2024铝合金氟铝酸盐转化膜的制备及其防腐蚀性能

为了寻找替代传统铬酸盐转化的处理工艺,采用由NaF,(NH4)2SiF6,(NaPO3)6和钛盐促进剂组成的转化液,在2024铝合金表面制备了一种氟铝酸盐化学转化膜,优化了转化液组分及转化工艺条件。结果表明:最优工艺为5.0 g/L NaF,5.0 g/L(NH4)2SiF6,0.9 g/L(NaPO3)6,0.5 g/L钛盐促进剂,pH值为4.7,室温,20 min;最优工艺所得氟铝酸盐转化膜由排列紧密且形状规则的晶体颗粒组成,表面覆盖有胶状物,膜层连续而致密、呈亚光,组成(质量分数)为7.53%O,48.87%F,19.11%Na,20.78%Al,0.79%Si,1.66%P,1.26%Cu;氟铝酸盐转化膜耐蚀性优良,最优工艺所得转化膜耐盐雾腐蚀达285 h,其使铝合金在3.5%NaCl溶液中的腐蚀电位增加了58 mV,腐蚀电流密度降为钝化前的1/9。

Preparation of Fluoroaluminate Conversion Coating on 2024 Aluminum Alloy and Evaluation of Its Corrosion Resistance

Chromium-free conversion coating was prepared on the surface of 2024 Al alloy using chemical conversion solution containing NaF,(NH4)2SiF6,(NaPO3)6 and inorganic accelerator.The composition of the conversion fluid and the conversion condition were optimized.It was found that the optimized coating was obtained when Al alloy immersed in conversion fluid consisted of 5.0 g/L NaF,5.0 g/L(NH4)2SiF6,0.9 g/L(NaPO3)6 and 0.5 g/L titanium-salt accelerator with a pH value of 4.7 at room temperature for 20 min.The conversion coating obtained under the optimized condition consisted of densely packed crystallites with regular shapes and was continuous,compact and matt,while its surface was covered by gel-like substances.Besides,the elemental composition(mass fraction) of the conversion coating was determined to be 7.53% O,48.87% F,19.11% Na,20.78% Al,0.79% Si,1.66% P and 1.26% Cu.Moreover,as-prepared fluoroaluminate conversion coating was able to increase the corrosion potential of the Al alloy by 58 mV and reduce the corrosion current density to 1/9 of that before passivation,showing good corrosion resistance(it was able to endure neutral salt-spray for up to 285 h).