ISSN 1067 8212, Russian Journal of Non Ferrous Metals, 2012, Vol. 53, No. 2, pp. 176–203. © Allerton Press, Inc., 2012.
POWDER MATERIALS AND COATINGS
On Conversion Coating Treatments to Replace Chromating for Al Alloys: Recent Developments and Possible Future Directions1 S. A. Kulinicha, b and A. S. Akhtara aDepartment of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1 bGraduate School of Engineering, Osaka University, 2 1 A12 Yamadaoka, Suita, Osaka 565 0871, Japan e mail: [email protected]
Abstract—Anticorrosive protection of Al alloys still depends heavily on the use of chromates, which are widely and universally employed as chromate conversion coating and chromic acid anodising pretreatments. The replacement of chromate based treatments with more environmentally compliant processes and materi als has been identified as a high priority. The aim of this paper is to review the most recent developments in the application of common inorganic protection layers based on conversion coatings for Al alloys. The review lists and discusses the majority of conversion coatings, including those formed through anodisation, on Al alloys as potential replacements to the most widely used treatments based on chromate chemistry.
Keywords: conversion coatings, aluminium alloys, anticorrosive protection, chromate free coatings, surface microstructure. DOI: 10.3103/S1067821212020071
1 1. INTRODUCTION CrCCs [1–6, 10, 12]. Since their publication a num ber of new results have appeared in the literature. Aluminium, which is widely used as a structural Therefore, this paper describes main trends and recent material owing to its low cost, excellent strength to reports on the corrosion protection of Al alloys by weight ratio and corrosion resistance, needs alloying means of conversion oxide layers (usually primer and to develop high strength, and this decreases the natural topcoat are applied on top of these layers), and covers corrosion resistance provided by its tenacious native a wide variety of conversion coatings. Based on the oxide layer [1–3]. The protection of Al alloys currently current knowledge, none of these conversion alterna depends heavily on the use of chromates. Chromic tives for CrCC are listed or suspected as carcinogens. acid anodising and chromate filming are widely Some recent trends in developing anodic conversion employed pretreatments, while SrCrO4 pigmented coatings on Al alloys are also mentioned. primers find almost universal application [4–6]. In the last few decades, chromate conversion coatings (CrCCs) have been the most common anticorrosive 2. ALUMINIUM ALLOYS treatments for Al alloys [1–3, 7–10]. Due to the tox AND CHROMATE CONVERSION COATINGS icity of Cr(VI), however, environmentally benign alternatives to CrCCs have been investigated exten 2.1. Aluminium Alloys and Their Microstructure sively [1–3, 6, 10–13]. Despite intense research activ Al is alloyed with various elements to improve spe ities on the subject in both industry and academia, no cific properties. Table 1 indicates examples of alloys equivalent substitute treatment has been found. The belonging to several “families” (mainly 2XXX, 6XXX main approaches that have been investigated as poten and 7XXX series) which find various applications. The tial replacements for CrCCs include conversion based concentration of each alloying element is a function of technologies (based on oxy anion analogues to chro the properties required from a particular Al alloy. mates [3, 12, 14–18], rare earth based inhibitors There are some discrepancies in the compositions pre [19–25] and anodising [1, 26–30]), sol gel coatings sented from different reports on the same alloys which [31, 32], organosilane based coatings [33, 34], coat can be explained by (i) different detection limits and ings based on conductive polymers [35–37] and accuracies of the analytical techniques used in differ even—to some extent—coatings deposited from ent works, (ii) probably some minor variations in the plasma [6]. chemical compositions of alloys provided by different During the last two decades, there have been sev suppliers. eral comprehensive reviews on replacements for Most of the recent research on inhibition and protec tion has been carried out in the context of AA 2024 T3 1 The article is published in the original. aluminium alloy [8, 16, 38–40, 55–60], which is the
176 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 177
Table 1. Chemical composition (wt %) of several typical aluminium alloys according to different reports (Al being always the balance element) Refer Alloy Cu Mg Si Mn Cr Zn Fe Ti ences AA2024 T3 3.8–4.9 1.2–1.8 0.5 0.3–0.9 0.1 0.25 0.5 0.15 [38] 4.4 1.5 0.5 0.6 – – <0.5 – [39] 4.47 1.42 0.07 0.61 <0.01 0.11 0.16 0.03 [40]a AA7075 T6 1.4 1.1 1.0 – 0.2 5.9 – – [41]a 1.78 2.91 0.03 0.02 0.21 5.6 0.25 0.04 [40]a AA6061 T6 0.2–0.6 0.5–0.9 0.5–0.9 0.2 – – – – [42] 0.35 0.95 0.5 0.15 0.15 0.25 0.7 0.15 [43] AA3003 H14 0.05–0.2 0.05 0.6 1.0–1.5 0.05 0.1 0.7 0.05 [44–46]a 0.12 0.017 0.18 1.05 0.002 0.012 0.52 0.009 [47] AA2014 T6 3.9–5.0 0.2–0.8 0.5–1.2 0.4–1.2 <0.1 – <0.7 – [25] AA6082 T6 0.1 0.6–1.2 0.7–1.3 0.4–1.0 0.25 0.2 0.5 0.1 [48] AA6016 T4 0.07 0.33 1.3 0.07 0.03 0.02 0.2 – [49] AA6060 T5 0.0031 0.499 0.396 0.018 – – 0.175 – [50]a AA7050 T7 2.14 2.0 – – – 6.26 – – [51] AA2618 T6 2.3 1.6 0.18 – – – 1.1 0.07 [52] AA5005 H12 0.05 0.95 0.4 0.1 – 0.1 0.6 0.1 [53] AA5052 H32 0.1 2.2–2.8 0.25 0.1 0.15–0.35 0.1 0.4 – [54] AA1050 4––0.3–––0.4– [46]a Note: a Other elements were also detected at lower levels. main alloy used in aircraft fuselage structures. It is also particles are usually noble with respect to the sur highly prone to corrosion [10, 55–60]. It is therefore rounding alloy matrix (Fig. 1c) and tend to localize not surprising that CrCCs have been developed and corrosion to the matrix phase at the particle periphery studied in detail on this alloy (referred to hereafter as on exposure to any aggressive aqueous environment 2024 Al) [7–9, 57–67]. Below we briefly describe [9,38]. Meanwhile, the Al–Cu–Mg particles (Fig. 1a) some mechanisms of CrCC growth on the microstruc are thought to be initially active with respect to the ture of 2024 Al. In principle, this knowledge can be matrix phase, which results in particle dealloying and generalized onto all Al alloys. In each particular alloy Cu enrichment [9, 10, 55, 66]. For instance, the parti different compositions, concentrations and combina cle in Fig. 1a appears to be partially dealloyed after tions of intermetallic second phase particles are found, contact with water during polishing. Later on, being which implies local differences in corrosion behaviour ennobled, such particles were reported to switch their and in the formation of conversion coatings. galvanic relationship with the matrix region [9, 66]. Thus, the 2024 Al surface is not chemically homoge The second phase particles in 2024 Al vary in size neous, leading to enhanced reactivity and corrosion of (most are in the range 0.1 μm to a few tens of μm in the alloy in the presence of water and electrolytes. diameter) and composition [38, 55, 60–63, 66, 67]. Recently the formation of conversion coatings in the These second phase particles are enriched in the vari vicinity of such intermetallic particles has been studied ous alloying elements (see Table 1) and are formed with special attention [9, 16, 39, 60, 63, 66]. during the original solidification of the cast billet; and there is no significant dissolution during subsequent processing. The predominant chemical types are 2.2. Chromate Based Corrosive Protection Al2CuMg (also referred to as S phase) and Al6(Cu, Fe, Mn), presented in Figs. 1a, 1b [38, 55, 60, 63, 66]. The term “conversion coating,” as used in the According to Buchheit et al. [55], the above men metal finishing industry, refers to the conversion of a tioned Al–Cu–Mg (S phase) and Al–Cu–Fe–Mn metal’s surface into a material that will more easily particles make up 61.3 and 12.3 number % of all parti accept applied primer and/or paint and offers corro cles on the surface of 2024 Al, the others being sion resistance in the event that the covering layer is Al7Cu2Fe (5.2%), (Al,Cu)6Mn (4.3%) and particles of damaged. Conversion coatings are rather thin (nor undetermined composition (16.9%). Most of these mally less than 600 nm on Al), quickly and easily
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 178 KULINICH, AKHTAR
(a) (b)
(c)
Fig. 1. SEM micrographs showing major surface features of polished 2024 Al alloy, (a) Al–Cu–Mg and (b) Al–Cu–Fe–Mn sec ond phase particles; (c) alloy matrix region. Scale bars indicate 2 µm. formed, easily scratched and, if used to enhance paint cipal provider of corrosion protection. Paint (the top adhesion, are coated shortly after being formed to pre layer) is applied for decorative purposes and is also the vent degradation of the conversion coating. A typical main barrier against environmental influences [11, 68]. coating system on Al alloy is shown in Fig. 2. It is nor The most successful conversion coating system mally comprised of three coating layers [11]. The pre used on aluminium and its alloys so far is based on treatment layer (usually conversion or anodised layer) hexavalent chromium, and is generally referred to provides corrosion protection and improved adhesion CrCC [4, 7–10, 60, 63, 66, 67, 69]. These coatings between the substrate and the primer. Composed of a provide good resistance against atmospheric corrosion pigmented organic resin matrix, the primer is the prin and are excellent bases for various primers and paints [1, 6, 10, 13, 20]. CrCCs are distinguished by the ease with which they are applied, their applicability to a wide range of metals and alloys and by their ability to improve the substrate corrosion protection by virtue of Paint a built in inhibitor reservoir (self healing property). Chromic acid anodising and CrCC processes are widely employed as pretreatments and SrCrO4 pig mented primers find almost universal application on Pigmented Primer airframe structures [1, 2, 4, 6, 10, 13]. It is thus the conversion layer and the primer in Fig. 2 that widely use chromates at present. However it has been recog Pretreatment Layer nized that they are both highly toxic and carcinogenic. The replacement of Cr(VI) based treatments with envi ronmentally compliant processes and materials has Al substrate been identified as a high priority [1, 4, 5, 10]. In the presence of aluminium, the Cr(Vl) is reduced to various Cr(III) compounds that form a soft gel like mixture. Initially it has an open porous struc Fig. 2. Schematic structure of a typical coating system on ture that easily accepts applied organic coatings aircraft Al alloy. [10, 17]. After eight hours, these coatings lose their
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 179 open porous structure due to the formation of an inor 0.4 ganic polymer [17, 57]. For this reason it is generally recommended that they be painted shortly after being conversion coated. The above mentioned inorganic 0.3 polymer is primarily responsible for the excellent cor rosion resistance shown by this conversion coating system. Traces of Cr(VI) left in the conversion coating 0.2 give the CrCC shades of yellow to dark brown colour. The coating colour indicates the relative coating mass or thickness of the coating, and Cr(VI) “heals” the 0.1 CrCC by reacting with any metal exposed by scratches or other damage to the coating [56, 58, 59]. The major Cr(VI)/(Cr(III) + Cr(VI)) drawback to the use of these coatings is that they will decompose when heated above 71–80°C because of 0 20 40 60 80 100 120 the loss of water of hydration which holds the polymer Sampie treatment time, s together [13, 17]. At the same time, the amount of sol uble Cr(VI) in CrCCs is limited (up to 10% of the total Fig. 3. XPS detected Cr(VI) incorporated into ferricyanide chromate in the coating) and decreases over time, free CrCCs on 2024 Al as a function of immersion time. restricting the use of such surface treatments to rela (From Refs. [63, 66]). tively mild climatic conditions [3]. Cr(VI) is one of the most effective aqueous corro build a conversion film [10, 57]. Specifically, this pro sion inhibitors for a range of metals including Al, Zn, cess involves reduction of Cr(VI) in solution to form steel, and Mg [4, 10, 59, 70]. The inhibition mecha )3+ nism involves formation of a protective mixed chro hydrated Cr(H2O 6 [14]. This species hydrolyzes in mium/metal oxide film, typically 0.1–1 μm thick, on the locally alkaline region at the alloy solution inter the metal surface [4, 10]. Today, the most commonly face forming hydrated Cr hydroxide, Cr(OH)3(H2O)3, used CrCC process for Al alloys is an acid treatment which polymerizes by forming Cr(III)–O–Cr(III) (pH = 1–2) based on a solution containing a source of linkages, resulting in a polymer “backbone.” Simulta Cr(VI) ions (chromate, dichromate or chromic, acid). neously, Cr(VI) binds via oxygen ligands forming The solution may also contain fluoride ions, which Cr(III)–O–Cr(VI) linkages, which are characteristic assists in the dissolution of the original oxide film, and of CrCCs [57]. The growth mechanisms, in relation to an accelerator, for instance, ferricyanide to facilitate the Al alloy microstructure, have been reported in the formation of the Cr(III) oxide [6, 10, 70]. In the detail elsewhere [9, 60, 61, 63, 66, 67]. The widely CrCC formation process, ferricyanide is used as a accepted model for CrCC growth indicates that the redox mediator [10, 59, 60]. Ferricyanide enhances process depends on the electrochemical characteris the Cr(VI) to Cr(III) reduction reaction kinetics, tics of the substrate [9, 60, 66, 67]. This is especially which are normally sluggish on Al, thus stimulating noticeable during the initial stage, when alloy local coating growth. Electrochemical testing of CrCCs microstructure dictates CrCC nucleation [9, 60, 66]. formed in the presence and absence of fluoride and Figure 4 shows local Auger point spectra, measured ferricyanide additions shows that these supplemental after 5 s (top spectra) and 30 s (bottom spectra) ingredients play an essential role in the formation of immersion into a CrCC bath at Al–Cu–Fe–Mn CrCCs with high levels of corrosion resistance [71]. intermetallic particles (a) and the alloy matrix (b) on According to the most recent studies, CrCCs are a 2024 Al. It is clearly seen that CrCC first deposits on mixture of hydrated amorphous Cr(III)–Cr(VI) the cathodic second phase particles (compare spectra oxides, hydrated Al oxide, ferri/ferrocyanides, small after 5 s over the matrix and particle in Fig. 4) [63, 66]. to moderate amounts of alloying elements from the It is believed that following this initial coating, the substrate and minor ingredients present in commercial cathodicity of the second phase particles decreases formulations. [10, 20, 57, 60, 67]. Coating thickness and the film growth continues on the alloy matrix (see ranges from a few tens of nm to 2–3 μm, being a func spectra after 30 s in Fig. 4) [60, 63, 66]. More complex tion of time and bath chemistry. The total amount of behaviour was reported for Al–Cu–Mg particles, Cr(VI) has been estimated to range from a few % to which are expected to be initially active with respect to 50% of the total coating Cr content, normally reach the Al matrix and dealloy [9, 38, 55, 66]. While some ing its maximum in the topmost layer (see e.g. Fig. 3) Al ⎯ Cu–Mg particles became coated within a few sec [8, 20, 63, 66]. The Cr(VI) ions are readily leached onds of immersion, acting similar to cathodic from the coating and act as corrosion inhibitors Al ⎯ Cu–Fe–Mn particles, other Al–Cu–Mg particles [20, 56, 59]. rapidly dealloyed during the initial immersion in the CrCC formation involves electrochemical reduc acidic bath (pH ~ 1.8) [66]. Subsequently, CrCC tion of Cr(Vl) followed by inorganic polymerization to deposited rapidly on the porous dealloyed particles,
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(b) (a) Al KLL
O KLL Cu LMM 5 s O KLL 5 s
Al KLL
30 s Intensity, a. u. Cr LMM 30 s Cr LMM
300 600 900 1200 1500 300 600 900 1200 1500 Kinetic energy, eV Kinetic energy, eV
Fig. 4. Point Auger spectra measured for 2024 Al after 5 and 30 s CrCC treatment from Al–Cu–Fe–Mn particles (a) and Al matrix (b). (From Refs. [63, 66]). because they were enriched in Cu and therefore acted particles, but they still contribute to coating inhomo as cathodic sites [66]. geneity on the matrix. The CrCC growth on the alloy matrix appears to follow the mechanism proposed by Brown et al. where 3. POTENTIAL REPLACEMENTS local galvanic couples are formed across the matrix by BASED ON CONVERSION COATINGS the various structural defects present [9, 72, 73]. The potential differences between these local regions are The main ingredients for various conversion coat smaller than those between matrix and second phase ing solutions are indicated in Table 2. The following
Table 2. Typical ingredients of most common conversion baths
Coating Primary ingredients Typical pH Activators Common additives References
CrCC CrO3 and/or chromates ~1.8–2.0 NaF or fluoro salts K3Fe(CN)6 accelerant [10] tCrCC Cr(III) salt ~4.0–5.0 NaF, K2ZrF6, H2O2 or KMnO4 as post oxidants [31, 74–76] or Na2SiF6
MoCC Na2MoO4 ~4.0– 6.0 NaF Na acetate – acetic acid buffer [77]
MnCC KMnO4 ~7.0–14.0 – Borax (buffer), LiCl, NaCl, [17, 78–80] LiNO3, NaNO3, Na2SiO3 (modi fiers)
VCC NaVO3 ~1.7 NaF K3Fe(CN)6 accelerant [14]
2+ 2+ 2+ – PCC Zn , H3PO4 ~3–4 NaF Ni , Mn , NO3 [18, 81]
CeCC CeCl3 or Ce(NO3)3 ~2.0 –H2O2 accelerant [23, 39, 52, 82]
ZrCC/ K2ZrF6 and/or ~2.0–4.0 HF, KF, NaF Polymer modifier [49, 83–85]
TiCC K2TiF6 or their acids or K2ZrF6
CoCC C(II) salt ~6.8–7.0 – NH4NO3 and formic acid as com [31, 86, 87] plexing agents, H2O2 as oxidizer
3+ – LiCC Li2CO3 and/or LiOH ~11.0–13.5 – Al or AlO2 [88–90]
Anodic H2SO4 Low pH Electric current Boric acid [91–93] oxide
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 181 sections contain more detailed descriptions of the coatings. Cr2p
3.1. Cr(III) Oxide Based Conversion Coatings Ideally, an alternative conversion coating to replace CrCCs should exhibit high resistance to dis 3 solution in an alkaline environment [3]. According to the Pourbaix diagram [94], the approximate stability limit of Al oxide is pH ~ 9. Other metal oxides, which 2 are involved in various conversion coatings, are stable Intensity, a. u. under more alkaline conditions: Cr2O3 (pH ~ 15), 1 CeO2 (pH ~ 9.5), ZrO2 (pH ~ 12.5), TiO2 (pH ~ 12), Co2O3 (pH ~ 13), SnO2 (pH ~ 12.5). The values are clearly in favour of any coating based on Cr(III) oxide, including CrCCs. Trivalent Cr conversion coatings 597.5 592.5 587.5 582.5 577.5 572.5 (tCrCCs) are worth studying since they are non toxic [74, 95] and also commercially acceptable alternatives Binding energy, eV for CrCCs for certain applications [96]. The newest Fig. 5. Effect of deposition pH and deoxidizing stage on Cr generation of tCrCCs was also reported to be resistant 2p peaks of tCrCC. (1): pH ~ 3.5; (2): pH ~ 4.5; (3): to thermal treatment [96], which is not the case for pH ~ 4.5 plus deoxidizing pretreatment in diluted (1 : 3) CrCCs [17]. Surprisingly, however, the number of HNO3 for 1 min. All samples were polished 2024 Al reports on tCrCCs applied to Al alloys is relatively treated in a Cr2(SO4)3 – Na2SiF6 bath for 20 min. small [31, 54, 74–76, 95, 98, 99]. tCrCCs on Al alloys were first developed by Pearl substrates. Thus, the important role of Cr(VI) as a stein and Agarwala [74, 95]. Attention was directed component in CrCCs (which are also based on towards tCrCCs as they do not require the use of toxic hydrated Cr2O3) is again confirmed [96]. However, if a Cr(VI) baths [74, 95], but with further post oxidation topcoat of organically modified silicates (ormosils) is they were considered to convert a part of the coating added, significant improvement is achieved [31, 76]. into Cr(VI). After film deposition, an oxidation treat Kachurina et al. [31, 76] have evaluated and compared ment converts part of the film to Cr(VI). Usually resistance behaviour of both single layers of tCrCC tCrCCs are prepared from an aqueous solution of and CrCC and multilayer coatings based on tCrCC water soluble Cr(III) compound and a fluoride com and CrCC both covered by a topcoat of ormosil. Salt pound (e.g., NaF, K2ZrF6, Na2SiF6). The combina spray and potentiodynamic polarization curve analyses tion of these reagents leads to the precipitation of have been used. The presence of the ormosil was found hydrous Cr2O3 [31, 74–76, 95, 97]. Normally the pH to enhance the corrosion resistance of the underlying is near or beyond the value for the precipitation of conversion coatings on 2024 Al [31, 76]. Moreover, basic compounds (~4.5) [54, 74–76], and samples are while the magnitude of the corrosion resistance for sin immersed for 10–15 min at about room temperature gle layer coatings was tCrCC < CrCC, multilayered [31, 74–76]. Figure 5 clearly shows how an increase in coatings composed of either tCrCC/ormosil or pH from 3.5 to 4.5 enhances tCrCC deposition in a CrCC/ormosil demonstrated very close performance Cr (SO ) –Na SiF bath (see spectra and ), while a 2 4 3 2 6 1 2 (tCrCC ≈ CrCC), implying that the environmentally deoxidizing pretreatment to thin the native oxide layer benign and non toxic tCrCC, if used in combination also plays an important role (see spectrum ). Interest 3 with a proper ormosil overcoat, may be a potential alter ingly, Yu et al. [98, 99] have recently proposed a tCrCC native for Cr(VI) based conversion coatings [31, 76]. process on 6063 Al, in which the substrate is treated at ° pH ~ 2.0 and 40 C in a bath based on KCr(SO4)2 [98, Another development was proposed by Pearlstein 99]. The pH used by this group differs from that used and Agarwala [74,95] and also studied by Delaunois in other studies of tCrCC deposition on Al alloys et al. [75, 97]. In order to improve the tCrCC perfor [54, 74, 75, 95, 97], however, an acidic bath is consis mance, it would seem beneficial to treat the as received tent with that used in [96] for deposition on zinc coatings with an oxidizing agent aimed at converting (pH ~ 1.8). According to Yu and coworkers [99], some part of Cr(III) to Cr(VI) [74, 75, 95, 97]. H O the addition of urea or (especially) thiourea into the 2 2 and KMnO4 were chosen for 0.5 to 5 min post oxida tCrCC bath could improve the performance of the tion procedures conducted at room temperature tCrCC on 6063 Al. Note that in all cases, the film [74, 75, 95]. While the tCrCC treated 7075 Al and growth mechanisms have not been investigated and/or 2024 Al provided corrosion resistance in excess of described in detail. 96 h (5% NaCl salt spray test), post oxidation pro Pure tCrCCs have been reported to perform poorly, vided corrosion resistance for about 336 h for both compared to CrCCs, on both Al [31, 76] and Zn [96] alloys [74]. Electrochemical measurements on sam
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 182 KULINICH, AKHTAR ples of 1050 Al, 5754 Al and 6082 Al coated by inhibitors, like metaborate, are present with the tCrCC and post treated in permanganate solution molybdate, significant corrosion inhibition of Al showed that the pitting potential and corrosion poten alloys can be observed [103]. tial of the post oxidized samples were close to those of There have been a number of studies on the suit CrCC [75]. The improvement was shown to be caused ability of MoCCs as protective layers on different met by Cr(VI) species formed in tCrCC during the post als [100, 104–106]. The MoCCs obtained were oxidation [74]. After a sample post treated in H2O2 reported to vary in colour, composition, thickness and was immersed in boiling water, the “self healing” Mo oxidation state [4]. Surprisingly, the number of capacity of Cr(VI) was observed [74]. Thus, coatings comprehensive reports on MoCCs on Al alloys is quite based on Cr(III) oxide with “self healing” abilities small [4, 77, 101, 105, 107, 108], and the research is provided by Cr(VI) inclusions can be produced with mainly focused on steel and Zn [4, 106, 109]. Gabe out using toxic chromate baths, which appears to be an and Gould [105] have prepared MoCCs on Al using attractive alternative to the CrCC process. electrodeposition, but no corrosion protection tests have been reported. Hinton [4] has prepared a gold 3.2. Molybdate and Tungstate Conversion Coatings coloured MoCC with blue interference on 7075 Al by (group VIB) immersion in de aerated 0.01 M Na2MoO4 at pH 8. A clear trend of increasing pitting potential in NaCl Cr, Mo and W belong to group VIB of the periodic solution with increasing immersion time in the molyb table and have many similar properties and character date bath was reported [4]. It was concluded that istics [100]. Molybdate and/or tungstate conversion MoCCs provide corrosion protection for Al alloys [4]. coatings (MoCCs and/or WCCs) were thus consid ered to be possible replacements to CrCCs, as similar The only study reporting that MoCCs provide pro chemistries are expected from Cr, Mo, and W oxyan tection comparable to that of chromium based com ions. Electro reduction of Mo(VI) or W(VI) species is mercial coating was published by Rangel et al. [77]. expected, yielding compounds less soluble in acid than Uniform MoCCs were deposited on pure Al without alumina or blocking the cathodic reactions [77]. any external polarization, using a buffer solution of Indeed, the early focus for alternatives was centered on Na2MoO4, acetic acid and sodium acetate [77]. The the metal oxyanion analogues of chromates, namely coatings were fully grown within 60 s of immersion, molybdates and tungstates, as well as permanganates and the solution pH range was from 4 to 6. Lower pH and vanadates [3, 4]. However, molybdate and tung values gave rise to hydrous blue Mo(V) compounds with state differ considerably from chromate in their poor protection ability whereas higher pH values did detailed reaction chemistry. Their ability to form iso not allow the coating to form, most likely due to insuf poly and heteropoly compounds appears to be unique, ficient thinning of the native Al oxide. Optimisation of as well as their ability to form the so called “blue” the coating produced a pitting resistance comparable to oxides which are mixed (V) and (VI) valence state that of CrCCs, namely pitting potential values obtained compounds [100]. with the produced MoCCs in chloride solutions were as high as those with conventional CrCCs [77]. Molybdate While Mn and Cr exhibit very broad ranges in pH incorporation into the MoCCs was evident from RBS where their reduced oxides are stable (according to the and XPS spectra, the main constituents of the MoCCs Pourbaix diagrams [94]), Mo, W and V (vanadate being MoO , some form of alumina and Mo(VI) [77]. based coatings are discussed below), exhibit relatively 2 narrow ranges in pH for stable reduced oxides. In this However, it was noted in many studies that when context the oxides of Mo, W and V will never provide molybdates are mixed with other compounds, better stability similar to that of Cr(III) oxide. However, the corrosion protection is achieved as a result of synergis oxo compounds of these elements can form very sta tic effects in conversion coatings [26, 110–113]. As an ble polyoxo metallate species with each other or phos example, a synergy with Ce treatment has been used to phates [10]. Therefore, these oxyanions are not ruled develop the so called “stainless aluminium” process out completely as Cr(VI) replacements. producing extremely resistant conversion coatings Molybdate based treatment has been the most based on Ce and Mo oxides on Al alloys [110–113]. widely investigated of this group, most likely because This treatment is described in more detail in the Ce of its non toxicity [4, 68]. The report by Wilcox and oxide section below (Section 3.6). Gabe has reviewed the corrosion inhibition by molyb Davenport et al. [15] have tested the performance dates [100]. It has been shown that molybdates are of coatings formed from a range of oxyanions: vana good inhibitors of corrosion for steel and Zn, which is date, tungstate, phosphotungstate, molybdate, per related to the formation of a Mo oxide film, with the manganate, phosphomolybdate, tungstosilicic acid Mo in a range of valency states [4, 100, 101].Com and molybdosilicic acid. All of the coatings were pre pared to CrCCs, molybdates and tungstates were pared using the same conditions (0.1 M oxyanion, reported to be less effective corrosion inhibitors for Al, 4 g/L HF, 10 min immersion at 60°C, pH = 2, 4, 6) on even in very dilute corrosive solutions [4, 51, 102]. 1100 Al and were tested using visual inspection and However, Wiggle et al. [103] have shown that if other X ray absorption near edge structure (XANES) [15].
