Phosphate Passivation Solutions Enhanced by Chemical Additives for Treatment of the Hot Dip Galvanized Steel
Abstract
To stabilize the surface of the galvanized steel it is important to apply a passivation treatment during its manufacturing. The general aim of the passivation treatment is to prevent the formation “storage stain” or “white rust”. For many decades, the chromium conversion treatments involving the use of chromic acid containing Cr6+ species have been used for their corrosion protection and adhesion promotion performances. Today all hexavalent chromates have been banned by the EC. Toxicological studies have evidenced the hazardous character of hexavalent chromium. Therefore more economical and environmentally friendly passivation solutions have been studied and used to replace the chrome passivations on galvanized steel.
Phosphate conversion coatings are commonly used on the galvanized steel. They form a thin protective film on steels surface. However, the formation of a stable and uniform coating for corrosion protection remains a challenge. More complexes passivating solutions with chemical activators added are the potential alternatives.
In this paper are presented the studies about such molybdate–phosphate passivation solutions as environmentally friendly alternative for chromium passivates. The results of treatments realized for different treatment times and more compositions were analyzed. Also the corrosion resistance of the passivated samples has been studied.
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