Phosphate-silicate composite coating formed on AM60 magnesium alloy

Abstract

A phosphate–silicate composite coating was investigated for AM60 magnesium alloy. The coating was achieved in a phosphate–permanganate solution which involved sealing in sodium silicate solution. The morphology, composition and corrosion resistance on the coatings were analysed. The phosphate–permanganate conversion coating was shown to exhibit network feature and plenty of cracks. The coating containing increased Mn element was produced using acetic acid to adjust the pH of the phosphating bath. The corrosion resistance of the coating was improved. The phosphate–silicate composite coating became more compact. The cracks on the phosphate–permanganate conversion coating were filled in by silica gel through sealing. The main elemental compositions of the phosphate–silicate composite coating were Mg, O, K, P, Mn and Si. The electrochemical polarisation curves demonstrated that the phosphate–silicate composite coating provided more effective protection against corrosion than that of phosphate–permanganate conversion coating and traditional chromate conversion coating.