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 183
Tungstophosphate, both alone or in combination with CrCCs [17, 78–80, 116]. In order to compensate for the vanadate, was shown to be the most promising substi deficiency, for 2XXX and 7XXX series alloys, the tute for chromate, even though detailed analyses of the MnCCs in these cases must be used with a unique coatings were not reported [15]. It is thought that opti organic seal, applied in an additional stage, to block the mization of treatment parameters, which are expected pores created during film formation [17, 78–80, 116]. to be different for different chemistries, may improve Such sealed coatings improve the anticorrosive perfor the performance of the above mentioned treatments. mance of the permanganate coatings [117] and dem Molybdates are also used as anticorrosive pigments onstrate corrosion resistances that very closely match in many paint formulations [4, 100]. The capability of the Cr(VI) based coatings [4, 17, 79, 80, 116]. molybdates to act synergistically with other inhibitors The MnCC system is based on reduction of has been recognized in the context of paint as well as Mn(VII) in solution, to a lower oxidation state, fol for conversion coatings [100]. lowed by precipitation as an oxide which coats and passivates the substrate. In this way it is similar to the CrCC process [17, 44], where chromates are reduced 3.3. Permanganate Based Coatings to Cr(III) oxide/hydroxide, along with metallic oxida In many respects, Mn is chemically similar to Cr. tion to Al oxide. Both the MnCC and CrCC processes Therefore, conversion coatings based on Mn chemis do not continue beyond the formation of a thin film tries are a logical approach to the problem of finding a [16, 17]. After drying the MnCC coated metal may be Cr(VI) free coating to replace CrCCs [17]. Perman painted, and there are reports of good paint adhesion ganate oxyanions were reported to be ineffective and filiform corrosion protection [17, 78–80, 116]. inhibitors on Al alloys at low and neutral pH [4, 114], There are also some differences between the CrCC whereas at high pH they acquire the properties of an and MnCC systems. CrCCs are inorganic polymers inhibitor [114]. This was attributed to the difference in and lose their properties over 71°C [17, 57], while solubility between Mn(IV) oxide (the main product of MnCC films are not polymeric, and therefore are less Mn(VII) reduction at high pH) and Mn(II) oxide [3]. affected by heat. Permanganates have been used to treat potable water Most studies indicate that the main form of Mn in systems, and they are on the list of materials allowed in – drinking water [17], implying that permanganate con MnCCs is hydrated MnO2 [3, 16, 17, 44]. The MnO4 version coatings (MnCCs) are promising as potential ion can be reduced to form MnO2, which is insoluble substitutes for CrCCs. under alkaline conditions. Polarization measurements Bibber [17, 78–80, 115, 116] has developed have shown that the protective properties of MnCCs are similar to those of CrCCs [118]. However, if MnCCs, based on the use of KMnO4, which are very effective in protecting Al alloys [17, 78–80, 115, 116]. Mn(VII) is reduced to Mn(II), an Mn(II) rich coat The initial coating process [115] involved successive ing may result in a decreased capacity to protect from immersions in NaBr, distilled H O, Al(NO ) /LiNO corrosion [3]. Interestingly, for Mg alloys MnCCs 2 3 3 3 have also been reported to provide similar corrosion and KMnO4 solutions [115]. The final stage involved sealing the porous oxide coating by immersion in a resistance to CrCCs [119–123]. Another successful solution of potassium silicate [115]. Such coatings development is based on a permanganate phosphate provided a level of corrosion protection that was sig treatment applied to Mg alloys [124, 125]. nificant but not as high as that provided by CrCCs Although MnCCs have been reported to be excellent [4, 115]. A practical disadvantage of the initially alternatives to CrCCs on Al alloys [17, 44–46, 116], and developed MnCC process was its large number of MnCC based formulations have been developed and treatment steps, many of which required elevated tem commercialized for dip bath pretreatment on Al in peratures. Some of these issues have been addressed in automotive and aerospace applications [64, 116, 126], a further development of the process [78–80]. The the information on such coatings is scarce and often number of immersion stages has been reduced by limited to patent claims [64, 78–80, 115]. While combining several of the reactive species (LiCl or Danilidis et al. [44–46] described MnCCs grown by the “no rinse” technique on 3003 Al and 1050 Al alloys, NaCl, LiNO3 or NaNO3, borax Na2B4O7 ⋅ 5H2O, sili and Bibber has reported excellent anticorrosive perfor cate Na2SiO3 ⋅ 5H2O and KMnO4) into one process solution [78–80]. The treatment temperature is mance of MnCCs on various Al alloys [17, 78–80, 116], between 38 and 82°C [78]; the treatment time depends the formation and growth mechanisms of such coatings on the temperature, and is about 1 min at 68°C and on Al alloys have not been studied extensively. 60 min at 25°C [78]. The pH of the bath without sili The growth of MnCCs on 2024 Al samples cate is between 9 and 10 while the pH of the silicate immersed in a borax KMnO4 bath (pH ~ 9) has been containing bath is generally between 12 and 14 [78]. studied recently [16]. For films formed at room tem The paint adhesion characteristics of such MnCCs are perature, growth ceases after ~2.5 h, while at elevated as good as, or better than CrCCs [17, 116]. With the temperatures similar films are grown in a few min [16]. exception of high Cu or Zn alloys (e.g. 2024, 7075 air The process has been shown to be electrochemical craft alloys), the corrosion characteristics are close to with MnO2 deposition first over the intermetallic sec
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(a) (b)
(c) (d)
Fig. 6. SEM images of surfaces of 2024 Al permanganate treated at 25°C (pH ~ 9). (a) Survey view after 20 min when particles are covered and PCC has just appeared on matrix; (b) Al matrix after 210 min; (c) Al–Cu–Fe–Mn particle after 210 min; (d) Al–Cu–Mg particle after 210 min. Scale bars are 5 µm. (From Ref. [16]). ond phase particles (Fig. 6a), similar to CrCC growth doubt about the “self healing” properties of pure [9, 60, 63, 66]. The MnCC formed over second phase MnCCs, given the absence of Mn species in a high, particles is considerably thicker compared to the coat oxidation state [16, 46]. ing over the alloy matrix throughout the range of time Buchheit et al. [127] attempted to introduce periods investigated (Fig. 6) [16]. Cracks are observed Mn(VII) into hydrotalcite like coatings described in in thicker MnCCs over the larger intermetallic parti more detail in Section 3.9. Hydrotalcite like coatings cles (Fig. 6c), but never on the Al matrix, where the were formed on 2024 Al by immersion in a LiNO3– coatings are relatively uniform and significantly thin KNO –LiOH bath, and then modified in a similar ner (Fig. 6b) [16]. 3 bath containing KMnO4, followed by a sealing treat Aluminium species and Mn(VII) have not been ment in a Mg acetate solution [127]. A “simulated detected by XPS in such MnCCs [16]. This differs scratch cell” test was performed in order to observe from an earlier study of “no rinse” MnCCs by Danil whether the incorporated Mn(VII) imparts “self heal idis et al. [44]. The apparent discrepancy may be ing” properties to the coating. Although the Mn mod explained by the differing methodologies used for ified hydrotalcite like coating displayed similar corro coating formation. For the “no rinse” processing, a sion properties to CrCCs, there is only indirect evi dence to support the possibility of Mn(VII) migration thin layer of KMnO4 solution was applied to the Al alloy substrate, which was then dried without any rins from the coating and subsequent inhibition of ing to reduce the amount of effluent. Thus, all chemi uncoated 2024 Al [127], which leaves the question cals and reaction products remain on the surface in unanswered. some form, including significant amounts of alumi Again, in agreement with the latest trend of using nates which otherwise could dissolve in water. Indeed, more complex baths for conversion treatment, the most the most recent report by Danilidis et al. confirms that recent work by Danilidis et al. [45] has investigated the after freshly prepared “no rinse” MnCC samples are electrochemical behaviour of “no rinse” MnCCs with subjected to rinsing with water, no Mn(VII) is detected various inorganic additions. While F– ions did not on their surface [46]. These latest reports thus raise improve coating properties, addition of Al3+ to 0.1 M
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KMnO4 as sulphate or nitrate resulted in increased coating morphology, VCCs are reported to be quite MnCC performance [45]. Thus, further explorations similar to CrCCs [14]. VCCs form in and over pits that of new baths with complex compositions are expected were formed during deoxidation and etching treat to improve MnCC treatments. ments, as well as over intermetallic particles and inclu sions present in 2024 Al. VCCs also contain a network of cracks that apparently due to shrinkage; similar 3.4. Vanadate Conversion Coatings cracks are known to develop in CrCCs [10, 60]. It is The early focus when looking for CrCC alternatives likely that such cracks in VCCs develop due to coating centered on the metal oxyanion analogues of chro dehydration, similar to the situation for CrCCs [14]. mates, i.e. molybdates, tungstates, vanadates and per In salt spray testing conducted according to the ASTM manganates [3, 5, 15, 17]. Molybdates have been most B117 standard, VCCs suppress formation of large cor investigated of the group, most likely because of their rosion pits for more than 168 h. However, the VCCs non toxic nature and wide use as paint pigments [4]. reported by Guan and Buccheit are not yet as protec Vanadate treatment was also among those proposed as tive as CrCCs [14]. Simulated scratch cell experiments an alternative to toxic Cr(VI) for the corrosion protec were carried out to determine whether VCCs exhibit tion of Al alloys. As mentioned in Section 3.2, there is a “self healing” characteristics [14]. The results showed relatively narrow pH range in which vanadium oxides release of V(V) from the VCC which then increases the are stable [94], but its ability to form poly vanadates is corrosion resistance of bare 2024 Al. Thus, VCCs expected to be important in the formation of stable appear to have “self healing” properties [14]. conversion coatings [10]. Hamdy and Beccaria have investigated a VCC treatment as a replacement for CrCCs on 6061 Al– Cook and Taylor [128] investigated the inhibitor 10% AI2O3 composite [131]. The corrosion resistance characteristics of 27 compounds considered as possi was measured using AC impedance spectroscopy and ble replacements for chromate pigments in paint. DC polarization. The process involves etching, oxide 2024 Al samples were exposed for 10 days to 0.6 M thickening by immersion in boiling deionized water, NaCl solution containing 3.4 mM of each candidate and a sealing stage in vanadate [131]. The oxide thick inhibitor, and NaVO3 consistently displayed the best ening