Release rates and environmental impact of zinc-nickel coatings in the automotive industry

Sammanfattning: At present the automotive industry is due to an EU directive, replacing hexavalent chrome on vehicles. This is an extensive job as hexavalent chrome is used all over the vehicle and to large extent on fasteners (screws, nuts, rivets etc.). Chrome (VI) is used as a passivating layer on mainly zinc-iron. When replacing the hexavalent chrome with a chrome (VI) free product, the passivating properties are reduced. One of the alternatives is to replace the zinc-iron coating with a zinc-nickel coating. This coating shows great promise from the corrosion resistance point of view. Zinc-nickel is a cathodically protecting coating, and will in principle dissolve to protect the substrate from corrosion. It is therefore important to understand how, and at what rates nickel is released from zinc-nickel coatings when exposed to a chloride-rich automotive environment. The potential environmental impact of nickel needs to be evaluated before Scania can introduce this alternative as corrosion protection. Tests by Scania have previously shown that contact allergy is not an issue for zinc-nickel coatings with Cr (III) passivation. Nickel release rates corresponding to 0,12 mg m-2yr-1 for zinc-15 % nickel coatings at a pH of 4,2 were determined from an accelerated corrosion test. Based on these values, less than 1 kg of nickel per year would be released from the Scania rolling stock if Scania were to introduce zinc-nickel coatings preferably on fasteners. This value is low compared to other sources of nickel release. In order to evaluate the toxicity of the released nickel, information about the chemical speciation, i.e. chemical forms, is needed. Total or dissolved metal are not good predictors of ecotoxicity of metals. Chemical speciation and bioavailability must be incorporated in toxicity testing. Total or dissolved metal may be used as a worst case approximation.