Effect of changing the silica coating pH on the corrosion characteristics of A6092/SiC/17.5p aluminum metal matrix composite in chloride media

This paper is a continuation to our previous study aiming at optimizing the silica conversion coating treatment for improving the corrosion resistance of A6092/SiC/17.5p aluminum metal matrix composite (AMMC) in Cl^- media. Previous results showed that the corrosion rate and the susceptibility to localized corrosion decreased four times due to barrier properties of the silica layer. The optimum concentration was determined to be 50 g/l. The aim of the current paper is to find the optimum silica coating pH that can provide the highest localized corrosion resistance for AMMC. Corrosion performance, morphology, and electrochemical characteristics of silica conversion coatings on AMMC were examined as a function of silica solution pHs. Electrochemical impedance spectroscopy and polarization testing correlated to macro- and microscopic surface examination and visual inspection. The optimum pH was determined to be ∼12.9 which is the neutral silica solution pH. It was found that this pH value is critical where increasing or decreasing it has a detrimental effect on the corrosion resistance due to formation of less protective or soluble silica compounds. At pH 9, the silica solution turns to "gel". Accordingly, processes active during silica conversion coating treatment are kinetically dependent and strongly influenced by silica solution pH.