities. The samples were not manipulated or exposed and the presence of electronic equipment, wiring and the magnetic fields generated by to air currents, thus reproducing the statics of a real facility. them, are similar to thoseThe firstof these issue facilities. encountered The wassamples the cylinder were not shape manipulated of the bending or exposed samples. In order to air currents, thusto have reproducing the best view the statics possible of of a thereal whiskers, facility. the images were taken from the side and The first issuenot encountered from the top was of the the sample, cylinder which shape gives of the depth bending to the picture.samples. The In order only extractable to have the best viewimage possible format from of the the whiske SEM is ars, grey the scaleimages picture were with taken no information from the side about and the height or not from the topthe of positionthe sample, of the which elements gives studied. depth In to order the topicture. try to implementThe only extractable an automatic whisker image format fromand the hillock SEM quantification, is a grey scale simple picture segmentation with no information tests have been about run, the and height element tracking or the position ofsoftware the elements has been studied. tried, In but order with to little try to success implement and with an automatic very noisy whisker and unexploitable and hillock quantification,results; no simple automatic segmentation counting and tests recognizance have been program run, and has element been encountered tracking to carry out this task. After checking the bibliography, it was found that all the previous searches software has been tried, but with little success and with very noisy and unexploitable re- on the same subject also used a hand counting system [2]. sults; no automatic countingFinally, for and this recognizan research, thece quantificationprogram has wasbeen semi encountered automatized. to Forcarry each sample, out this task. Afterpictures checking of a certainthe bibliography, marked zone it werewas takenfound monthly that all inthe both previous stress-free searches (R) and bending on the same subject(F) states;also used the studya hand was counting performed system with [2]. a twelve month time lapse. The quantification Finally, for thiswas research, based on only the fourquantification images without was anysemi overlaps automatized. between For them. each Reduced sample, images were pictures of a certainmandatory marked as zone the shapewere take of then monthly sample is in cylindrical both stress-free so the focus (R) and cannot bending be on the entire (F) states; the studyimage; was a performed reduced frame with was a tw appliedelve month for the time two types:lapse. 686The× quantification600 pixels in the case of was based on onlystress-free four images (R) samples without and any 1024 overlaps× 400 between pixels in thethem. case Reduced of bending images (F) samples. were mandatory as the shapeThe of first the step sample was tois identifycylindrical the hillocksso the focus on this cannot frame be and, on inthe a secondentire stage, the image; a reducedwhiskers frame was were applied enumerated; for the two no size types: restriction 686 × 600 had pixels been in applied the case for of the stress- whisker length. Then, the evolution over time of each of the hillocks and whiskers identified was controlled. free (R) samples and 1024 × 400 pixels in the case of bending (F) samples. For each sample, pictures were taken monthly in both F and R states for twelve months; The first stepalso, was a firstto identify picture wasthe takenhillocks immediately on this frame after theand, sample in a second preparation stage, (month the 0), when whiskers were enumerated;the cable trays no had size been restriction stored for had around been two applied months for after the manufacture.whisker length. Then, the evolution overIn order time to proceedof each toof the the control hillocks of the and hillocks whiskers and whiskersidentified over was time, con- the evolution trolled. For each ofsample, each of pictures these identified were take itemsn monthly was followed in both in each F and inspection. R states It for should twelve be noted that months; also, a firstthe picturespicture takenwas taken were coveringimmediately the exact after same the areasample each preparation month with the(month same conditions 0), when the cableand trays with had identical been parametersstored for around in every two test, months so the whiskers after manufacture. identified and controlled were In order to proceedthe same to month the control after month. of the Figures hillocks6 and and7 show, whiskers respectively, over time, the evolution the evolu- of a whisker tion of each of theseand aidentified hillock over items time was in a foll whiteowed alkaline in each electrolyte inspection. sample It withoutshould compressivebe noted stresses that the pictures (astaken received). were covering In Figure 6the, it canexact be same observed area how each the month whisker with under the study same remains con- constant at a length of around 14 µm, while the whisker density observed in the image does not. In ditions and with identical parameters in every test, so the whiskers identified and con- Figure7, it can be observed how the hillock decreases from 130 to 80 µm over the first five trolled were the same month after month. Figure 6 and Figure 7 show, respectively, the evolution of a whisker and a hillock over time in a white alkaline electrolyte sample with- out compressive stresses (as received). In Figure 6, it can be observed how the whisker

