Fundamental research on corrosion resistance of precoated steel sheets for automobiles.

Abstract

Corrosion behaviors of Precoated sheets for automobiles were studied basically. Weight loss of unpainted specimens, paint adhesion and perforation resistance were measured. Corrosion of unpainted sheet in a salty environment is cathodically controlled. Cathodic reactions are retarded effectively by corrosion products with a low electrical conductivity, such as Zn(OH)2 and ZnCl2•4Zn(OH)2. Al, Mg, Ni, Co or Mn alloyed Zn coated sheets exhibit a good performance because of the formation of above mentioned corrosion products. Paint adhesion depends greatly on the nature of the underlying phosphate layer. During cathodic electrodeposition of Zn coated sheet part of the phosphate film dissolves and gets defects because of high alkalinity. On the other hand, Zn-Fe and Zn-Ni alloy coated sheets have the defects repairing effect, and therefore show a better paint adhesion than Zn plated sheet. Perforation corrosion is caused by the formation of differential aeration cells in several car components. Perforation resistance was evaluated by the tests in a combined corrosive environment using jointed panel specimens. Heavil Zn coated sheets and primer coated sheets demonstrated a high resistance.