Interrelationship between steel surfaces phosphatability, and corrosion resistance

The phosphatability of a steel surface and, hence, the corrosion protection achieved is related to the quality of that steel surface. It is the intent of this paper to determine what parameters of the steel surface influence phosphatability. This was done by examining the influence of steel surface roughness and contamination on zinc phosphate coating quality, and by determining the relationship of phosphate coating weight and density to the corrosion resistance of painted steel. A high correlation was found between the amount of corrosion creepback of phosphatized and painted steel substrates and the amount of organic carbon present on the surface of the steel. The carbon, analyzed by Auger Electron Spectroscopy, average approximately 50A in depth and is not removed by phosphate precleaning operations. The carbon inhibits the formation and development of phosphate coatings which are required to provide satisfactory corrosion resistance.