Influence of fluoride ions on the amorphous phosphating of aluminium alloys

The influence of the F− ions containing compounds: NaF, HBF4, H2SiF6, NH4F·HF and their concentrations on formation of phosphate coatings on Al in МоО42− ions containing solutions, as well as protective properties of the phosphate coatings in a 0.5 M Na2SO4 solution have been studied. The studies of film composition by the XPS method have shown, that the phosphate coating consists of the metal phosphates from aluminium alloy and the Mo (IV, V) phosphates, which are formed during reduction of МоО42− ions. The mass of phosphate layer and that of etched metal depend on the nature of fluoride ions, which can be arranged in the following order according to the decrease in strength of their influence: H2SiF6 > NH4F·HF > HBF4 > NaF. Polarization measurements in a 0.5 M Na2SO4 solution and the calculated electrochemical parameters testify that phosphated Al samples exhibit a lower corrosion rate and a higher corrosion resistance as compared to non-phosphated Al substrate.