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under study remains constant at a length of around 14 µm, while the whisker density observed in the image does not. In Figure 7, it can be observed how the hillock decreases months, going down to around 75 µm during another 4 months; hillock density remains fromunder 130 tostudy 80 µm remains over theconstant first five at a months, length of going around down 14 µm, to around while the 75 whiskerµm during density another constant over the whole time of study. 4 months;observed hillock in the densityimage does remains not. In consta Figurent 7, over it can the be wholeobserved time how of the study. hillock decreases from 130 to 80 µm over the first five months, going down to around 75 µm during another 4 months; hillock density remains constant over the whole time of study.

FigureFigure 6. ExampleFigure 6. Example 6. of Example evolution of ofevolution of evolution a whisker of of ina a whisker awhisker white alkaline in a whitwhit electrolyteee alkaline alkalinesample electrolyte electrolyte without sample sample compressive without without com- stresses com- over time. pressivepressive stresses stresses over over time. time.

Figure 7. ExampleFigure 7. of Example evolution of ofevolution a hillock of in a a hillock white alkalinein a white electrolyte alkaline sampleelectrolyte without sample compressive without compres- stresses over time. sive stresses over time.

Figure4. Coating 7. Example Characterization of evolution of Results a hillock in a white alkaline electrolyte sample without compres- sive stresses over time.

4. Coating Characterization Results

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4. Coating Characterization Results 4.1. Coating Thickness Analysis 4.1. Coating Thickness Analysis Figure8 corresponds to microscope observations of the coatings at different magnifi- cationsFigure (×100 8 corresponds×200 and ×to500), microscope to identify observat in eachions of case the the coatings relevant at different measurements. magni- fications (×100 ×200 and ×500), to identify in each case the relevant measurements.

FigureFigure 8. ThicknessThickness analysis analysis by byoptical optical microscope microscope of the of different the different samples: samples: white alkaline white coating alkaline coating (Sample(Sample 1 ),), yellow yellow trivalent trivalent zinc zinc electrolyte electrolyte (Sample (Sample 2), acid 2 ),zinc acid electrolyte zinc electrolyte (Sample 3 (),Sample hot-dip 3), hot-dip galvanizing (Sample 4) and zinc-nickel (Sample 5). Remark: Decimal separator in these images is galvanizing“,” instead of (“.”.Sample 4) and zinc-nickel (Sample 5). Remark: Decimal separator in these images is “,” instead of “.”. The white alkaline coating seems to have quite a regular thickness, with the meas- urementsThe whitedisplaying alkaline an average coating thickness seems toof around have quite 14 µm. a regularOn examining thickness, the coating, with the mea- surementssectioned hillocks displaying were observed an average (×500 thickness image), having of around a diameter 14 µm. of On 30 µm examining and a height the coating, sectionedof around hillocks11 µm; these were hillocks observed can ( ×be500 described image), as having having a a diameter semi-spherical of 30 µshape.m and The a height of around 11 µm; these hillocks can be described as having a semi-spherical shape. The yellow trivalent zinc electrolyte (sample 2) also has a steady thickness of around 15 µm; in this

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case, some sectioned hillocks have also been found, presenting a similar shape description and size as for sample 1. The acid coating (sample 3) is slightly thicker than the above ones, and reaches around 20 µm; a thin layer (of around 7 µm) can be seen on the external side, which is not part of the coating, but seems rather to be the void created by the thermal retraction of the Bakelite cylinder that had been filed using polishing dusts during this process. For the hot-dip-galvanized coating (sample 4), the coating is notably ten times thicker than the above ones, and the external surface seems more irregular (×200-magnification image). In this case, no trace of hillocks was found, as the undulations found correspond to the coating irregularities. Finally, the zinc–nickel coating (sample 5) has an average of 14 µm thickness, similar to the ones observed in all the above samples except the hot-dip-galvanized one. The presence of several cracks at a space from each other can be observed in all the coating (×200 magnification). As this crack damage does not appear in the SEM images used for the whisker study (see Section5), and has a high hardness of over 350 HV (see Section 4.2), it has been assumed that the cracks are a product of the cutting and polishing steps.

