Influence of pH on the Formation, Composition and Protectiveness of Fluoride Conversion Film Deposited on AZ31 Magnesium Alloy in KF Solution

Abstract

In deaerated 0.1 M KF solution at pH 5, pH 7.5 and pH 9, fluoride films were deposited on AZ31 magnesium (Mg) alloy at − 1.4 V vs saturated calomel electrode (SCE). The morphology, phase composition, elemental and compositional depth distribution of different fluoride films and their influences on the corrosion resistance of AZ31 Mg alloy were explored in detail. The results indicated that the films deposited at weak acidic and nearly neutral pH consisted of two layers with amorphous Mg(OH) 2 and MgF 2 as the inner layer, and Mg(OH) 2 , MgF 2 and KMgF 3 crystals the outer layer. The film deposited at alkaline pH was a monolayer structure consisting of amorphous Mg(OH) 2 as the major constitute and MgF 2 as the minor constitute. The protectiveness of fluoride film was intimately related to its composition and structure. The content and crystal regularity of KMgF 3 were the two main factors influencing the protectiveness of fluoride film. The fluoride film deposited at pH 7.5 offered the best protection to AZ31 Mg alloy.