On Conversion Coating Treatments to Replace Chromating for Al Alloys: Recent Developments and Possible Future Directions1 S
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ISSN 10678212, Russian Journal of NonFerrous 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, 21A12 Yamadaoka, Suita, Osaka 5650871, Japan email: [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, chromatefree 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 strengthto 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 conversionbased concentration of each alloying element is a function of technologies (based on oxyanion analogues to chro the properties required from a particular Al alloy. mates [3, 12, 14–18], rareearthbased inhibitors There are some discrepancies in the compositions pre [19–25] and anodising [1, 26–30]), solgel coatings sented from different reports on the same alloys which [31, 32], organosilanebased 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 2024T3 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 AA2024T3 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 AA7075T6 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 AA6061T6 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] AA3003H14 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] AA2014T6 3.9–5.0 0.2–0.8 0.5–1.2 0.4–1.2 <0.1 – <0.7 – [25] AA6082T6 0.1 0.6–1.2 0.7–1.3 0.4–1.0 0.25 0.2 0.5 0.1 [48] AA6016T4 0.07 0.33 1.3 0.07 0.03 0.02 0.2 – [49] AA6060T5 0.0031 0.499 0.396 0.018 – – 0.175 – [50]a AA7050T7 2.14 2.0 – – – 6.26 – – [51] AA2618T6 2.3 1.6 0.18 – – – 1.1 0.07 [52] AA5005H12 0.05 0.95 0.4 0.1 – 0.1 0.6 0.1 [53] AA5052H32 0.1 2.2–2.8 0.25 0.1 0.15–0.35 0.1 0.4 – [54] AA10504––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 2024Al) [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 2024Al. 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 secondphase 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 2024Al surface is not chemically homoge The secondphase particles in 2024Al 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 secondphase 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 Sphase) 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 abovemen metal finishing industry, refers to the conversion of a tioned Al–Cu–Mg (Sphase) 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 2024Al, 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 NONFERROUS METALS Vol. 53 No. 2 2012 178 KULINICH, AKHTAR (a) (b) (c) Fig. 1. SEM micrographs showing major surface features of polished 2024Al alloy, (a) Al–Cu–Mg and (b) Al–Cu–Fe–Mn sec ondphase 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.