4.2. Coating Hardness Table2 gathers the results of the Vickers hardness test on each of the five samples. Table 2. Vickers hardness.

Average Sample Ind. 1 (HV) Ind. 2 (HV) Ind. 3 (HV) Ind. 4 (HV) Ind. 5 (HV) Hardness (HV) White alkaline (1) 87.4 84.4 83.5 94.6 50.7 80.2 Yellow trivalent (2) 102.4 130.2 113.5 102.5 93.7 108.5 Acid zinc electrolyte (3) 53.7 53.5 52.2 54.3 48.6 52.5 Hot-dip galvanizing (4) 148.5 142.3 144.5 206.1 172.2 162.7 Zinc-nickel (5) 376.1 326.5 383.3 303.9 390.2 356.0

Sample 5 seems to have the greatest hardness, a fact that could explain its extreme fragility and the cracks observed with the optical microscope (see Section 4.1); this value is high compared to the other coatings. However, this result looks coherent considering that the sample 5 coating is an alloy of zinc and nickel. Sample 3, acid electrolyte coating, has the lowest value with an average of around 52 HV. The alkaline electrolyte coatings 1 and 2 have values of around 80 HV and 108 HV, respectively. The hot dip galvanized coating has an intermediate average value of 162 HV.

4.3. Rugosity of the Coatings Table3 gathers the results of the superficial rugosity tests on each of the five samples. Samples 3 and 5 have the lowest rugosity values, which matches with the flat and smooth surface observed by SEM techniques (see Section5). Sample 4 is the one with the most irregular surface, also in agreement with SEM images.

Table 3. Superficial rugosity of the coatings.

Average Sample Rug. 1 (µm) Rug. 2 (µm) Rug. 3 (µm) Rug. 4 (µm) Rug. 5 (µm) Rugosity (µm) White alkaline (1) 0.38 0.39 0.26 0.31 0.41 0.30 Yellow trivalent (2) 0.39 0.31 0.38 0.41 0.31 0.28 Acid zinc electrolyte (3) 0.27 0.21 0.30 0.32 0.30 0.22 Hot-dip galvanizing (4) 0.47 0.55 0.62 0.49 0.67 0.44 Zinc-nickel (5) 0.31 0.22 0.25 0.32 0.22 0.21 Metals 2021, 11, 325 13 of 21 Metals 2021, 11, x FOR PEER REVIEW 13 of 22 Metals 2021, 11, x FOR PEER REVIEW 13 of 22

