Corrosion mechanisms and protection of Zn-Mg coatings applied on steel

Due to its high corrosion resistance, zinc coated steel is largely used in wide range of industries such as household appliances, construction or automotive. However, the sensitivity of zinc to aggressive environments, the intensive consumption of zinc and the increase in coating price are so many factors that are limiting the use of zinc coatings. A lot of coatings formulations have consequently been studied for many years in order to reduce the consumption of zinc and its cost in coatings. The addition of magnesium into the metallic coating was proved to improve corrosion resistance in atmospheric conditions without increasing thickness[1,2]. However, there is a lack of information about the behavior of Zn-Mg coatings in immersion, namely in the case of water retention inside hollow bodies. Moreover, the high dissolution rate of magnesium due to its high electronegativity is a critical parameter that affects the corrosion mechanism of the coating. Thus, the study focused on the in-depth understanding of the dissolution process, in order to develop an anti-corrosion system considering the complex behavior of the substrate.
Three axes were developed in order to achieve this aim. On a first part, the surface corrosion and the sacrificial protection mechanism were studied at both global and local scales [3,4]. On a second part, the efficiency of corrosion inhibitors was investigated by means of electrochemical, analytical and surface analysis techniques in order to reduce the dissolution rate of the substrate in immersion conditions. Benzotriazole (BTAH) was found to be the most effective molecule and its inhibition mechanism in chloride solution was analyzed in details [5]. By regarding the dissolution and inhibition protection mechanism of Zn-Mg coated steel, a coating with BTAH-layered double hydroxide was developed and its efficiency was discussed.
[1] P. Volovitch, C. Allely, K. Ogle, Understanding corrosion via corrosion product characterization: I. Case study of the role of Mg alloying in Zn-Mg coating on steel, Corros. Sci. 51 (2009) 1251-1262. doi:10.1016/j.corsci.2009.03.005.
[2] N.C. Hosking, M.A. Ström, P.H. Shipway, C.D. Rudd, Corrosion resistance of zinc-magnesium coated steel, Corros. Sci. 49 (2007) 3669-3695. doi:10.1016/j.corsci.2007.03.032.
[3] J. Rodriguez, L. Chenoy, A. Roobroeck, S. Godet, M.G. Olivier, Effect of the electrolyte pH on the corrosion mechanisms of Zn-Mg coated steel, Corros. Sci. 108 (2016) 47-59. doi:10.1016/j.corsci.2016.02.041.
[4] F. Andreatta, L. Fedrizzi, J. Rodriguez, M. Mouanga, A. Lanzutti, M.G. Olivier, Corrosion protection by zinc-magnesium coatings on steel studied by electrochemical methods, (2018) 16-17. doi:10.1002/maco.201810554.
[5] J. Rodriguez, M. Mouanga, A. Roobroeck, D. Cossement, A. Mirisola, M.G. Olivier, Study of the inhibition ability of benzotriazole on the Zn-Mg coated steel corrosion in chloride electrolyte, Corros. Sci. 132 (2018) 56-67. doi:10.1016/j.corsci.2017.12.025.