Formation of Iron Orthophosphate Colloids and Anodic Films on Iron in Phosphate Solutions

Abstract

Equilibrium diagrams for iron(III)-and iron(II)-orthophosphate systems were developed for the purpose of clarifying the effect of pH and electrode potential on the corrosion and surface treatment of steels in a solution containing phosphate ion. It has been confirmed from both the diagrams and experiments that iron (III) ion is precipitated in the form of FePO4 in an acidic phosphate solution, and that the precipitate changes to Fe(OH)3 with the increase of pH value. A titration experiment was performed to measure the formation constant, K, of iron(II) phosphate colloid. The value of K for the reaction, 3Fe2++2H2POrPe3(P04)2+4W, was found to be 10-7,8. The elec-trochemical conditions for the formation of an Fe3(P04)2 film on iron were investigated with the aid of an E-pH diagram developed for the iron-phosphate system. The anodic Fe3(P04)2 film was formed on iron in a neutral phosphate solution at a potential of active dissolution. The film growth was measured by in situ ellipsometry.