Table 4 gathers4.4. Chemical the semi-quantitative Composition composition of each of the five coatings. It can Table 4 gathers the semi-quantitative composition of each of the five coatings. It can be noted that coatingTable 4 (hot-dip-galvanized)4 gathers the semi-quantitative is the one that composition has the highest of each percentage of the five of coatings. It can beZinc, noted followed that coating bybe notedcoating 4 (hot-dip-galvanized) that 3 (acid coating electrolyte) 4 (hot-dip-galvanized) is with the one85.72 that wt%. has is theCoatings the one highest that 1 and haspercentage 2 the (alkaline highest of percentage of Zinc,white followed alkaline byandZinc, coating yellow followed 3 trivalent (acid by electrolyte) coating respectively) 3 (acid with electrolyte) seem85.72 towt%. have with Coatings an 85.72 almost 1 wt%. and identical Coatings2 (alkaline zinc 1 and 2 (alkaline whiteconcentration alkaline ofandwhite around yellow alkaline 80 trivalentwt%. and Concerning yellow respectively) trivalent coating seem respectively) 5 (zinc-nickel),to have an seem almost it is to understandable haveidentical an almost zinc identical zinc concentrationthat the zinc concentrationofconcentration around 80 wt%. is of lower, around Concerning as 80this wt%. cois atingan Concerning alloy 5 (zinc-nickel), with coating approximately it 5 (zinc-nickel),is understandable 11 wt% it isof understandable thatNickel. the In zinc each concentration coatingthat the we zinc can concentrationis lower,also find as sm this isall lower, ispercentages an asalloy this with is of an oxyg alloyapproximatelyen with and approximatelycarbon, 11 rangingwt% of 11 wt% of Nickel. Nickel.from 1 toIn 4each wt% coatingIn and each 6 towe coating 16 can wt%, also we respectively. canfind also small find percentages small percentages of oxygen of and oxygen carbon, and ranging carbon, ranging from 1 from 1 to 4 wt%to and 4 wt% 6 to and16 wt%, 6 to 16respectively. wt%, respectively. Table 4. Chemical composition of the coatings. Table 4. ChemicalTable composition 4. Chemical of the composition coatings. of the coatings. Sample Zn (wt%) Ni (wt%) C (wt%) O (wt%) Total (wt%) Sample Sample Zn (wt%) Zn (wt%) Ni (wt%) Ni (wt%) C (wt%) C (wt%) O (wt%) O Total (wt%) (wt%) Total (wt%) White alkaline (1) 80.96 - 15.27 3.77 100 White alkalineWhite (1) alkaline (1)80.96 80.96- -15.27 15.273.77 3.77100 100 Yellow trivalent (2) 79.86 - 15.87 4.27 100 Yellow trivalentYellow (2) trivalent (2)79.86 79.86- -15.87 15.874.27 4.27100 100 Acid zinc electrolyte (3) 85.72 - 11.36 2.92 100 Acid zinc electrolyteAcid zinc (3) electrolyte85.72 (3) 85.72- -11.36 11.362.92 2.92100 100 Hot-dip galvanizingHot-dip (4) galvanizing (4)92.26 92.26- - 6.63 6.631.11 1.11100 100 Hot-dip galvanizing (4) 92.26 - 6.63 1.11 100 Zinc-nickel (5)Zinc-nickel (5) 73.73 73.7311.74 11.7410.61 10.61 3.92 3.92 100 100 Zinc-nickel (5) 73.73 11.74 10.61 3.92 100 5. Whisker and5. Hillock Whisker Quantification and Hillock Quantificationand Evolution and Evolution 5. Whisker and Hillock Quantification and Evolution 5.1. First Observations5.1. First from Observations Preliminary from Analysis Preliminary of the Samples Analysis of the Samples 5.1. First Observations from Preliminary Analysis of the Samples From a first generalFrom analysis a first general of the analysis pictures of obtained the pictures monthly, obtained preliminary monthly, preliminary conclu- conclusions sionsFrom can bea firstdrawncan general be by drawn analyzing analysis by analyzing ofFigures the pictures 9–13. Figures It obtained 9must–13. Itbe must monthly,taken be into taken preliminary account into account that conclu- at thatthe at the moment sionsmoment can when be drawn whenthe study by the analyzing studystarted, started, theFigures cable the 9–13. trays cable It had traysmust been hadbe takenstored been into storedfor approximatelyaccount for approximately that at twothe two months momentmonths after when their theafter manufacturing.study their started, manufacturing. the cable trays had been stored for approximately two months after their manufacturing.

Figure 9. ExampleFigure of9. surfaceExample evolution of surface in evolution white alkaline in white electrolyte alkaline (sample electrolyte 1) under (sample compressive 1) under stresses com- (F); (a) month 1, Figure 9. Example of surface evolution in white alkaline electrolyte (sample 1) under com- (b) month 3.pressive stresses (F); (a) month 1, (b) month 3. pressive stresses (F); (a) month 1, (b) month 3.

Figure 10. Example of surface evolution in yellow trivalent zinc electrolyte (sample 2) without Figure 10. ExampleFigurestresses 10. (R); of Example surface (a) month evolutionof surface 1, (b) month evolution in yellow 3. in trivalent yellow zinctrivalent electrolyte zinc electrolyte (sample (sample 2) without 2) without stresses (R); (a) month 1, (b) month 3.stresses (R); (a) month 1, (b) month 3.

