Investigations on hopeite- and phosphophyllite-containing phosphate coatings on steel

Abstract

A series of phosphate coatings on steel produced by different procedures and containing differing contents of hopeite and phosphophyllite, are investigated by SEM, XRD, XRF on removed coatings, AES, ESCA, and EDX microanalysis. The morphology of the coatings is mainly determined by the procedure employed (spray or immersion). The hopeite/phosphophyllite ratio of the layers depends on the phosphating bath composition and is quantitatively determinable with sufficient accuracy by XRD, XRF on the removed phosphate layers, and AES. Further results are that an immersion time of more than 60 s does not considerably increase the layer thicknesses and that the first-grown crystalline layers on the steel sheets are hopeite-enriched.