Mechanism of Automotive Steel Corrosion in Salt Spray, Scab, Filiform and Atmospheric Conditions

THE LOCI OF CORROSION PROPAGATION AND PAINT DELAMINATION in various types of automotive steels have been determined. Several test conditions were compared. In salt spray corrosion of phosphated steel the alkali stability of the phosphate crystals is an important parameter. An important property of the paint film is the retention of adhesion under wet, alkaline conditions. In scab corrosion selective dissolution of Zn₃ (PO₄)₂ is observed, initiated at the phosphate-coating interface. In filiform corrosion the stability of the phosphate-metal interface is decisive. The locus of corrosion in phosphated galvanized steels is different. In electrogalvanized steel the phosphate crystals delaminate from the metal. In hot-dip galvanized steel corrosion propagation is along the zinc-steel interface. In electrodeposited zinc-nickel coatings microcracking was observed and release of the alloy from the steel. A good correlation was observed between the corrosion in the cyclic and atmospheric tests.