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FigureFigure 11. 11.Example Example of of surface surface evolution evolution in in acid acid zinc zinc electrolyte electrolyte coating coating (sample (sample 3) 3) without without stresses stresses Figure 11. Example(R); of surface(a) month evolution 1, (b) month in acid 9. zinc electrolyte coating (sample 3) without stresses (R); (a) month 1, (b) month 9. (R);Figure (a) month11. Example 1, (b) monthof surface 9. evolution in acid zinc electrolyte coating (sample 3) without stresses (R); (a) month 1, (b) month 9.

Figure 12. Example of surface evolution in hot-dip galvanizing zinc coating (sample 4) under Figure 12. ExampleFigure 12. of Example surfacecompressive of evolution surface evolution in stresses hot-dip (F);in galvanizing hot-dip (a) month galvanizing zinc1, (b) coating month zinc (sample 12.coating (sample 4) under 4) compressive under stresses (F); Figure 12. Example of surface evolution in hot-dip galvanizing zinc coating (sample 4) under (a) month 1,compressive (b) month 12. stresses (F); (a) month 1, (b) month 12. compressive stresses (F); (a) month 1, (b) month 12.

Figure 13. Example of surface evolution in sample 5 under compressive stresses (F); (a) Figure 13. Example of surface evolution in sample 5month under 1, compressive (b) month 8. stresses (F); (a) Figure 13.FigureExample 13. Example of surface of evolutionsurface evolution inmonth sample 1, in (b 5sample) under month compressive5 8.under compressive stresses (F);stresses (a) month (F); (a 1,) (b) month 8. month 1, (b) month 8. • As can be appreciated in Figures 9 and 10, in the cases of samples 1 and 2, the white • As can be appreciated• alkaline in Figureselectrolyte 9 and coating 10, in and the thecases yellow of samples trivalent 1 and zinc 2, electrolytethe white coating, respec- • As can be appreciatedAs can in be Figures appreciated 9 and 10, in Figuresin the cases9 and of 10 samples, in the 1 casesand 2, of the samples white 1 and 2, the alkaline electrolytetively, coating a significant and the yellow growth tr ivalentof whiskers zinc electrolyteon the surface coating, can respec-be noted from the first alkaline electrolytewhite coating alkaline and electrolyte the yellow coating trivalent and zinc the electrolyte yellow trivalent coating, zinc respec- electrolyte coating, tively, a significantrespectively,months. growth It was aof significant whiskers also noted growthon that the in ofsurface the whiskers comp canressed onbe thenoted samples surface from (F), can the the be first noted density from of whiskers the first tively, a significant growth of whiskers on the surface can be noted from the first months. It was alsowas noted greater that than in the in compthe non-compressedressed samples (F),ones the (R). density of whiskers wasmonths. greater It was than also in thenoted non-compressed that in the comp onesressed (R). samples (F), the density of whiskers was greater than in the non-compressed ones (R).

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months. It was also noted that in the compressed samples (F), the density of whiskers was greater than in the non-compressed ones (R). • As shown in Figure 11, for the 3rd coating, the acid zinc electrolyte coating, no visual change was observed before the 9th month, where the appearance of new elements indentified as zinc whiskers on the surface could be appreciated (highlighted in white or light gray in the photo). No hillocks were found during the study, the whiskers appearing directly. • In Figure 12, for the hot-dip galvanized coating (sample 4), it can be appreciated that its appearance is different from the rest; a much rougher surface is observed. However, no whiskers were detected during the entire observation period. • Figure 13, for sample 5, which corresponds to the zinc-nickel coating, did not present any representative growth of whiskers on the surface during the period studied. The surface of this coating is also smoother, as in the other electrolyte coatings. This means that the influence of each of the five coatings on whisker and hillock growth is different, even though the substrate material is exactly the same. While coatings 1 and 2 (white alkaline electrolyte coating and yellow trivalent zinc electrolyte coating, respectively) exhibit a surface with numerous hillocks and whiskers from the beginning, samples 4 and 5 (hot-dip galvanizing and zinc-nickel) do not show any evolution and do not present any whiskers on their surface. Finally, sample 3 (acid zinc electrolyte coating) incubates the ZW phenomena for the first 9 months after it appears.

