A chromium-free passivation solution for A356 aluminum alloy die castings was developed based on orthogonal test. The bath composition were determined as follows: hexafluorozirconic acid 1.5 g/L, hexafluorotitanic acid 0.7 g/L, hydrogen fluoride 0.05 g/L, cobalt nitrate 0.1 g/L, allylthiourea 0.2 g/L, cerium(III) chloride 0.1 g/L, and pH 4.0. Uniform, intact, colorless, and transparent passivation films could be formed after treating with the given bath at 25 °C for 120 s, which were able to endure potassium dichromate dropping corrosion up to 200 seconds, and had only 9% of corroded area after a neutral salt spray test for 168 hours.%通过正交试验开发出一种A356铝合金压铸件用无铬钝化配方:氟锆酸1.5 g/L,氟钛酸0.7 g/L,氟化氢0.05 g/L,硝酸钴0.1 g/L,烯丙基硫脲0.2 g/L,三氯化铈0.1 g/L,pH 4.0.采用该配方在温度25 °C下钝化120 s可获得均匀、完整、无色透明的钝化膜,耐重铬酸钾点滴时间可达200 s,中性盐雾试验168 h后的腐蚀面积分数仅为9%.
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