Electrochemical properties of corrosion products formed on Zn-Mg, Zn-Al and Zn-Al-Mg coatings in model atmospheric conditions J. Stoulil*, T. Prosek, A. Nazarov, J. Oswald, P. Kriz and D. Thierry The electrochemical properties of corrosion products formed under sodium chloride deposits on zinc coatings alloyed with aluminium and magnesium have been studied using impedance spectroscopy (EIS), scanning Kelvin probe (SKP) and photoluminescence (PL) techniques. The low-energy band gap identifed in corrosion products on hot-dip galvanized steel was associated with their higher electric conductivity and ef fciency of the rate-controlling oxygen reduction reaction. It was attributed to the presence of ZnO, zincite. The formation of ZnO was hindered by the alloying. The alloyed coatings were covered by more compact layers of corrosion products with lower electric conductivity and better barrier properties. 1 Introduction Hot-dip zinc coatings alloyed with aluminium such as Zn-5Al and Zn-55Al have been used for industrial applications since decades. New coating alternatives containing magnesium with substantially reduced corrosion rates compared to hot dip galvanized (HDG) steel have recently emerged [1–4]. A number of explanations of the inhibiting effect of magnesium have been proposed. Mg is known to provide a complex microstructure. Large potential differences of up to 0.5 V [5] between the phases may be observed for Zn-Al coatings with a multiphase structure, where Zn-rich phases are preferentially attacked by corrosion. This effect may be intensifed in the presence of oxidising agents, e.g. nitrates [6]. The potential difference is shrinking in the presence of chlorides that activate the phases rich in Al [7]. Tanaka et al. reported substantial refnement of the structure of Zn-6Al coating after addition of Mg and Si, which made corrosion much more uniform [8]. Higher homogeneity after addition of Mg was also observed by Li et al. [9]. Erdavan et al. noted that a Zn-Al-Mg coating worked better as a sacrifcial electrode than a Zn-Al coating [10], probably due to the higher dissolution rate of the former coating. With respect to Zn-Al-Mg coatings, Mg is primarily dissolved; later, Al also dissolved at alkalinized cathodic sites [11]. A detailed mechanism of the action of magnesium in a chloride cyclic corrosion test was suggested by Hosking et al. [1]. Released Mg 2þ ions migrated to cathodic sites and buffered them by the formation of Mg(OH) 2 . This allowed a compact layer of simonkolleite, Zn 5 Cl 2 (OH) 8 .H 2 O, to spread from anodic locations over the whole surface. Simonkolleite is otherwise unstable at higher pH levels. This phenomenon was later confrmed by titration experiments performed by Volovitch et al. [12]. On the contrary, while monitoring binary Zn-Mg alloys, Prosek et al. reported the lowest corrosion losses for zinc alloys with 4–8 wt.% of Mg, the surface of which was almost exclusively covered by hydrozincite, Zn 5 (OH) 6 (CO 3 ) 2 , after exposure under non-rinsing conditions under chloride depos- its [13]. Schuerz et al. consider hydrotalcite (LDH), Zn 6 Al 2 (CO 3 ) (OH) 16 .4H 2 O, to be the key corrosion product in the corrosion protection of Zn-Al-Mg coatings [14]. Similarly arranged layered structures were also reported by Volovitch et al. [11] and described in details by Persson et al. [15]. Keppert et al. studied an infuence of pH on composition of corrosion products [16,17] and an infuence of presence of other anions [18]. Volovitch et al. noted bonding and removal of CO 2 and SO 2 4 from the corrosion environment [11] as one of the positive role of magnesium, which is, however, in confict with the generally perceived positive infuence of CO 2 on the corrosion of Zn alloys, J. Stoulil Institute of Chemical Technology, Technicka ´ 5, 166 28 Prague, Czech Republic E-mail: jan.stoulil@vscht.cz T. Prosek, A. Nazarov, D. Thierry Institut de la Corrosion/French Corrosion Institute, 220, rue Pierre Rivoalon, F-29200 Brest, (France) J. Oswald Institute of Physics, Czech Academy of Science, Cukrovarnicka ´ 10/112, 162 00 Prague 6, (Czech Republic) P. Kriz University of South Bohemia, Jeronymova 10, 371 15  Ceske ´ Bude ˇjo- vice, (Czech Republic) Materials and Corrosion 2015, XXX, No. XXX DOI: 10.1002/maco.201408058 1 www.matcorr.com wileyonlinelibrary.com © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim