The alkaline stability of phosphate coatings II: in situ Raman spectroscopy

Abstract In situ Raman spectroscopy was used to study the dissolution of phosphate layers on zinc coated steel in dilute sodium hydroxide solution. The Raman intensity of the symmetrical stretching vibration of the phosphate anion was used to quantify the relative amount of phosphate on the surface. During exposure to an alkaline electrolyte, the intensity was found to decrease in a near linear manner, indicative of the removal of the phosphate anion. Crystal layers containing secondary ions Mn 2+ and Ni 2+ showed an improved stability compared to monocation zinc phosphate layers. The formation of amorphous Mn-hydroxide was observed on phosphate layers containing manganese.