Modifications of the phosphate coatings microcrystalline structures induced by dehydration and rehydration processes

Bare and electrogalvanized steel sheets were subjected to phosphatation in three different phosphatation baths. The aim of the work was to characterize and to compare the behaviour of pure hopeite, phosphophyllite and manganese modified hopeite crystals during dehydration and rehydration processes. Scanning electron microscopy, thermal analysis and X-ray diffractometry were employed to study the obtained phosphate coatings. Thermogravimetric analysis showed that all the considered phases underwent the total loss of two water molecules above 150° C. X-ray diffraction measurements showed the formation of bihydrated phases which appear very similar independently from the use of the phosphatation bath. After exposure to a hydrating environment only the pure hopeite bihydrated crystals underwent a rehydration process which led to the tetrahydrated phase and consequently to a decreased paint-phosphate adhesiveness. On the contrary both the bihydrated phosphophyllite phase and the bihydrated manganese modified hopeite phase revealed their inability to return to the completely hydrated crystal and consequently improved paint adhesion.