Formation of black chromate conversion coatings on pure and zinc alloy electrolytic deposits: Role of the main constituents

Preparation of electrolytic zinc coating covered by a chromate conversion coating is of great interest for automobile companies. Apart from the protective properties required, the most important factor in this field is the aesthetic aspect. Principally, black conversion films are now used. The purpose of this study is to understand and explain the mechanism of formation of chromate conversion coatings and how the black colour is obtained. Electrochemical measures showed parameters that influence the kinetic formation of the chromate layers. The use of surface analysis methods (SEM, ESCA, GDOS) showed the role of anions in a chromate electrolyte and the effect of the transition elements in zinc electrodeposits. The chromate him is made of two layers superimposed: a very thin black oxide layer at the zinc/chromate coating interface and a gel type layer at the surface. The oxide layer containing the transition elements is the essential factor for the formation of the black colour. (C) 1997 Elsevier Science S.A.