A Zinc and Manganese Phosphate Coating on Automobile Iron Castings

A modified phosphate coating on automobile iron castings was described in this paper. The phosphating bath was modified by adding sodium molybdate. The microstructure of the phosphate coating was remarkable refined as the addition of Na2MoO4 increases to 1.5-2.0 g/L. As a result, the corrosion current of the coatings measured by electrochemical polarization decreases as Na2MoO4 increases, which indicates the increase of the corrosion-resistance of the coatings. The modified phosphate coating was used to automobile castings as an intermediate protect layer before final painting. It was shown that the coating containing molybdate can improve the adhesion of painting onto the automobile casting. Salt spray test and atmospheric corrosion test indicated that the anticorrosion performance of the paint plus phosphate coatings on automobile castings was also improved significantly.