Corrosion Protection Fundamentals of Metal-Finished Steel Sheets for Automotive Applications

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.