step substantially enhanced the corrosion resis performance [128]. Electrochemical impedance spec tance of the conversion coating produced. Moreover, troscopy indicated that the corrosion performance of V was reported to incorporate into the thickened oxide NaVO3 is similar (within an order of magnitude) to layer. The formed Al V oxide appeared to resist corro that of Na2CrO4 [128]. More recently, Iannuzzi et al. sion after 60 days of immersion in NaCl [131]. studied the inhibition of Al alloy corrosion by vana dates [129, 130]. Vanadium speciation was shown to be A vanadate sealing procedure was reported to have very complex and vital to the inhibition efficacy a positive effect on Co based conversion coatings [129, 130]. Metavanadates reduce the kinetics of oxy applied to 2024 Al and 7075 Al [40]. The sealing was gen reduction to an extent similar to chromates. How reported to result in penetration of V through the con ever, decavanadates were shown to be poor inhibitors version oxide and deposition of a thin sealing coating of the oxygen reduction reaction [129, 130]. within the pores and on the external surface of the Co containing coating [40]. Thus, vanadate treat As mentioned in Section 3.2, Davenport et al. [15] ments seem to fulfill their promise as sealers applied have tested the performance of coatings formed from a in combination with other chemistries. This agrees range of oxyanions. Vanadate conversion coatings with the above mentioned report of Davenport el al. (VCCs), especially in combination with tungstophos [15] in that vanadate treatments can give positive phate layers, were among the most corrosion resistant synergistic effects in combination with other chemis of all the oxyanions tested [128]. However, only visual tries. VCCs have also demonstrated potential on Mg inspection was used to compare coating performance, substrates [132]. According to the potentiodynamic and there was no systematic optimization of process polarization test, VCCs display superior corrosion parameters for the different oxyanion conversion coat resistance compared to cerium and phosphate based ings [15]. conversion coatings for AZ61 alloy [132]. Organic Guan and Buchheit [14] have taken the most sys coatings with vanadate pigments have shown promise tematic and comprehensive approach for developing a on Zn alloys [133]. VCC process. VCC coatings on 2024 Al were formed by immersion in a bath containing a mixture of 3.5. Phosphate Based Conversion Coatings NaVO3, K3Fe(CN)6 and NaF (similar to the coating process for CrCCs). A range of bath chemistries were The phosphating process has been developed over a examined: bath pH varied from 1 to 9, immersion times period of more than a century [81, 134] and efforts to ranged from 1 to 10 min. In terms of corrosion resis optimize the coating process are ongoing [135]. Phos tance, coatings formed from baths with 10–100 mM phate conversion coatings (PCCs) are widely used for NaVO3, 3 mM K3Fe(CN)6, and 0–2 mM NaF with a substrates such as steel, galvanized steel, and alloys of pH of 1.7 yielded the best results [14]. In terms of aluminium and magnesium [134]. Two major applica
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ganic/organic additives and accelerators used in phos phating baths have been summarized [81]. Combina tions of conversion coatings such as vanadate phos phate [15] and cerium oxide phosphate [144, 145] are also under investigation. PCCs on steel have been studied extensively [81] but there are fewer reports on PCCs for Al alloys. A study of zinc phosphate (ZPO) coatings on pure Al indicated that zinc substitutes for aluminium, deposit ing on the surface during the initial period of coating growth, followed by precipitation of ZPO [146]. In a neutral aqueous solution the ZPO coated Al has a high corrosion resistance, but under acidic conditions corrosion behaviour is observed [146]. The titanium 2 µm phosphate surface activation treatment before phos phating has been studied [147]. The Al surface pre treatment (degreasing, etching, or electropolishing) Fig. 7. SEM micrograph showing Al–Cu–Mg second before activation determines the extent of titanium phase particle surrounded by the matrix region on a phosphate adsorption on the Al surface. Precipitating 2024 Al surface after dipping for 60 s in a commercial zinc zinc phosphate crystals nucleate on these titanium phosphate coating solution at 25°C. There are ZPO coat phosphate particles, and grow laterally, resulting in flat ing crystals at the edge of the particle but not over most of its central area. (From Ref. [154]). crystals on the Al substrate. Different surface pretreat ments and variations in the phosphate coating solution composition have significant effects on the coatings tions include use as a lubricating film for cold extru formed [148, 149]. During zinc phosphating there is sion and as a pretreatment before paint is applied [3, co precipitation of cryolite (contains Al and F) and 134]. In this review the focus will be on the latter appli zinc phosphate. A larger amount of mass transfer in cation; for this situation, the corrosion resistance and solution during the coating process increases the for paint adhesion characteristics of the coating are mation of cryolite and decreases ZPO precipitation important and the coatings used contain primarily [150]. The effect of a silane rinse applied to phos zinc phosphate (ZPO). ZPO coatings have been stud phated Al alloy was compared with the conventional ied extensively compared to other more recently chromic acid post treatment and the silane rinse developed conversion methods (Sections 3.2–3.9). appears to provide a similar corrosion resistance to the The reaction mechanisms are similar to other conver Cr(VI) treatment [151]. Other alternatives to the sion coatings; etching of the substrate metal by the Cr(VI) post treatment include rinsing the phosphated acidic coating solution causes an increase in local metal in a mixture of Zr, V, F, and phosphate ions [152] solution pH, resulting in deposition of insoluble metal or a solution containing Li, Cu, and Ag ions [153]. phosphates [18, 134, 136]. The nature of the metal in Alloy microstructure influences the formation of ZPO the precipitated phosphate compounds depends on coatings. During the early stages of the coating process the metal ions in the coating solution (e.g. Zn2+, Fe2+, (2024 Al alloy) ZPO crystals form on the Al–Cu–Mg Mn2+) [18]. second phase particles, rather than on the matrix or on the Al–Cu–Fe–Mn particles, with the initial nucle PCCs have low operational costs [137], mild envi ation occurring at interfaces between Al–Cu–Mg par ronmental toxicity [6, 137], and provide a good base ticles and the matrix (see Fig. 7) [154]. for organic coatings [138]. From a corrosion resis Segregation of Cu from the bulk to the alloy oxide tance perspective, CrCCs are superior to PCCs interface occurs as a result of acid etching for 2024 Al [139, 140]. The main use of phosphating processes on alloy and an appropriate amount of Cu at the interface Al is in situations where mixtures of metals, such as Al, decreases the crystal size and increases the corrosion steel, and galvanized steel are being treated in the same resistance of the ZPO coating formed after the acid process, for example in a car body assembly [3, 136, etch pretreatment [155]. Adding Cu2+ to the ZPO 140, 141]. The presence of both Zn and Fe in the coat coating bath at concentrations up to 10 ppm causes ing bath makes the process easier to control; however, significant changes in coating morphology, adhesion, the amount of Al should not exceed a specific percent and corrosion protection [156]. The addition of age of the total surface area being treated [3]. In order yttrium oxide accelerates the phosphating process and to be compatible with cathodic electrophoretic depo also decreases the ZPO crystal size, resulting in a more sition technology the phosphate coating must be stable compact coating [157]. Substantial changes are under alkaline conditions [81]. Several studies have observed in coating morphology and stability against indicated that the phosphate coating dissolves in high corrosion for the 2024 Al alloy as the amount of Ni2+ pH environments [81, 142, 143]. The many inor in the coating solution increases through the 0–
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µ µ (a) 1 m (b) 1 m
µ (c) 1 m
Fig. 8. SEM micrographs showing Al–Cu–Mg second phase particles surrounded by the matrix region on a 2024 Al surface after dipping for 120 s in zinc phosphate coating solutions which contained varying amounts of Ni2+ and Mn2+: (a) no Ni2+ or Mn2+, (b) 2000 ppm Ni2+ and (c) 2000 ppm Mn2+. (From Ref. [161]).
2000 ppm range [158]. The addition of Ni2+ improves the different microstructural regions on the 2024 Al the corrosion resistance of the resulting coatings in surface (see Fig. 8) [161]. It would be useful to develop spite of an accompanying decrease in coating thick similar mechanistic knowledge for the chemical com ness. Ni2+ has two main roles in the ZPO coating ponents involved in other conversion coating pro mechanism. First, the rate of increase in local solution cesses. Such information would enable a more effi pH is limited by the slower kinetics of reactions involv cient design process (as opposed to trial and error) for ing Ni2+ compared to Zn2+, leading to thinner ZPO a multilayer coating to replace CrCCs. coatings when Ni2+ is present in the coating solution. Second, most Ni2+ deposition occurs during the later Surface polishing of 2024 Al prior to phosphating stages of the coating process, by nickel phosphate dep results in an Al–Cu–Mg particle surface that contains osition and formation of a Ni rich corrosion resistant metallic Cu as well as an overlayer of aluminium and oxide [159, 160]. Aluminium fluoride precipitates magnesium oxide, while larger amounts of aluminium during the early stages of the coating process [159]. oxide are present on the Al–Cu–Fe–Mn particles and For the ZPO coating process with Ni2+ there is signifi matrix. When dipped in the acidic ZPO coating solu cant precipitation of zinc oxide at the Al–Cu–Fe–Mn tion, the oxide covering the Al–Cu–Mg particle is intermetallic particle while phosphate deposition pre etched most easily, and metallic Cu near the surface dominates at other regions of the surface [160]. Both makes that region most cathodic, allowing more coat Ni2+ and Mn2+ additives decrease the coating crystal ing deposition compared with the other regions. The size, with Mn2+ causing a greater reduction, and the oxides on the Al–Cu–Fe–Mn and matrix regions are coating formed from the Mn2+ containing solution is similar, thereby confirming that the observed differ thicker than that formed with Ni2+ [161]. Patterns of ences in ZPO coating characteristics at these two deposition across the surface are affected significantly regions arise from their underlying electrochemical by the ZPO coating solution composition. Ni2+ in the properties. Immersion of a ZPO coated 2024 Al sam coating bath causes greater ZPO precipitation at the ple in corrosive NaCl solution for extended periods Al–Cu–Mg particles compared to the matrix while indicates that the ZPO provides better protection to Mn2+ gives the most even distribution of coating across the second phase regions than to the matrix [162].
