MOLYBDATE-BASED ALTERNATIVES TO CHROMATING AS A PASSIVATION TREATMENTFOR ZINC

Zinc-plated parts are typically passivated with chromate-based solutio ns to reduce corrosion. Chromates, however, are a cause of environment al concern, for their toxic effects on plants and wildlife, and allerg ic 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 proj ect set up by the Danish Government in 1989, at the Centre of Advanced Electroplating (CAG)in Denmark. Procedures used for the alternative p rocess are similar to those used in chromating. The corrosion protecti on provided by the process on zinc-plated parts is comparable to chrom ates. Depending on test conditions, especially pH value, the molybdate /phosphate process was found to be better than chromate at low pH valu es, equal to chromates in outdoor exposure tests and prohesion tests, but not as good in neutral tests, such as salt spray. This edited vers ion of a presentation from the joint AESF/EAST Session at SUR/FIN(R) ' 94-Indianapolis, discusses the effectiveness, cost, stability, chemist ry, layer composition, and performance of this alternative process.