EFFECT OF STEEL SURFACE-COMPOSITION ON THE STRUCTURE AND QUALITY OF PHOSPHATE COATING

The quality of phosphate coatings deposited on steel surfaces vary qui te widely depending on the surface condition of the substrates. A syst ematic study of a set of commercial steel samples with essential simil ar bulk composition, reveals that wide variation in coatings morpholog y may be attributed to the differences in steel surface composition vi z the amount of surface carbon and iron oxides. The ratio of surface i ron and carbon seems to be a reliable index in grading steel sheets wi th respect to phosphatability. XPS study indicates that the surface Fe /C ratio decreases systematically from a value of 0.41 to 0.15 for the steel samples with best quality phosphates coating to those with poor quality phosphate coating. The morphology of phosphate coatings was d etermined by scanning electron microscopy (SEM) and the bulk compositi on of steel samples was determined by vacuum emission spectroscopy.