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3.6. Cerium and Rare Earth Oxide Based Coatings film with persistent corrosion resistance could be formed by exposure of an Al alloy to a CeCl3 solution Among potential CrCC replacements, rare earth [176, 177]. The corrosion resistance appeared to be inhibitors have attracted significant attention and attributed to suppression of the reduction reaction shown promise as corrosion inhibitors on Al alloys kinetics or, for long coating formation times, suppres [3, 4, 19, 20, 22, 24, 25, 163–167]. They form a variety sion of the anodic kinetics and elevation of the pitting of compounds and exhibit multiple valence states. The potential [20, 176, 177]. metals are generally considered non toxic [166, 168]. Hinton et al. were among the first to examine rare The mechanism was revised by Campestrini et al. earth salts as corrosion inhibitors [ 19, 24, 163, 165]. [39], who performed a detailed investigation using var ious surface science techniques to follow the CeCC The coatings based on Ce oxides (CeCCs) have growth resulting from immersion of 2024 Al into a been documented extensively and are applied by one bath of CeCl3–H2O2 [39]. The authors state that the of two methods: (i) immersion in aqueous CeCl3 or second phase particles do not act as preferential (ii) electrochemical activation [3, 20, 39, 48, 52]. nucleation sites, while deposition of Cu on the the sur Despite the simplicity of Hinton’s original process face of 2024 Al is a necessary condition in order to (immersion), it is not attractive commercially, because form a thick cerium oxide film. The deposits over the it requires a long immersion, on the order of days particles after short immersions are shown to be [3, 24, 39, 163, 165, 169, 170]. To lower the immersion oxide/hydroxide corrosion products enriched in chlo time, galvanostatic polarization in aqueous or organic ride rather than Ce oxides [39]. Thus, according to solutions of Ce salts was proposed [3, 39]. Another way Campestrini et al. [39], the cathodic nature of the sec to decrease the time is based on increasing the temper ond phase particles in 2024 Al is not a sufficient condi ature of the Ce salt bath [52, 111, 112, 170–172]. A tion for preferential precipitation of CeCC over the par simple and fast process (~10 min) was developed based ticles. The presence of Cu rich smut evenly distributed on an H2O2 accelerator added to the CeCl3 bath over the surface enhances the CeCC precipitation rate, [39, 173]. The acceleration effect of H2O2 may be which involves deposition and coalescence of small Ce related to the rapid increase in pH resulting from H2O2 oxide particles across the entire surface [39]. reduction [21]. H2O2 also enhances the oxidation of Ce(III) to Ce(IV) in solution, which leads to films The formation of CeCC may occur through a num with higher Ce(IV) content [21]. However, the stan ber of different steps [39]. First thinning of the natural dard H2O2–CeCl3 treatment appeared to be mainly Al oxide and deposition of Cu rich smut need to occur effective on Cu rich alloys (2XXX and 7XXX series). in order to facilitate precipitation of small coating par Therefore, other accelerators have been investigated ticles. The latter particles subsequently coalesce, for other alloys [39]. Studies of CeCC treated Al alloy forming a thin and compact layer. Further immersion composites indicate that the CeCC treatment signifi in the coating solution causes film thickening, and the cantly improves the corrosion properties of the com film may be eventually removed at some sites as a con posites [42, 174]. sequence of a poorer adhesion to the substrate caused by the Cu rich interface [39]. The reason for the Film deposition in the original immersion process accelerating effect of Cu is not yet clear, although it was thought to initiate with island growth on sites of seems that the effect of the Cu particles is more than a local cathodic activity, usually associated with inter simple cathodic activation of the surface [39]. In addi metallic particles and grain boundaries [21, 24, 163, tion, it is suggested that the Cu particles can be formed 165, 172]. It was reported that the pH value at the during either the surface preparation treatments or the cathodic sites is increased due to oxygen reduction or early stages of the immersion in the conversion bath. hydrogen evolution; the hydroxyl ions formed attack Therefore, the morphology of the CeCC depends on the metal surface and react with the metal ions in the the amount of deposited Cu and its distribution on the solution, which leads to a precipitation of mixed surface, i.e. homogeneous or concentrated at the loca oxide/hydroxide on the surface [21, 24, 39, 163, 165, tion of the second phase particles [39]. 173, 175]. The coating thus starts to deposit at cathodic intermetallic particles and subsequently pro The coating composition is usually described as a ceeds to grow over the Al matrix [21, 24, 165]. Depo mixture of CeO2, Ce(OH)4 and Ce(OH)3 [20, 39, 41, sition continues over the particles throughout the pro 82, 145, 173, 178]. The Ce(IV) oxides detected in the cess, resulting in highly cracked regions where the film coating were thought to result from oxidation of is thicker compared to other regions on the surface. Ce(III) in oxygenated alkaline solutions. In addition, The film morphology resembles that of MnCC depos the valence of the deposited Ce is very sensitive to the ited at pH ~9 on 2024 Al from a KMnO4 borax bath solution pH and degree of aeration [176]. Poor depo [66]. It has been suggested by others that the first step sition is observed under de aerated conditions, and a of film deposition involves the oxidation of Ce(III) to solution pH higher than 8.7 results in deposition of Ce(IV) in the bath solution [176], which then can pre mainly CeO2 [176]. This was recently confirmed by cipitate as insoluble CeO2 due to the local pH increase systematic studies (Yu et al. [173]) on the role of H2O2 resulting from the cathodic oxygen reduction [176]. A and pH in the cerating process. Hydrogen peroxide
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 189 may act as an oxidizing or reducing agent for the While some groups reported lower protective per Ce(III)–Ce(IV) system depending on the solution formance of CeCCs [41, 144, 145, 174, 180], some pH. In the lower pH range, Ce(III) is the more stable others report a similar level of protection as CrCCs form and Ce(IV) may be reduced to Ce(III) by H2O2. [52, 181, 182]. For example, the comparative analysis At higher pH values, Ce(IV) is more stable, particu by Debala et al. [52] concluded that the boiled CeCC larly when oxidizing agents such as O2 or H2O2, are treatments in aqueous Ce(NO3)3 are as effective for present. Ce(III) may be oxidized to Ce(IV) by H2O2 or corrosion resistance as the traditional chromating pro O2, if a sufficient concentration of oxygen is available. cess, but the interest for industrial purposes is limited A very recent report by Scholes et al. [82] based on because of long application times. On the other hand, direct titrations of CeCl3–H2O2 coating solutions with the H2O2–CeCl3 conversion coating process produces OH– (to simulate the pH increase at the surface of Al effective corrosion resistance layers, (containing on immersion) gives support to the complexation of Ce(IV)), thinner than the ones obtained by traditional H2O2 with Ce(III) ions in solution [82], which implies processing, but very interesting for industrial applica even more complex chemistry for deposition of the tions due to limited environmental impact [52]. coatings. Adding other inorganic components into the con Johnson et al. used TEM to investigate the effect of version baths can improve the protective properties of different deposition parameters on the microstructure the CeCCs. For example, a mixture of CeCl3, KMnO4 of CeCCs formed on 7075 Al [41, 144, 145]. Addition and SrCl2 was used to anodise 2024 and 7075 Al, of glycerol results in finer particles, which leads to which improves corrosion resistance and also improved corrosion resistance. Phosphate sealing also decreases the treatment time [183]. Corrosion resis enhances the corrosion resistance, converting the as tance is further improved by subsequent treatment of deposited coating to hydrated Ce phosphate [144, 145]. the coated surface in an alkaline solution containing The addition of a phosphate post deposition sealing molybdate, nitrite and metasilicate ions and another step leads to substantial improvements in the salt fog solution containing a polyfunctional silane [183]. The performance of spontaneously formed coatings [41]. final coating produced is a mixture of the oxides and Hamdy applied various combinations of CeCl3 and sil hydroxides of Ce, Sr and Al, which are intermixed icate treatments to AA6061 Al2O3 composites [174]. with molybdate, silicate and nitrite anions. In the most Improved corrosion resistance was attributed to the corrosion resistant form, the coating layer includes a formation of protective oxide films acting as a barrier hydrophobic silane topcoat [183]. to oxygen diffusion to the Al surface [174], and the Another example of CeCCs with corrosion proper author confirms that Ce oxide has an important role in ties comparable to those of commercial CrCCs was the passivation mechanism. The best performance was proposed by Mansfeld et al. in the so called “stainless demonstrated by samples first silicate treated and aluminium” process [111, 171]. This is one of a few then sealed with CeCl3. Decroly and Petitjean [48] most developed alternatives to CrCC treatment on Al also agreed that a post deposition sealing may be nec in the literature and it provides excellent corrosion essary for CeCCs in order to compete with CrCCs in protection. Samples are first immersed in a boiling terms of corrosion performance [48]. Ce(NO3)3 solution, then dipped in aqueous CeCl3, One important feature of CeCCs is their potential also at 100°C, followed by potentiostatic polarization for active corrosion protection (or “self healing”) in a de aerated Na2MoO4 solution. Al alloy surfaces [20]. This property may result from the interconver treated in this manner resisted pitting in aerated NaCl sion of Ce(III) and Ce(IV) species, which is possible solution for 60 days [171]. Mo(VI) appears to incorpo in an aqueous solution according to the Pourbaix dia rate into the coating, and may be capable of repairing gram [20, 94], recently updated [173, 175]. The “sim minor damage to the coating via a redox mechanism. ulated scratch cell”, previously designed by Zhao et al. Thus, the protective abilities of the complex coatings for CrCCs [59], was used to study CeCCs for evidence developed in the “stainless aluminium” process [111, of “self healing” [20, 127]. The CeCCs were formed 112, 171] appear to arise both from the coatings them on 2024 Al by modifying hydrotalcite Al2O3 coatings selves acting as insulating barriers and the “self heal with Ce(IV) compounds. Results of experiments con ing” ability of the incorporated Mo(VI). This was ver ducted with the “simulated scratch cell” show that, ified by Kendig and Thomas [113] who studied the when Ce(IV) is introduced into the hydrotalcite con process in detail. They concluded that the Ce and Mo version coating [179], a classic “self healing” response compounds in the “stainless aluminium” process is observed. Specifically, Ce(IV) species incorporated [111, 112, 171] play synergistic roles in corrosion pro into the hydrotalcite coating are dissolved by an tection. If only one of the compounds is used (either attacking solution, transported to active defect sites on Ce or Mo), the same level of protection cannot be bare aluminium surfaces, and then reduced and pre obtained. It was thus hypothesized that the Ce and Mo cipitated to inhibit further corrosion [20, 127]. How compounds within the film provide two complemen ever, whether the same “self healing” takes place in tary functions. The Ce is likely to form very stable and CeCCs not containing hydrotalcite is still an open insoluble oxides which reinforce the hydrothermally question. grown Al oxide film, while the Mo(VI) species are
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 190 KULINICH, AKHTAR believed to provide “self healing properties” [113]. treatment [191]. An advantage is the short length of However others [184] concluded that Mo(VI) was treatment, with immersion times reported to be simply leached into solution and did not participate in between 10 and 600 s [49, 50, 85, 192]. Short immer active corrosion protection. It is therefore quite sur sions are a distinct advantage on a continuous produc prising that the “stainless aluminium” process, which tion line. Such coatings are effective in promoting the provides extremely high corrosion protection, is not adhesion of organic coatings [4, 53] and in preventing well studied in terms of mechanisms occurring during under film corrosion [190]. Although the technology coating formation, species incorporated and chemis has mainly been used for can stock, it has been devel tries involved. oped to a stage where the coatings can be effective in In conclusion, while CeCC treatment alone providing extra corrosion protection and improving appears not to provide as versatile and universal pro paint adhesion for architectural applications (frames, tection as CrCC does, in combination with additives panels, etc. in building industry) and high strength Al or another treatment, corrosion performance compa alloys [190]. rable to CrCCs can be achieved. The most recent stud Notwithstanding the above mentioned usage and ies [22, 23, 164, 185] report spray deposition of CeCC numerous patents [190, 191, 193, 194], the coatings of on 2024 Al, and a successful combination of spray this type are not well studied and their morphology, deposited CeCC with a phosphate post treatment that formation and properties are not extensively reported produced dense coatings of hydrated Ce phosphate in the literature. From XPS, AES and SIMS results, which survived the ASTM B117 salt spray corrosion Schram et al. [85] reported a two layer structure for a test for 336 h [185]. Similar improvements were ZrCC consisting of Al oxides close to the metal inter reported by Zhang and Zuo for electrochemically face and compounds containing Zr, O, and F near the deposited CeCCs after application of a phosphate surface. The surface itself was coated by a polymer post treatment [186]. Ce salt “dopants have been pro film, which derived from the polymeric compound posed for use as inhibitors incorporated into sol gel used in the conversion solution. The thickness did not coatings on Al alloys [187, 188]. Similarly, rare earth exceed 10 nm and was independent of the dipping metal compounds were reported to have the potential time [85]. The Zr content in the layer was very low and as corrosion inhibiting pigments in paints [4]. Thus, Zr and F were always present together. In the top layer, the CeCCs and salt pigments show considerable the lateral distribution of both Zr and F was homoge potential as alternatives to CrCCs and chromate neous, while in the bottom layer, the elements were based pigments. present only in pitted areas. The two layer model is in agreement with an earlier study by Deck and Reich gott [195], who performed AES depth profiling after a 3.7. Zr and Ti Based Coatings (Group IVB Oxides) ZrCC treatment of 3003 Al. TiCCs have a similar Among the Cr(VI) free coatings developed so far, coating structure: an inner Al oxide layer and outer those based on Zr and/or Ti (and Hf to a lesser extent) inorganic layer with Al, F, Ti and O [195]. The polymer are in commercial use as they meet the requirements was incorporated throughout the coating but was of the container industry [47, 50, 83–85, 189–192]. present in higher concentrations at regions closer to The Pourbaix diagrams for Ti and Zr indicate that the the outer surface. stability limits of TiO2 and ZiO2 are at approximately The characterisation studies by Nordlinen et al. pH 12 and 12.5 respectively [94], compared to pH 9.0 [50] and Lunder et al. [84] showed that Zr–Ti based for Al2O3, implying that the mixed Zr /Al and/or conversion layers (ZrTiCCs) formed on 6060 Al are Ti /Al oxide layers (ZrCCs and TiCCs, respectively) highly heterogeneous. Under the conditions applied, resist corrosion more effectively than pure Al2O3. the Zr–Ti based conversion layer studied did not cover The coating baths used are normally aqueous fluo the Al surface fully: coating and deposition occurred at rozirconic or/and fluorotitanic acids [47, 49, 50, 53, and in the vicinity of the second phase particles 83–85, 189–191]. Organic polymers (such as poly [50, 84]. The deposition results from a local increase acrylic acid, carboxy vinyl polymer, ammonium poly in pH at these sites due to oxygen reduction and acryiate, polyvinyl alcohol, acrylic emulsion and so on hydrogen evolution. Consequently the conversion lay [4, 47, 50, 83, 85]) are also added to improve corrosion ers formed exhibit considerable variations in thickness protection and establish a base for subsequent painting across the surface. Preferential nucleation of the [47, 50, 53, 83, 85]. Furthermore, a small amount of ZrTiCC occurred on and around the intermetallic hydrofluoric acid is frequently added in order to particles, resulting in reduced cathodic activity of the enhance dissolution of the naturally formed aluminium particles and inhibiting further film growth [50]. In oxide film. Fluoroacid based treatments may be applied contrast, coatings reported by Deck et al. [83] formed in a conventional fashion (dipping) [50, 83–85, 192] or from both fluoroacids (with Ti or Zr) on extruded 0636 by “no rinse” techniques [47, 83, 189]. These coatings Al were reasonably uniform when the appropriate poly are also used as a treatment for temporary protection meric compound was added to the bath [83]. Reducing of Al alloys, before mechanical shaping, adhesive the bulk pH or increasing the convection in the conver bonding or welding and/or permanent conversion sion bath also inhibit growth of this layer [84].
