Black passivation films without hexavalent chromium on top of electroplated layers of zinc and zinc alloys

Typical layers with cathodic corrosion protection for steel are zinc and zinc alloys. These layers are usually chromated and, for special applications, an additional organic or inorganic topcoat is applied. The most attractive finish is either black or clear. For aesthetic reasons, black coatings must be used for parts to be mounted in close vicinity of other black parts. Trivalent clear chromatings with good corrosion protection are nowadays well established in the production lines, but trivalent black chromatings are not. Development efforts were made to identify and overcome the problems and/or to find suitable workarounds. This paper will introduce a complete line of new products for producing black parts with high cathodic corrosion protection. This is: black chromiting on top of acid zinc and on top of alkaline Zn, Zn/Fe and Zn/Ni.