5.2. Quantification of as Received “R” Samples The following graphs in Figure 14 show the evolution of the hillocks and whiskers in the samples without efforts (R). Regarding sample 1, the white alkaline electrolyte coating, it can be observed how both graphs show a certain dispersion due to the high degree of uncertainty of the hand counting system. However, it can be noticed that the quantity of hillocks remains relatively constant for the all period considered, whereas the whisker evolution seems to increase in a relatively linear way. It can be noticed that the average density of the hillocks is around 6 × 10−4 hillocks/µm2, while a whisker evolution of between 4 × 10−4 and 1.6 × 10−3 whiskers/µm2 was observed. The trend of sample 2, the yellow trivalent zinc electrolyte coating, is clearer than the previous one, with a relatively constant hillock density during the study period and a visible increase in the whisker density. The uncertainty is also present, in the same order of magnitude as in the previous sample. An average hillock density of 7 × 10−4 hillocks/µm2 and a whisker progression of between 1 × 10−4 and 2.75 × 10−4 whiskers/µm2 are observed. Metals 2021, 11, 325 16 of 21

14 Figure 14. Evolution of hillocks and whiskers in as received “R” samples.

If a comparison is made between samples 1 and 2 (white alkaline electrolyte coating and yellow trivalent zinc electrolyte coating, respectively), the hillock density is prac- tically constant and very similar, but the difference in the whisker density is relevant. Tests show that even at the beginning of the study it is possible to detect a one order of magnitude difference in the amount of whiskers, around 4.4 × 10−4 whiskers/µm2 in white alkaline electrolyte (sample 1) and 2.4 × 10−5 whiskers/µm2 in yellow alkaline (sample 2); in the 12th month, a 10-fold difference was observed, reaching densities of 1 × 10−3 whiskers/µm2 and 2 × 10−4 whiskers/µm2 for samples 1 and 2, respectively. As shown in the graphs, sample 3 (acid zinc electrolyte coating) did not show any hillocks or whisker progression over the first 8 months. However, traces of a beginning of growth were

1

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encountered in the 9th month. As the study covered just 12 months, a clear tendency could not be established; however, the existence of sample 3, the acid zinc electrolyte coating, proves the possibility of a delayed appearance of whiskers without a clear presence of hillocks prior to them at around 10 months of incubation (2 months prior to the sample reception plus the first 8 months of study). According to the graph (Figure 14), it seems that the density is as great as in sample 1, around 1.2 × 10−3 whiskers/µm2 and constant during months 9th to 12th. However, the length of the whiskers identified is insignificant in comparison with coatings 1 and 2 (Figures9–11). Given this fact, plus the short duration of the evolution study, the density obtained needs to be treated with caution. As previously mentioned, samples 4 (corresponding to hot-dip galvanized) and 5 (zinc-nickel coating) did not show any significant evolution on the hillocks or whiskers in the as received state (R) during the period of study, so the quantification for these samples gave residual values practically equal to zero. In Section2, coating thickness has been coupled with texture, and it was also stated that thinner coatings favor whisker growth [9] by influencing the coating texture [21]. In the five cases studied, the steel substrate was exactly the same, and the thickness of the white alkaline and acid electrolyte coatings were of 14 and 20 µm, respectively, while the hot-dip galvanized was around 100 µm. In view of this, it seems logical that the white alkaline electrolyte presented a lower density of ZW and hillocks than the acid electrolyte. Regarding the hot-dip galvanized coating, whose rugosity was superior to that of the rest of the coatings, the much thicker zinc layer justifies the low presence of whiskers and hillocks.

