Resolving White Spotting Problems (NUBBING) on Electrogalvanised Metal During Metal Pretreatment

During the chemical surface treatment of electrogalvanized metal, the formation of white spots or “nubs” can seriously affect the smoothness of the cathodic electropaint, necessitating sanding prior to the final application of top-coat. Contrary to common belief, these white spots are not always produced in the zinc phosphate stage of the metal pretreatment process. They are frequently produced from the interaction of the electrogalvanized surface with the cleaners used in the body shop, pre-wipe stations and alkali cleaning stages prior to phosphating. The chemical nature of the white spots is dependent upon cleaner formulation and their formation is greatly accelerated by surface contamination of microparticulate iron (from metal finishing operations). This paper documents some work to date in resolving such problems in an assembly plant.