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Fedrizzi et al. studied the performance of ZrCCs or Co based conversion coatings (CoCCs) are currently TiCCs and CrCCs on Al alloys [53, 196]. Sheets were used in the marine and auto industries [40, 198]. The pretreated in fluorozirconate or fluorotitanate baths coating bath does not contain fluoride and is at a neu and, for the sake of comparison, in a traditional chro tral pH; therefore, oxidants are the only bath compo mate bath; these samples were then studied (with and nents which may cause etching. Aqueous coating solu without covering organic coatings) using electro tions contain a Co(II) salt, an ammonium salt, an chemical impedance spectroscopy. The corrosion pro inorganic complexing agent, an organic complexing tection of the Zr or Ti based pretreatments was less agent, and an oxidizing agent, leading to the formation effective compared to traditional CrCCs; however of highly stable Co(III) complexes which are incorpo their performances can be improved by a proper bath rated in the oxide conversion coating [31, 40, 87]. It is formulation and treatment procedure, as shown by believed that the complexes have the general formula corrosion tests performed on samples without organic Na3[Co(NO2)2 (X)4] (X = OOCH, OOCCH3, etc.), coating [53, 196]. In fact organic coatings adhere based on the reagents listed in the experimental por more strongly to TiCCs compared with traditional tion of the patents [86, 87, 197]. The soluble inhibitor CrCCs, when the processing parameters are opti complexes formed may migrate throughout the coat mized for TiCCs [53]. Meanwhile, Deck et al. [83] ing and/or over the metal surface, facilitating a “self compared ZrCCs and TiCCs on 6063 Al with com mercial Cr phosphate conversion coatings and con healing” repair process similar to that exhibited by cluded that the performance of the former was on par CrCCs [31, 87]. with Cr based treatments [83]. According to the patent literature, CoCC baths con A recent study by Andreatta et al. has probed the tain an organo cobalt nitrite complex generated mechanism of ZrTiCC formation on 6016 Al at room through the reaction of a metal nitrite with a Co(II) salt, temperature and pH = 4.2 (immersion times ranging H2O2 (oxidizer) and an accelerator [31, 86, 87, 197] at from 1 to 10 min) [49]. The deposition of the ZrTiCC elevated temperatures (~60–65°C). This process has been shown to be an electrochemically driven pro results in a coating that has an open porous rigid struc cess. The existence of cathodic sites on the alloy sur ture, which is generally 120–300 nm thick [40]. This face is the driving force for the formation of the con coating then can be subjected to an additional sealing version layer. While immersed in the conversion bath, step which may involve organic compounds, although the naturally formed oxide film on Al is removed or aqueous vanadates or tungstates are preferred as they thinned due to the presence of fluorides in the solu improve corrosion resistance to a greater extent tion. Subsequently, deposition of the conversion layer [40, 87]. Thickness of the CoCC on 2024 Al and initiates on cathodic second phase particles [49]. The 7075 Al alloys was reported to increase rapidly as the film exhibits lateral growth in the region surrounding immersion time was extended; however, the rate of the particles and eventually covers the entire 6016 Al thickness increase slowed for times longer than 5 min surface [49]. [40]. Additional sealing with vanadate resulted in pen Addition of phosphate compounds to baths was etration of V through the oxide and deposition of a reported to improve pitting corrosion protection and thin sealing coating within the pores and on the exter paint adhesion [193, 194, 196]. A passivating post nal surface of the CoCC [40]. In general, however, rinse was applied to PCCs [193] by dipping the coated details of CoCC growth have not been studied exten substrate in a solution of aluminium fluorozirconate sively. before the application of paint. Improved performance was achieved for Al sheets by treating in a conversion A major drawback of the above described CoCC bath with K2ZrF6, Ca(OH)2, Na2B4O7, HNO3, KF process is that additional tanks are required for seal and Na5P3O7 [194]. The protection given by ZrCCs ing, which could have a considerable cost impact. In could be improved further by adding P2O5 to the bath an alternative Schriever process, Co(III) in the form of [196], which resulted in incorporation of P in the )3+ external coating layers, and thus better barrier protec Co(NH3 6 is used to form the CoCC in one step tion. ZrCCs and TiCCs have yet to be widely tested in [199]. However, a disadvantage of this approach is the the types of environments in which CrCCs have been very careful pH control required in order to keep the proven effective. In general, Zr salts are non toxic Co hexamine complex stable [3, 199]. except for those containing fluorine; nevertheless such coatings have been used on Al cans for a few decades, Good paint adhesion and excellent corrosion resis and no health problems have been reported [4]. tance have been reported for CoCCs [31, 87]. At the same time, there are indications of poorer paint adhe sion for sealed CoCCs [3]. Although Co is less toxic 3.8. Co Based Conversion Coatings than Cr(VI), there are nevertheless some health con Another alternative to chromating was developed cerns associated with Co [40]. Thus, additional studies by Schriever [86, 87, 197] and is based on Co chemis are necessary before these coatings can be used exten try. Originally developed for aerospace applications, sively.
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3.9. Li Based Conversion Coatings LiCC was lower than the one of surfaces covered with CrCC [88]. The corrosion protection provided by the Several reports have appeared in the literature on LiCC on 2024 Al is thus reported to be comparable Li based conversion coatings (LiCCs) [89, 90, 200–203], with the corrosion protection from CrCCs [88]. but the nature of such layers is not understood well. A few early reports described the unexpected passivity of Al in alkaline solutions containing Li salts [204, 205]. 3.10. Anodised Layers as an Alternative Subsequently Rangel and Travassos reported a conver sion coating which forms on Al after immersion in a Anodic films electrochemically formed on Al sur faces are also a type of conversion oxide. It is well Li2CO3 alkaline solution [200]. The formation process is dependent on the concentrations of oxygen and Li+ known that a thick oxide layer can be electrochemi ions, and the pH of the bath at room temperature cally grown on an Al substrate by anodic oxidation [200]. Restricting the amount of available oxygen pre (anodising) [93]. Electrochemical anodising of Al is vented formation of the coating [200]. The film pro performed using the sample as the anode immersed duced was remarkably resistant to Cl– ion attack [200]. typically in 1.5 M H2SO4 with a steel cathode [30, 93]. In later work, the same authors [203] produced LiCCs As constant current density is applied to the electro chemical cell, the Al plate undergoes oxidation, react at the open circuit potential using either Li2CO3 or LiOH as a base solution. An electrochemical study was ing with water and forming Al oxide. Typically, a tena undertaken with the objective of clarifying the mecha cious and dense oxide layer forms close to the metal nism of coating growth [203]. The protective character with a more porous outer oxide layer [30, 92]. The of LiCCs was suggested to result from formation of outer oxide layer is up to 99% porous with the pores lithium aluminate, which acts as a pore filler in a extending to a distance of approximately 20 nm from matrix of aluminium hydroxide. Coatings were found the Al surface [30, 92]. The thickness and morphology to be more stable in the absence of carbonate species of the anodised layer depends on the applied current, [203]. voltage, temperature and anodising time [29, 30, 92, 93, 208, 209]. The properties of the oxide layer are also In a series of works, Buchheit et al. have developed related to the electrolyte and the Al alloy [30, 91, 93, hydrotal cite like LiCCs on Al alloys by applying var 3+ 208, 210]; surface pretreatments (such as electropol ious alkaline baths containing Li2CO3, LiOH and Al ishing and acid etching) also affect the quality of [90, 201, 206, 207]. The approximate composition and anodic layers [211]. Electrochemically grown Al stoichiometry of such coatings is similar to the com oxides provide extra barrier protection due to the ⋅ pound Li2Al4CO3(OH)12 3H2O [201, 206]. The coat highly capacitive behaviour of Al oxide. However, the ings were reported to form in a matter of minutes as processing costs are substantial. compact thin films on alloy surfaces [201]. Such Li carbonate hydroxide hydrate coatings demonstrated Anodised films do not generally provide sufficient thermal stability up to 150°C [206]. As the tempera corrosion protection without an additional protective ture increases further, interlayer H O is expelled fol layer, but they are an excellent paint base [93]. The 2 wear resistance and hardness of anodised films can be lowed by loss of structural H2O and CO2 [206]. Sealed hydrotalcite coatings formed under hot, alkaline and improved by operating at a decreased electrolyte tem oxidizing conditions have sufficient corrosion resis perature and an increased current density. This process tance to prevent pitting formation during 168 h of salt is referred to as “hard anodising” [93]. The properties spray exposure [179, 207]. Similar results have been of the hard anodised coatings can be further improved reported recently by Anica( i et al. [89] for LiCCs by incorporating solid film lubricants such as Teflon or MoS [93]. formed in a LiNO3–KNO3–LiOH bath with further 2 treatment in Ce(III) or Mn(VII) solutions. To further The “black anodic coating” consists of a porous Al improve corrosion resistance the coatings were sealed oxide layer containing a UV resistant mineral pigmen with compounds such as transition metal salts or seal tation and is an example of an anodising based process ants typically used on anodic films. The sealed coat for space applications [13, 28, 212]. These coatings ings were reported to protect 2024 Al from pitting for exhibit better scratch resistance than black paints. The 336 h in the salt spray test [202]. Thus, LiCCs show process has three main steps: (i) anodisation; (ii) par potential when combined with other chemistries. tial filling of the pores in the Al oxide (achieved by Castro et al. reported comparison of 2024 Al alloy electrolytically depositing Sn or chemically obtained surfaces coated with LiCCs and CrCCs [88]. All coat Ni or Co sulfides); (iii) colmatation of the pores by ings were painted with a primer epoxy resin. Electro hydration in hot water, forming hydroxides which chemical corrosion measurements indicated that occupy more volume than Al oxide and thus tight up LiCC coated surfaces have lower corrosion current the porous structure. Various tests have proven the density, larger polarization resistance and less negative effectiveness of this process in protecting Al alloys for corrosion potential than CrCC coated surfaces [88]. space applications [13, 28, 212]. The LiCC coated surfaces were highly porous; never The absence of a direct replacement for CrCCs theless, the corrosion rate of 2024 Al coated with places a greater importance on anodic (and other Al