5.3. Quantification of Compression Stressed Samples by Bending “F” The following graphs in Figure 15 show the evolution of hillocks and whiskers in the samples with compression stresses applied by bending moment (F). In sample 1, despite the dispersion, it can be noticed that the quantity of hillocks remains relatively constant during the 12 months of study, around 6 × 10−4 hillocks/µm2; however, this value was slightly higher in the 1st month of study (approximately 1.2 × 10−3 hillocks/µm2). At the same time, the whisker evolution also showed a quasi- constant value after the 2nd month of study around 1.4 × 10−3 whiskers/µm2; however, its presence in month 1 was practically inexistent while it rose to 1.8 × 10−3 whiskers/µm2 in month 2, to go down slightly the aforementioned constant value over the rest of the time. This fact indicates that there could have been a 2 month period when the first hillocks ap- peared and from them whiskers grew after a certain incubation time, the process stabilizing after the 2nd month. If samples 1 in “R” and “F” states are compared, it can be noted that, while the hillock density in both cases presents similar values of around 6 × 10−4 hillocks/µm2, the whisker density in the bended samples (R) was higher during the whole study (approximately 1.4 × 10−3 whiskers/µm2) although the samples without efforts (R) tended to its value at the end of it (4 × 10−4 whiskers/µm2 at month 1 and 1.6 × 10−3 whiskers/µm2 at month 12 were observed). This confirms the aforementioned hypothesis from the literature that bending compression stresses, at least in this coating (white alkaline electrolyte), favor the growth of zinc whiskers by accelerating it; on the other hand, no effect of bending stresses have been noticed in relation to the hillocks. The effects observed on samples 2 (yellow trivalent zinc electrolyte coating) follow the same patterns as the ones described above for samples 1. In this case, quasi-constant values of around 7 × 10−4 hillocks/µm2 and 1.5 × 10−3 whiskers/µm2 have been found, which compared to 7 × 10−4 hillocks/µm2 and a whisker progression of between 1 × 10−4 and 2.75 × 10−4 whiskers/µm2, confirms that the aforementioned bending compression stresses favor whisker growth, but not in hillocks. In this case, the incubation period of 2 months observed for coating 1 was not noticed (it may well have taken place in the storage time of the samples prior to the study). Metals 2021, 11, 325 18 of 21

15 Figure 15. Evolution of hillocks and whiskers in compression stressed samples by bending “F”. In sample 3 (acid zinc electrolyte coating), no hillocks or whisker progression were no- ticed during the first 8 months. However, traces of the beginnings of whisker growth were encountered in months 9 to 12, although their density was negligible (5 × 10−5 whiskers/µm2), so no tendency could be established. In this case, the com- pression bending stress effect observed in samples 1 (white alkaline electrolyte) and 2 (yellow trivalent zinc electrolyte) that favors whisker growth was not present. In fact, the whisker growth after 8 months was higher in the effort-less samples, but given the presence of this effect at the end of the study, further conclusions should not be made regarding this matter in coating 3.

2 Metals 2021, 11, 325 19 of 21

Sample 4 (hot-dip galvanized) did not show any significant evolution of the hillocks or whiskers under compression bending forces (F) during the period of study, as occurred in the as received state (R), so the quantification for these samples gave residual values practically equal to zero. However, sample 5 (zinc-nickel coating) showed a constant hillock density of around 1.5 × 10−5 hillocks/µm2 during the whole study from month 3, but no whiskers grew from them at any time, so it can be stated that in this case the compression bending stresses have a slight influence on hillock growth but not on whiskers. Again for the bended samples, for the 14 and 20 µm thick white alkaline and acid electrolyte coatings, respectively, the results are as stated in the literature: the white alkaline electrolyte presented a lower density of ZW and hillocks than the acid electrolyte, given the fact that thinner coatings favor whisker growth [9] by influencing the coating texture [21]. Additionally, the hot-dip galvanized samples, whose thickness was of 100 micros and with a rugosity superior to the rest of the coatings, the much thicker zinc layer justifies the low presence of whiskers and hillocks.

