Anodic spark deposition of P, Me(II) or Me(III) containing coatings on aluminium and titanium alloys in electrolytes with polyphosphate complexes

The aim of this study was to characterise the correlation between the molar ratio n=[polyphosphate]/[metal cation] in aqueous electrolyte and the growth parameters, composition and surface distribution of elements of anodic coatings on aluminium and titanium alloys. Coatings were formed in electrochemical cell using the anodic spark deposition method. The two-component electrolytes contained sodium hexametaphosphate (Na6P6O18) or sodium tripolyphosphate (Na5P3O10) and acetates of Mg(II), Zn(II), Ba(II), Mn(II), Ni(II), Pb(II), Y(III), Eu(III) were used. Coatings were characterised by electron probe microanalysis, laser mass spectrometry and X-ray diffraction. It was shown that the n value essentially affects the composition and thickness of coatings when lower than a certain value n0, which lies in the range from 2.5 to 10 depending on the metal cation in the electrolyte. Coatings formed in electrolytes of n<1 consist mostly of electrolyte species. The influence of n on composition and parameters of the coatings is explained in terms of deposition and thermolysis of polyphosphate complexes on the anode surface owing to electric breakdown.