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 193 oxides, e.g. boehmite) films for corrosion protection containing KMnO4 were relatively thin (2–3 μm) but of Al [1, 3, 12, 26, 213]. Therefore, it is desirable to exhibited a high corrosion resistance [1]. Anodic films develop anodic films with improved corrosion resis formed in the presence of vanadates, molybdates and tance, which may be achieved using two approaches: zinc thioglycollate displayed poor resistance to pitting (i) finding improved sealing treatments; (ii) direct on exposure to neutral salt spray [1]. The films incorporation of inhibitors into the anodic film [1]. obtained from solutions containing either sodium molybdate or molybdic acid were very thin (<1 μm) Sealing of the anodised film is necessary to improve and displayed poor corrosion resistance [1]. A two its abrasion and corrosion resistance [178, 214, 215]. step process, in which aluminium samples were first The porous oxide film is sealed by precipitation of immersed in Ni(NO ) with Ce or Nd salts at 90°C hydrated metal oxide species in the pores. This is nor 3 2 and then dried and anodised, has been recently mally accomplished by boiling in hot water or NaOH, reported by Li and coworkers to improve both coating steam treatment, dichromate (or other active inhibi morphology (density) and corrosion resistance [27]. tor) sealing or lacquer sealing [93, 178, 210, 214]. Ce(III) salt sealed anodised 2024 Al was reported to Anodic films developed in sulfuric acid with perform in a similar manner to anodised and chro Na2MoO4 additives (for 2024 Al) showed some prom mate sealed 2024 Al [178, 214]. Pearlstain and Agar ise in corrosion tests compared to Al oxide layers wala attempted to incorporate Cr(VI) into anodised obtained by anodising in pure sulfuric acid [216], but 2024 Al through non toxic chemistry [74, 95]. After their corrosion resistance was lower than that of anodising, the samples were sealed by boiling in a anodic films obtained in chromic acid [216]. However, Cr(III) bath followed by a 2 min immersion in aque at high molybdate concentrations, the anodic layer ous H2O2 to oxidize incorporated Cr(III) to Cr(VI). from sulfuric acid solution was significantly improved The panels were then exposed to salt spray for over and its performance was similar to the chromic acid 3000 h before any signs of corrosion were observed. anodic layer [219]. Addition of KMnO4 to sulfuric acid also improved the anodic layer formed on 2024 Al A number of electrolytes have been tested to pro [220], in agreement with the earlier work [1]. The use duce anodic films with excellent properties [29, 209, of KMnO4 made it possible to obtain corrosion resis 213, 215–218]. Thompson et al. tested anodic films tance similar to chromic anodisation [220, 221]. A formed from boric acid with H2SO4 on 2024 Al and combination of both film porosity and inhibitor incor 7075 Al [213]. The reported corrosion resistances for poration into the oxide layer are considered the key fac these films were inferior to those for films formed by tor in improved corrosion performance [26, 218–220]. anodising in chromic acid [213]. Monfort et al. [217] have studied the growth of coatings produced by ano More recently, Moutarlier et al. [26] have investi dising pure Al under sparking conditions in gated the development of anodic films on 2024 Al in dis odium tetraborate solution with Na MoO (pH = 10). KOH/Na2SiO3 electrolytes. The coatings consist of an 2 4 Al rich inner layer, formed by growth of anodic alu Thicker films, Mo(VI) incorporation, and decreased mina near the metal/coating interface, and a Si rich layer porosity are favoured at lower molybdate con outer layer, formed by deposition of silica gel at the centrations [26]. At higher molybdate concentrations coating surface [217]. However, the corrosion protec (>0.4 M), the formation of Mo oxide hinders the tion performance was not reported. Recently, Zhang growth of Al oxide and Mo(VI) incorporation [26]. and coworkers have reported a new process of phos The thickest anodic layer, formed from 0.3 M molyb phoric/boric/sulfuric acid anodising, which produces date solution, demonstrated the highest corrosion a thicker film with high porosity and large pores [91]. resistance [26]. The anodic films have higher corrosion resistance The work of Delaunois et al. [75] is an example of compared to those obtained by anodising in boric/sul anodising successfully combined with other conver furic acids or in phosphoric acid [91]. Zabillaga et al. sion treatments. The authors applied anodisation to [29, 209] reported one step produced (in oxalic acid) tCrCC covered 1050 Al and 5754 Al surfaces [75]. films containing polyaniline and TiO2 nanoparticles to The procedure led to enhanced corrosion resistance of show a lowered passive current density on 2024 Al. the tCrCC (up to 1–2 × 106 Ω cm2 as measured by Direct incorporation of inhibitors was attempted EIS) [75]. This value is generally characteristic of anti by several groups [1, 216, 218, 219]. Smith et al. [1] corrosive coatings surviving 168 h of salt spray test (e.g. reported promising anticorrosive behaviour of CrCCs). Thus, anodic oxidation, combined in differ ent ways with various chemistries, can lead to 2014 Al panels anodised in H2SO4 baths with Ce(III) and Min(VI) inhibitors [1]. They used a standard acid improved anticorrosive properties of conversion layers. bath (150 g/L H2SO4) to which various inhibitors were added at a concentration of 7.5 g/L. The films pro 4. ON ACTIVE CORROSION PROTECTION duced using Ce(IV) electrolytes were consistently IN CONVERSION COATING thinner than those formed in Ce(III) (3–5 μm com pared to 7–12 μm) but appeared to provide better pro In order for a coating to provide adequate corro tection [1]. The anodic films produced from solutions sion protection, the coating must be uniform, pore
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Table 3. Active corrosion protection (“self healing”) “stainless aluminium” conversion coating produced behaviour relative to the most known conversion coatings by the Mansfeld process [111, 112, 171]: Ce(III) was described proposed to reinforce Al oxide by forming insoluble Coating Acting species References Ce oxides and Mo species were proposed to act as active inhibitors [113]. Table 3 lists the major conver CrCC Cr(VI) [10, 58, 59, 66] sion coatings (and active species) where ACP is CeCCa Ce(IV) [20, 127] expected or reported. The mixed conversion coating a produced in the Mansfeld (“stainless aluminium”) MnCC Mn(VII) [127] process [111, 113, 171], based on the synergistic action VCC V(V) [14] of Ce(III) and Mo(VI) species, is also mentioned in MoCC Mo(VI) [3, 15, 77, 107] Table 3, even though the active role of the Mo(VI) “Stainless Al” Mo(VI)b [113] incorporated into such protective layers was suggested by Kendig and Thomas [113] and then was debated CoCC Co(III) [31, 87] [184]. The tCrCCs only possess ACP if a post treat tCrCC Cr(VI)c [74, 75, 95] ment oxidation is applied, which converts some of the Notes: a Only post treated hydrotalcite coatings. Cr(III) in the topmost layer into Cr(VI) [74, 75, 95]. b Species hypothesized. Finally, indications of ACP in CeCC and MnCC sys c Only after post oxidation of “pure” tCrCC. tems were only observed when CeCC [20, 127] or MnCC [127] post treatments had been applied to hydrotalcite Al2O3 coatings. free, adhere well to the substrate, and possess “self It thus appears that ACP has been discovered in healing” properties for applications where physical several conversion treatments other than CrCCs. damage to the coating may occur. “Self healing”, or Moreover, coatings such as CeCCs and MoCCs active corrosion protection (ACP), is a very important appear to contain a reservoir of active inhibitor, component of CrCC performance and a necessary although the subject requires further investigation. aspect of alternative coatings for commercial compet The ability of conversion coatings to actively heal their itiveness with CrCCs. Indeed, conversion coatings minor damages considerably enhances their anticor based on pure Cr(III) oxide (no “self healing”) per rosive performance. However, it should be mentioned form less effectively than CrCCs [31, 76, 96]. SrCrO4 that protection at areas where the paint/conversion pigments for paints/primers have also demonstrated coating is damaged depends on both the leachability of excellent performance, because Cr(VI) is leachable the inhibitor (which in turn depends on the solubility and provides ACP to coating systems [1, 4, 5]. CrCCs and migration of “self healing” species) and the are noteworthy because of their ability to “self heal”, inhibitor efficiency (i.e. the minimal concentration if mechanical or chemical damage occurs, provided required to inhibit corrosion) [1]. Thus incorporation the damage is not too severe. “Self healing” involves of active species into conversion coatings is not suffi several discrete processes: release of Cr(VI) from the cient to provide real efficient ACP to conversion coat coating, its transport through solution, and its action ings. Therefore, more detailed studies are necessary to at the site of damage (typically pits). Thus, even develop understanding for the effectiveness and though ACP is a temporary property of CrCCs (over behaviour of inhibitors incorporated into the chrome time Cr(VI) inclusions tend to leach, and ACP dimin free conversion coatings listed in Table 3. ishes), it is to much extent this property of keeping a reservoir of active inhibitor inside that makes CrCCs so widely used as anticorrosive protection. 5. EFFECT OF ALLOY MICROSTRUCTURE An apparatus known as the “simulated scratch The surface microstructure of Al alloys has a signif cell” was devised by Zhao et al. [59] to examine CrCCs icant influence on the conversion film morphology for evidence of “self healing”. The cell consists of two and properties. The formation of uniform conversion Al alloy surfaces, one coated and one bare, separated layers is often reported to be impaired by second by a gap of several mm filled with an aggressive solu phase intermetallic particles [16, 49, 50, 60, 84, 162]. tion. If a coating exhibits ACP, the inhibiting agent will As mentioned in Section 2.1, 2024 Al contains two be released into solution, interact with the uncoated major types of second phase particles, embedded surface, and stifle corrosion. When CrCCs were within the alloy matrix. The second phase regions tested, corrosion of the uncoated Al was decreased have differing chemical compositions and properties [59]. More recently similar tests have been applied to compared to the matrix. The CrCC distribution across several other conversion systems and the results indi the microstructure after 5 s of immersion resembles cate potential for ACP [20, 127]. the coating pattern for PCC coatings containing Ni2+ The report of Kendig and Thomas [113] was per (more deposition on the second phase particles com haps the first mention of ACP in a non Cr(VI) based pared to the matrix) [66, 161]. On the other hand, the conversion coating. The authors suggested that coating distribution is similar for the CrCC (30 s Ce(III) and Mo(VI) have synergistic roles in the immersion) and the PCC coating containing Mn2+,
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 ON CONVERSION COATING TREATMENTS TO REPLACE CHROMATING 195 where there is a more even coating distribution across performance levels of different conversion coatings the different microstructural regions. When there is a (or pigments). The use of different evaluation meth smaller amount of deposition, precipitation occurs ods is another important issue. The methods most fre mainly at the second phase particles for CrCCs and quently used are salt spray exposure (accelerated salt PCCs. As more precipitate accumulates on the sec spray testing in accordance with ASTM B117 [222]) ond phase regions, less metal is exposed to the coating and electrochemical impedance spectroscopy (EIS). solution, so diminishing the pH increasing reduction The coating resistance, as measured by EIS, can be reactions and rate of coating deposition. There is no used as an indication of the presence of flaws and equivalent retardation in coating deposition at the cracks in the coating at which corrosion cells might matrix because the coating coverage there is less exten form. Meanwhile the spray test involves a visual judge sive. The cumulative result is a more even coating dis ment of how coatings survive or fail in aggressive tribution across the surface. While CrCCs and PCCs media over time. Normally, corrosion protection have similar patterns of growth, MnCCs appear to dif properties of coated Al alloys are evaluated by expos fer: throughout the coating process, the amount of ing the samples to a salt fog atmosphere generated by deposition over second phase particles is greater com spraying 5 wt % aqueous NaCl solution at 35°C for pared to the alloy matrix (Fig. 6) [16]. In this case the 168 h. According to the ASTM B117 standard, a pro slowing of reduction reactions at the second phase tective conversion coating should survive 168 h of this regions would not drastically retard coating deposition test with little or no pitting [222]. because the basic conditions required for precipitation In an extensive study, Buchheit et al. [64] investi are maintained by the high pH of the coating solution. gated the relationship between these two evaluation Similarly to MnCCs, ZrTiCCs deposited on 6016 Al methods. Corrosion resistance was measured for were reported to be thicker over intermetallic particles thirty three inorganic commercial conversion coat than on the metallic matrix, while no difference in ings on five different Al alloys using the two different potential was found across the surface of the coatings methods [64]. Parameters defining the pitting resis from analysis using scanning Kelvin probe force tance were developed [64]. Examination of the data microscopy [49]. This implies that no driving electro showed that EIS provides a more quantitative measure chemical force is present on the samples. Highly het of corrosion protection, while the salt spray test is erogeneous ZrTiCC layers were also reported by two more qualitative [64]. Moreover, the visual judgment other groups [50, 84], with coating located mainly at in the latter differs between evaluators, though this and in the vicinity of the second phase particles. does not influence the results, as relative performance In the case of CeCCs, the most systematic work by between different coatings is usually presented [64]. Campestrini et al. [39] has demonstrated that the dis The latter explains why some discrepancies in the tribution of coating on 2024 Al depends strongly on results appear in the literature. Examination of the Cu smut distribution over the substrate surface. The data showed that both EIS and salt spray test could be smut may form as a result of surface pre treatment or sensitive discriminators of corrosion protection, but during the first stages of immersion in