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Water penetration into the paint film of car bodies causes cosmetic corrosion. The circumstances experienced by driven cars can be classified into three groups, corresponding to high, medium and low water migration, respectively. In the first group a lot of water enters into the paint/substrate interface and lifts up the film. The stability of the phosphate layer influences the adhesiveness. In the second group salt water brings about alkaline blister formation. The corrosion resistance of the substrate against alkali in cathodically polarized state determines the paint adhesion. The third group corresponds to the scab corrosion, which can be suppressed well by zinc coating layer.

Corrosion protection mechanism was investigated on newly developed-metal-finished steel sheets for automobile applications. Such alloying elements to zinc as Ni, Co, Mn, Mg and Al retard oxygen reduction reaction because of their stabilizing effect of Zn(OH)2 films, leading to improved corrosion resistance. Wet adhesion of cathodic primed full painted system is governed by alkali-induced dissolution of phosphate and zinc coatings during electrocoating. Fe in coating layer improves wet adhesion because of its high alkali-resistance. Both Fe and Ni have a pore blocking effect on phosphate films, with resultant a superior paint performance. The factors affecting perforation of jointed panels were also discussed.