Comparison of the nucleation and growth of a phosphate conversion coating on Zn[sbnd]Al and Zn[sbnd]Al[sbnd]Mg coatings under the influence of a corrosion inhibitor film

Abstract

In this study a film of Ca-sulfonates, often found in chemical post-treatments as a corrosion inhibitor, is deposited on ZnAl and ZnAlMg coatings by dip coating and subsequently cleaned under the usage of an alkaline cleaner. The cleaned zinc coatings are activated and phosphated in comparison to the respective layer system without a prior applied Ca-sulfonate film. The surface analysis shows less efficient cleaning behavior of the Ca-sulfonate film on ZnAlMg. This behavior is related to the high polarity of the Mg-rich oxide layer of ZnAlMg. The kinetics of the conversion coating are influenced by remains of the Ca-sulfonate film leading to varying distances of crystal nuclei and the formation of inhomogeneous crystal morphology. It is concluded, that the strength of interaction between surface active corrosion inhibitors and the oxide layer of the coating is relevant for the outcome of a cleaning step and consequently for the conversion coating. When this is applied to the automotive processing of hot-dipped galvanized sheet steel, the results indicate that zinc coatings with different alloying elements may also show different cleaning properties when coated with temporary corrosion protection products, which in turn holds the potential of affecting the outcome of the phosphate conversion coating.