Improving the corrosion resistance of LY12 aluminum alloy via a novel Mo–Zr–Ti composite conversion coating

Abstract

To improve the corrosion resistance of LY12 aluminum alloy, a novel Mo–Zr–Ti composite conversion coating on its surface was prepared. The surface morphology, elements distribution, phase compositions and the corrosion resistance were systematically determined, and the relation between them were established. The results indicate that the optimal conversion temperature and conversion time are 40 °C and 15 min, respectively. The surface morphology of the composite coating under the best conversion condition is continuous and dense. The major elements are Mo, Zr, Ti, Al, O and F, and their distribution between the inner and outer coating are distinction. The phase compositions mainly consist of MoO _3 , Mo _2 O _5 , TiO _2 , ZrO _2 , Al _2 O _3 , AlF _3 ∙3H _2 O, Na _3 AlF _6 and ZrF _4 , and the nucleation and growth mechanisms are proposed. Additionally, the composite coating can significantly reduce the I _corr and increase the film resistance R _f in the temperature range from 0 to 60 °C through electrochemical test, and its activation energy E _a increases from 25.7 to 34.5 kJ ∙ mol ^−1 . Compared with Al matrix, the average corrosion rate of the composite coating is reduced by 65% to 85% in the full immersion test. Therefore, the composite conversion coating greatly improves the corrosion resistance of Al matrix.

Keywords

• aluminum alloy
• Mo–Zr–Ti; conversion coating
• conversion temperature
• conversion time
• corrosion resistance