Abstract(s) :

(Anglais) Galvanised steels represent one of the most extensively used types of steel encountered in automotive applications, due to its great set of mechanical properties. Zinc is extensively used as a sacrificial coating in the case of the corrosion protection of steel in a wide range of applications other than the automotive industry, such as building and house hold appliance industries. However, the steel protection brought by the zinc layer is not assured in aggressive corrosive environments, such as NaCl-containing media [1]. In addition, there is a lack of information about the sacrificial protection provided by the Zn-rich layer to the underlying steel substrate: at which extent the corrosion protection can be attributed to a sacrificial effect or to the precipitation of stable Znbased corrosion products over the exposed steel surface [2]? In all cases, in the last twenty years, new families of protective compounds have been tested in order to improve the corrosion resistance of zinc coatings without increasing their thickness or resorting to additional protective layers [3]. Despite the ban of the use of Cr(VI) in European Union since 2000, hexavalent chromate salts have still been employed as corrosion inhibitors for zinc coatings worldwide [4,5]. Hence the actual interest to identify or develop corrosion inhibitors that behave efficiently when the steel substrate is exposed upon mechanical removal of the Zn-rich coating - during transportation and storage of the material, in particular. In this work, the zero-resistance ammeter (ZRA) was applied to study the galvanic corrosion behaviour of a physical model comprising hot-dip galvanised steel (HDG) coupled to mild steel. The ZRA technique is well-known as suitable for studying galvanic corrosion and defining strategies for its control, since relevant semi-quantitative electrochemical information can be extracted. The HDG/mild steel model was designed to simulate the galvanic coupling formed in case of severe removal of the Zi-rich layer and following local exposition of the steel substrate. In this case, despite the cathodic protection of the steel provided by the sacrificial Zn coating, the intense galvanic corrosion process leads to significant consumption of the Zn-rich phases. By employing ZRA, the inhibitive effects of several compounds were evaluated on the HDG/mild steel pair exposed to 0.05 M NaCl solution. Promising corrosion inhibitors, such as sodium molybdate, were shown to significantly reduce the consumption rate of the sacrificial Zn layer. The insights provided on the inhibition mechanisms of galvanised steel corrosion constitute a first step towards the development of highly corrosion resistant coatings.