Effect of Steel Surface Composition on the Structure and Quality of Phosphate Coating

SUMMARYThe quality of phosphate coatings deposited on steel surfaces vary quite widely depending on the surface condition of the substrates. A systematic study of a set of commercial steel samples with essentially similar bulk composition, reveals that wide variation in coatings morphology may be attributed to the differences in steel surface composition viz the amount of surface carbon and iron oxides. The ratio of surface iron and carbon seems to be a reliable index in grading steel sheets with 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 determined by scanning electron microscopy (SEM) and the bulk composition of steel samples was determined by vacuum emission spectroscopy.