Corrosion performance of Zn–Mg–Al coated steel in accelerated corrosion tests used in the automotive industry and field exposures

Abstract

The corrosion performance of Zn–Mg(1–2%)–Al(1–2%) (ZMA) coatings has been compared to zinc–iron alloy (galvannealed, GA) and zinc–aluminum coating (Zn–5Al, Galfan) as well as to conventional zinc coatings produced by hot‐dip galvanization (HDG) and electrogalvanization (EG). For this purpose, cosmetic samples (painted and uncoated) and hem‐flange panels were produced. Their corrosion performance was compared in three different accelerated corrosion tests, as regularly used by the automotive industry, e.g., VDA621‐415, N‐VDA (VDA233‐102), and Volvo STD 423‐0014. As can be concluded from our results, the behavior of ZMA coatings was strongly dependent on the testing conditions as well as on the configuration of the samples. The advantageous effect of ZMA coating was more pronounced in open situations than in confined ones, irrespective of the testing conditions. ZMA coatings provided a significant improvement in comparison to conventional coatings in tests involving a significant salt load such as VDA621‐415 or neutral salt spray especially on cosmetic configurations. By contrast, the beneficial effect of ZMA coatings was less obvious in tests with lower salt load (VDA233‐102, Volvo STD423‐0014), particularly when considering cosmetic corrosion on painted samples and corrosion in confinement. Interestingly, no significant differences were observed between samples with varying Al and Mg content in the metallic coating (1–2% each). The results were compared to data from field exposure at stationary sites.

The corrosion behavior of ZnAlMg alloys in maritime environments

“…In a series of corrosion tests under laboratory conditions, ZnAlMg has been shown to achieve significantly better corrosion protection than coatings consisting of pure Zn . This was found to be the case not only in terms of corrosion of the alloy itself but also with regard to delamination and rust creep under ZnAlMg coatings with a painted finish .…”

ZnMg and ZnMgAl alloys for improved corrosion protection of steel: Some new aspects

“…Furthermore, the corrosion properties of zinc coatings can be improved by alloying it with one or more suitable elements [2][3][4][5][6][7].…”

Coil-coated Zn–Mg and Zn–Al–Mg: Effect of climatic parameters on the corrosion at cut edges

“…The higher sensitivity of HDG to chloride compared to Zn-Mg(-Al) has been demonstrated previously [25,26]. In addition to the role of permanent wetness discussed above, it explains the huge difference between HDG and Zn-Mg(-Al) in conditions of the neutral salt spray test (NSST), where the latter material shows up to 20-fold lower mass loss [2][3][4][5]27].…”

“…Currently, hot dip Zn-Al-Mg coated steel materials are coming to the market for building and automotive applications, and Zn-Mg coatings produced using physical vapour deposition (PVD) are under development [1][2][3]. Several corrosion studies have demonstrated superior protective properties of such coatings in the bare state (non-painted), in both open and confined configurations [2][3][4][5][6][7][8]. Data on the corrosion performance of painted Zn-Mg(-Al) materials are, however, scarce and mainly concern paint systems that are used in the automotive industry.…”