the conversion EIS is more discriminating in the extremes of coating bath. The CeCC morphology strongly depends on the performance [64]. The probability of achieving a pass distribution of the deposited Cu smut particles, which ing salt spray result was shown to increase with the cor can be either uniformly spread out or concentrated rosion resistance value measured by EIS [64]. Thresh mainly at the second phase particles. Others com old values of this parameter define the minimum value pared CeCCs produced on 6061 Al and 2618 Al by for which a given coating can be expected to attain a two different techniques: the coatings obtained by passing result in a 168 h salt spray test [64]. The boiling in a Ce(III) solution covered the alloys uni thresholds differ as a function of the alloy, e.g. between formly, while those obtained by immersion in a 2 × 106 and 5 × 106 Ω cm2 for 2024 Al and 6061 Al, and 1.5 × 107 Ω cm2 for 7075 Al [64]. Ce(III)–H2O2 bath demonstrated preferential cover age at the second phase particles [52]. Interestingly, Earlier, Davenport et al. tested the performance of for CeCCs spray coated on desmutted 2024 Al and several conversion coatings formed from a range of subjected to a phosphate post treatment [185], inho oxyanion analogues: vanadate, tungstate, phospho mogeneity in thickness was not reported. tungstate, molybdate, permanganate, phosphomolyb date, tungstosilicic acid and molybdosilicic acid [15]. The coatings were formed under the same conditions 6. COMPARATIVE STUDIES (i.e. same oxyanion concentrations, pH, temperature OF CONVERSION COATINGS and immersion time) on 1100 Al and were tested using AND FUTURE DIRECTIONS visual inspection and X ray absorption near edge Work on systematic comparison of various conver structure (XANES) [15]. Visual inspection was the sion coatings (and similar analysis for primer/paint principal technique used to measure coating perfor pigments) is very important in developing alternatives mance and potential as a substitute for CrCCs [15]. to Cr(VI) based chemistries. However, there have Systematic studies and detailed analyses of the coat been only a few published studies that compare the ings were not reported [15]. Moreover, the disadvan
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Table 4. CrCC corrosion resistance from electrochemical also systematically investigated the inhibitor charac impedance spectroscopy (EIS) and salt spray performance teristics of twenty seven compounds considered as (from ASTM B 117 standard tests) from different literature possible replacements for chromate pigments in aero sources space paints. 2024 Al samples were exposed for Resistance Time to pits Refer 10 days to 0.6 M NaCl solution containing 3.4 mM of Alloy from EIS, Ω cm2 observed in spray test ence each candidate inhibitor to reveal their comparative action [128]. It would be useful to follow the 2024 A1 4.4 × l05–1.6 × 108 >168 h [64] approaches described in these works in studies of the 7075 A1 3.87 × l05–2.1 × 108 >168 h [64] comparative behaviour of various conversion coatings 6061 A1 2.8 × l05–3.2 × l08 >168 h[64] on Al alloys. 2024 A1 – >336 h [17] Table 4 shows data on the corrosion resistance 7075 Al – >336 h [17] properties of various CrCCs, as measured by the two 6061 A1 – >336 h [17] methods above mentioned (EIS and salt spray test). × 6 Tables 5 and 6 present similar data for various non 1050 A1 2 l0 –[75]chromate conversion coatings. As seen in Table 4, cor 5754 A1 2 × l06 –[75]rosion resistance as high as ~106 Ω cm2 (EIS) can be 2024 A1 1.58 × 105 ~168 h [76] achieved by applying CrCCs on Al alloys. CrCCs also 2024 A1 1.3 × 106 >168 h [3] pass the neutral salt spray test (ASTM B 117 stan × 4 dard), which requires minimal pitting during 168 h of 1100 A1 2.2 10 >168 h [90] exposure (see Table 4). This is consistent with the 6061 A1 ~107 >168 h [90] observations and conclusions of Buchheit et al. described above [64], as corrosion resistance values measured for CrCCs in different works are normally tage of the approach is that conversion coatings based over the thresholds proposed in Ref. [64]. on differing oxyanion chemistries require different conditions (oxyanion concentrations, pH, tempera As seen in Tables 5 and 6, practically each of the ture, and immersion time) to achieve optimal protec above mentioned conversion systems demonstrates tive properties; therefore, comparisons made in this comparable results, both corrosion resistance up to study do not reflect the optimal performance of the ~106 Ω cm2 and higher (where such measurement conversion coatings. results are available) and survival in spray test for at least 168 h. Thus, a conclusion can be made from In a recent work, Prokeš and Kalendová systemat Tables 5 and 6 that formally both requirements for ically investigated and compared the anticorrosive anticorrosive coatings (passing 168 h spray test and efficiency of thirty three epoxy resin based coatings corrosion resistance from EIS at least as high as and fifteen silicone resin based coatings (with differ ~106 Ω cm2) can be satisfied by several chromate free ent content and kind of pigments incorporated) on conversion systems or by combinations of a few treat steel substrates [223]. Earlier, Cook and Taylor [128] ments (two or more chemistries).
Table 5. Corrosion resistance from electrochemical impedance spectroscopy (EIS) and salt spray performance (from ASTM B 117 standard tests) for non chromate conversion coatings based on Cr(III), phosphating, Mn and V oxides Corrosion resistance Time to pits observed Coating Alloy Reference from EIS, Ω cm2 is spray test tCrCC, post oxidized with KMnO4 2024 A1 – ~336 h [74, 95] '' 7075 A1 – ~336 h [74, 95] '' 1050 A1 l.58 × l05 –[75] '' 5754 A1 2 × 106 –[75] tCrCC, post treated with anodic oxidation 1050 A1 106 –[75] '' 5754 A1 2 × l06 –[75] tCrCC/ormosil 2024 A1 2 × 106 ~168 h [76] PCC high purity Al 105 –[146] MnCC 2024 A1 – >336 h [17] '' 7075 A1 – >336 h [17] '' 6061 A1 – >336 h [17] VCC 2024 A1 ~106 >168 h [14]
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Table 6. Corrosion resistance from electrochemical impedance spectroscopy (EIS) and salt spray performance (from ASTM B 117 standard tests) for non chromate conversion coatings based on Ce, Ti, Zr, Co, Li oxides or anodising Corrosion resistance Time to pits observed Coating Alloy Reference from EIS, Ω cm2 is spray test CeCC 3003–A1 5.24 × 105 –[186] CeCC, sealed with phosphate 7075–A1 – ~336 ha [41] '' 2024–A1 – ~336 h [185] '' 3003–A1 1.52 × 106 –[186] Stainless Al 6061–A1 >107 1440 hb [171] TiCC and ZrCC Al extrusion – ~1000 h [190] CoCC/ormosil 2024–A1 2.5 × l05 > 1 6 8 h [ 3 1 ] LiCC 1100–A1 2.3 × 105 >168 h [90] '' 6061–A1 1.7 × l05 >168 h [90] '' 2024–A1 – >168 h [179, 207] LiCC, sealed with Ce(III) or Mn(VII) 2024–A1 – >192 h [89] LiCC, sealed with Ce salts 2024–A1 8.13 × 105–2.1 × 107 ~336 h [202] LiCC, sealed with Al salts 6061–Al 1.5 × l06–108 ~336 h [202] Anodised layer sealed with Cr(III)c 2024–A1 – >3000 h [74,95] Anodised layer with Ce(IV) added 2014–A1 – >116 h [1] Notes: a 80% of samples are reported to pass; b in 0.5 N NaCl environment; c sealed and then post oxidised with H2O2.
On the other hand, it can be seen from Tables 4–6 based conversion coating was shown to demonstrate that the data on both EIS corrosion resistance and superior corrosion protection to that of CrCC [225]. spray test results for various conversion coatings are from different sources and are in general measured under somewhat different conditions. Thus, caution 7. CONCLUDING REMARKS must be exercised when making comparisons. It is The search for low toxicity alternatives to CrCCs for necessary to conduct new extensive studies in order to passivation of Al alloys has been extensive. Over many measure numerous coatings under identical conditions, years a variety of possibilities were examined but no and to consider various aspects of coating behaviour candidate was found to be completely suitable. Not and properties, including ACP behaviour. Moreover, withstanding the increasing activity in the non conver such tests of conversion treatments should be probably sion coating research field (including organosilane made with organic coatings (paint coats), rather than based chemistries, electroactive polymers, sol gel coat relying solely on testing without paints [126]. ings), inorganic conversion coating systems, both alone Tables 4–6 also indicate a general tendency for and in combination with other chemistries, are still combined chemistries (e.g. tCrCC coated with ormo among the most attractive and desirable candidates to sil layer, LiCCs sealed with proper sealants, anodic replace CrCCs on Al alloys. Though none of the alter layers properly sealed, etc.) to have corrosion resis natives described in this work represent universal com tance very close to, if not superior to, that of CrCCs. mercial substitutes for CrCCs, an understanding of the This shows definite promise and points towards a factors determining coating properties may lead to the probable direction for future research on improved development of new protective coatings. conversion replacements for CrCC. Typically, non chromate conversion alternatives do Finally, development of new conversion systems, not provide the same level of corrosion protection as based on both organic and inorganic chemistries may CrCCs. Though the performance characteristics of result in a successful replacement for CrCCs. Two many of them appear favourable, they are not yet recent works describe coatings on Zn and Mg alloys widely used. A number of such Cr(VI) free coatings [224, 225]. In the first report, Jacques et al. [224] stud exhibit similar performance to CrCCs under certain ied a heptanoic acid—based conversion coating on conditions; however, matching the ease of application zinc. On AZ91D magnesium alloy, a tannic acid— and high performance of CrCCs using non chromate
RUSSIAN JOURNAL OF NON FERROUS METALS Vol. 53 No. 2 2012 198 KULINICH, AKHTAR formulations has been difficult. While many alterna active inhibitors can be beneficial, but more work is tive coatings offer very good corrosion protection in needed in this direction. selected environments (and some are already used It should be mentioned that a significant amount of commercially), their performances relative to CrCCs the work done on conversion coatings for Al alloys is in all types of environments (including field tests) have proprietary. Thus there is still much research to be not been completely assessed. For many of these alter done in order to improve understanding of the surface natives, the problems of formulation and large scale reactions between Al and the coating solution, the processing details have not yet been considered. For effect of the alloy microstructure and treatment this reason, it is still difficult to evaluate some of them parameters, etc. Such knowledge would facilitate opti in terms of cost. Therefore, the development of new mization of the coating preparation procedures. The conversion technologies would be facilitated by more recent studies [16, 39, 49, 60, 63, 66, 159, 160] of coat extensive studies comparing various candidate coat ing formation mechanisms as related to alloy micro ings (and combinations of thereof) with those based on structure, alloy pretreatment and immersion bath chromates. To date, the number of such reports is rel chemistry are believed to open a new avenue in the atively small. research, helping further to develop and improve such coatings. Analysis of the available corrosion protection data indicates that the existing alternative conversion tech It is also worth mentioning that the long term nologies will not individually provide an effective effects on health, in terms of prolonged exposure to replacement for CrCCs. In future developments, a any of the alternatives discussed above, are mainly mixture of treatments (possibly also involving non unknown at this stage. This may be an important fac conversion coatings) is likely to be used. This would tor in developing alternative processes at a commercial provide more than one type of corrosion protection. level. The main reason for avoiding CrCCs is the An example is the Bibber process, which progressively health problems associated with their long term use. improves the resistance of the natural Al oxide film by Such effects can take many decades to develop and be introducing Mn(IV) oxide and various sealers [17]. recognized. Some of the above alternatives may even Another useful approach combines compounds with tually prove to be damaging to both human health synergistic inhibiting properties (the Mansfeld pro and/or the environment. However, this should not cess, also known as the “stainless aluminium” treat deter research into replacements for CrCCs, while any ment) [111–113, 171]. This method provides ade harmful effects associated with the use of alternatives quate Al alloy protection, but is not commercially must be studied in parallel. attractive as it is time consuming (approximately 6 h) and requires high temperature treatments in hot solu ACKNOWLEDGMENTS tions, including an electrochemical step with polariza tion applied [112, 113, 171]. Other typical examples The authors are grateful to Prof. K.A.R. 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