Résumé :

For the last decades, the development of Zn-Mg coatings has been of a great interest due
to the improved corrosion resistance that they confer to steel substrate compared with
classical galvanization. However, the immersion conditions have been pointed as harmful
due to the high electronegativity of Mg and its pH dependence. The objectives of this PhD
thesis are to complete the understanding of the corrosion mechanisms of Zn-Mg coating
applied on steel in immersion conditions, by considering both sacrificial and barrier
protection perspectives, and also to develop a self-healing coating for its smart corrosion
protection. Firstly, the morphological and electrochemical depth profiles of the coating
were carried out by combining surface analyses and local electrochemical techniques. The
sacrificial protection mechanism was proved to be related to the preferential dissolution of
Mg in the Zn-Mg layer, thus leading to the formation of Mg-rich oxides/hydroxides
contributing to the inhibition of corrosion processes both on the metallic coating and the
steel. Secondly, the corrosion mechanisms of Zn-Mg coated steel in immersion in an
aqueous electrolyte were investigated by focusing on the influence of the initial electrolyte
pH and its modification due to the corrosion phenomena. In immersion in a neutral and
non-buffered solution such as NaCl, Mg dissolves preferentially compared with Zn, thus
involving the buffering of the pH around 10-11 after several h of immersion. Although
insoluble oxides such as Zn5(CO3)2(OH)6, MgCO3 and Mg5(CO3)4(OH)2 are formed when the
pH threshold value is reached, the formation of non-protective ZnO occurs leading to a
critical electrochemical activity of the substrate. An anticorrosion system with self-healing
ability was therefore developed considering the reactivity of the substrate. In a first step, a
screening of the inhibitors was established and benzotriazole (BTA) was unambiguously
proved to be the most efficient compound of the tested panel. An inhibition mechanism of
BTA was therefore proposed based on electrochemical and analytical measurements
combined with surface analyses. The alkalinization of the medium near the metallic surface
induced by the Mg dissolution was assumed to involve the formation of a complex between
Zn2+ and the anionic form of benzotriazole BTA-. This chemical entity reinforces the
corrosion product layer composed of zincite ZnO which precipitates on the surface and
forms a layer with barrier properties. Finally, layered double hydroxides modified with
benzotriazole (LDH-BTA) were synthesized and characterized before to be incorporated in
an epoxy resin. The addition of LDH-BTA in the coating was demonstrated maintaining
reasonable barrier properties in the same time as providing a self-healing ability. That was
explained by the LDH-BTA capability to trap chloride ions and to release benzotriazolate
ions thanks to an anion-exchange mechanism.