Phosphate conversion coatings on steel

Abstract

A mechanism for the formation of crystalline phosphate conversion coatings on steel is proposed from the results of this investigation. X-ray diffraction results identify the crystal types formed from both Zn and Mn phosphating baths. The effect of process parameters, such as metal surface finish and soluble iron content of the phosphate bath, on crystal size and surface layering are discussed. Differential thermal analysis shows the water of crystallization of the phosphate precipitates to be driven off at about 325° C with corresponding contraction and fracture of the surface crystal. Corrosion data was determined and correlated with the crystal morphology. Scanning electron micrographs were taken of the various crystal systems produced to confirm conclusions drawn from the data collected.