工艺参数对AZ31镁合金磷化膜耐蚀性能及表面形貌的影响

以磷酸二氢锰和无氟添加剂为主要成分，通过化学沉积的方法在AZ31镁合金表面获得了均匀且无氟、无镍和无铬的磷化膜。采用硫酸铜点蚀测试、扫描电镜及电化学极化曲线表征手段，详细地研究了成膜温度、游离酸及酸比工艺对AZ31镁合金磷化膜耐蚀性能及表面形貌的影响。结果表明：在成膜温度95℃，游离酸FA 4-5， 酸比TA/FA 15~20的条件下，可获得晶粒<20 μm的致密磷化膜，耐CuSO₄点蚀时间>5 min。磷化AZ31镁合金的自腐蚀电位比未处理基体正移110 mV，自腐蚀电流密度降低3个数量级。成膜温度<75℃时，不能得到完整的磷化膜；成膜温度≥75℃时，随着成膜温度的升高，磷化膜颗粒得到细化，膜层更加致密，进而有效地抑制AZ31镁合金的阳极溶解和阴极析氢，提高了耐蚀性能。但升高成膜温度，加速磷酸盐的水解，容易产生大量的磷化渣，而游离酸的控制，能够有效减少磷化渣的产生，降低生产成本，提高膜层质量。

EFFECTS OF PARAMETERS ON SURFACE MORPHOLOGY AND CORROSION RESISTANCE OF PHOSPHATE FILM FOR AZ31 MAGNESIUM ALLOY

A phosphate solution, free of chromate, fluorides and nitrite, was prepared to form the corrosion resistance conversion film on the AZ31 magnesium alloy surface via chemical deposition, and the effects of film-forming temperature and free acid on surface morphology and corrosion resistance of the film were investigated via CuSO₄ pitting corrosion test, scanning electron microscopy and potentiodynamic polarization curves. The denser film with grain of 20 μm, whose time of CuSO₄ pitting corrosion resistance is longer than 5 min, is obtained on the following process: film-forming temperature 95℃, FA 4~5, ratio of TA/FA 15~20. The E_corr with the film is shifted positively 110 mV(vs. SCE), the I_corr is decreased approximately by three orders compared with that of the bare substrate AZ31 respectively. The results show also that the existence of the film has a great inhibitive action on anodic dissolution and restraint action on the cathodic hydrogen evolution. The dense film is not obtained when the film-forming temperature is lower than 75℃, but increasing with the film-forming temperature, the grain is refiner and the film is denser, which can improve corrosion resistance performance of AZ31 magnesium alloy. Unfortunately, higher film-forming temperature is easy to accelerate hydrolysis of the phosphate and cause plenty of residues phosphide. But the decreasing of residues phosphide can be effectively controlled by controlling the adding amount of the free acid, which further improve quality of the film and decrease the cost.