6. Conclusions and Future Work In this report, the following five VALDINOX® patented EASYCONNECT system cable trays made out of the same base steel wires but finished in different coatings, which cover a wide range of the commercial offer available at present, have been characterized mainly with respect to their zinc whisker and hillock evolution over 12 months. • White alkaline electrolyte coating. • Yellow trivalent zinc electrolyte coating. • Acid zinc electrolyte coating. • Hot-dip galvanizing zinc coating. • Zinc-nickel coating. From the bibliographic review, it was concluded that parameters such as grain size, substrate hardness, microstrain, light and magnetic fields and humidity do not influence the zinc whisker growth (ZW) phenomena. The following variables favor ZW growth: ap-plied external compressive stresses, residual stresses, temperature, corrosion, organic contaminants, thin coatings and thin substrates; in contrast, cyanide electrolytes reduce ZW growth rate by producing less residual stress and chrome inhibits the onset of ZW, but does not prevent them. It was also concluded that elongated Zn grains in the coating tend to raise ZW growth rates more than equiaxial ones, and that stress relaxation might be the driving force of ZW growth. Secondly, the main parameters of the five aforementioned coatings were characterized in depth. It should be pointed out that: • The hot-dip galvanized coating, as already known, is very different from the electrolyte ones (the rest); it is approximately five to seven times thicker and its rugosity is also higher. On the other hand, its composition does not differ much from the rest of the zinc coatings, except for the zinc-nickel one, even though its oxygen content is substantially lower than in the rest. • The zinc–nickel coating stands out from the rest for its approximately 11% of zinc substitution by nickel (73% Zn and 11% Ni, instead of around 80% to 85% Zn). The hardness of this coating stands out, it being twice as hard as the hot-dip galvanized one and nearly four times harder than the rest of the coatings, which can make it more brittle. Finally, a complete study of hillock and whisker density evolution with and without presence of compression stresses, which is one of the most influencing parameters, was completed. After a preliminary analysis, even though the substrate material was exactly the same, different conclusions for the different coatings regarding whisker and hillock growth could be made: Metals 2021, 11, 325 20 of 21

• After the first 3 months of study, it was clear how the white alkaline electrolyte and the yellow trivalent zinc electrolyte coatings showed a significant growth of ZW and hillocks in the as-received samples as well as in the compressed ones; the whisker density was higher in the compressed samples, but the hillocks were in a similar range in both cases. • The acid zinc electrolyte coating did not present any ZW or hillock presence during the first 8 months. After the 9th month, however, small whiskers appeared on the surface, though no presence of hillocks was found during the whole study. • The hot-dip galvanized coating presented a much rougher surface than the rest. Nevertheless, no ZW or hillock appearance was observed during the 12 months, either in the as-received or in the compressed samples. • The zinc–nickel coating did not present any whisker or hillock growth on the surfaces during the period studied, either in the as-received or in the compressed samples. In general terms, regarding the four electrolytic coatings which have similar thick- nesses, zinc contents (there is a 12% approx. substitution in Zn–Ni coating) and which have been applied on the same substrate, it can be concluded that the white alkaline and yellow trivalent coatings had a quite similar behavior during the 12 months of study, presenting hillocks and zinc whiskers from the beginning and throughout the whole period of study. These two coatings were the most affected ones, the effects on the yellow trivalent coating being slightly worse when compressive stresses are applied. In comparison with the aforementioned ones, the zinc–nickel coating showed the best behavior, in general terms, with respect to hillock and whisker evolution over the 12 months of study. Nevertheless, although its behavior was qualitatively slightly worse, the acid zinc electrolyte coating (sample 3) also showed good results despite the delayed appearance of whiskers from the month 9. Finally, in the case of the hot-dip galvanized coating, no presence of zinc whiskers or hillocks was detected during the period of study. The study carried out covered just 12 months, a far shorter time than the in- service life of cable trays in data centers. A necessary future study might be to extend the study over a longer period of time, to determine whether incubation times of several years might appear on those coatings that did not show any hillock or whisker presence during this study.

Author Contributions: Conceptualization, B.A. and J.S.; methodology, B.A., J.A.Á. and J.S.; software, M.R. and A.I.C.; validation, B.A., G.B. and J.S.; formal analysis, B.A. and S.C.; investigation, B.A., M.R. and A.I.C.; resources, G.B., J.S. and B.A.; data curation, M.R., A.I.C. and B.A.; writing—original draft preparation, B.A., M.R. and J.A.Á.; writing—review and editing, B.A., J.A.Á. and S.C.; supervision, J.A.Á., J.S. and G.B.; project administration, B.A. and J.S.; funding acquisition, B.A. and J.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by “CONSEJERÍA DE INNOVACIÓN, INDUSTRIA, TURISMO Y COMERCIO of Gobierno de Cantabria”, grant “Cheques de Innovación para el año 2018” and The APC was partially funded by a discount offered by Metals Editorial Office to celebrate the journal’s 10th anniversary and partially by the authors discount vouchers for paper reviews to MDPI. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is property of the company VALDINOX Ltd. and the authors of the article. For data request please contact the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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