Molybdate based Alternatives to Chromating as a Passivation Treatment for Zinc

Zinc-plated parts are typically passivated with chromate-based solutions to reduce corrosion. Chromates, however, are a cause of environmental concern, for their toxic effects on plants and wildlife, and allergic effects on workers who come in contact with them. A molybdate-based alternative has been developed that can be used to replace chromates in a wide range of applications.

The process has been tested in a project set up by the Danish Government in 1989, at the Centre of Advanced Electroplating (CAG)in Denmark. Procedures used for the alternative process are similar to those used in chromating. The corrosion protection provided by the process on zinc-plated parts is comparable to chromates.

Depending on test conditions, especially pH value, the molybdate/phosphate process was found to be better than chromate at low pH values, equal to chromates in outdoor exposure tests and prohesion tests, but not as good in neutral tests, such as salt spray. This edited version of a presentation from the joint AESF/EAST Session at SUR/FIN(R) '94-Indianapolis, discusses the effectiveness, cost, stability, chemistry, layer composition, and performance of this alternative process.