微量Cl^-和温度对7150-T76铝合金电化学腐蚀性能的影响

Effect of Trace Chloride and Temperature on Electrochemical Corrosion Behavior of 7150-T76 Al Alloy

利用开路电位、循环极化法和腐蚀形貌表征研究了微量Cl^-与温度对7150-T76超高强度铝合金电化学腐蚀性能的影响。结果表明:低温低Cl^-浓度的溶液中,铝合金主要发生点蚀;温度升高、Cl^-浓度增大,晶间腐蚀的倾向逐渐变大。微量Cl^-(20 mmol/L)致使开路电位(OCP)显著负移;温度升高OCP逐渐降低,且在60~70℃温度范围内发生突变,表明腐蚀机理发生变化。还分析了循环极化曲线的各个电位和电流密度参数随Cl^-浓度和温度的变化。点蚀转换电位Eptp的出现表明被侵蚀后的铝合金表面的钝化是分步进行的,Eptp随Cl^-浓度的增大逐渐负移。自腐蚀电流密度随温度的升高先增大再减小,而自腐蚀电位逐渐负移,均可归因于高温溶液中溶解氧减少的缘故。此外,也论证了自腐蚀电位和再钝化电位的差值ΔE3(Ecorr-Erep)作为评价局部腐蚀发展程度标准的局限性。

3.5%（mass fraction） Na Cl has been intensively used as an electrolyte for evaluating the corrosion behavior Al-alloys for decades due to the simulated sea water being close to the real service environment, particularly for the carrier- based aircrafts. But this medium has several limitations such as the lack of acid substances and the absence of pitting potentials in polarization curves for some alloys. According to Arrhenius equation, the corrosion rate is promoted by temperature. In fact, one of the most crucial factors in the corrosion of Al alloys is temperature which has a strong impact on the stability and properties of passive films. In the present work, the influence of traceCl^-and temperature on the electrochemical corrosion of 7150- T76 Al alloy was investigated by measurements of open circuit potentials（OCP） and cyclic polarization curves as well as observation of corrosion morphology. The results showed that the localized corrosion of 7150 Al alloy was promoted by the increase of Cl^-concentration and temperature. In the solutions with relatively low temperature and low chloride concentration, pitting corrosion was the main corrosion form,while for higher temperature and higher chloride concentration, the corrosion of the alloy gradually turned to be intergranular corrosion. The presence of pit transition potential Eptpreflects a stepwise propagation of corrosion on very narrow fronts with a sequence of active tip- passive walls.OCP shifts to the negative direction with the increase of Cl^-concentration and temperature, respectively. Moreover, OCP decreased drastically with increasing temperature above 60 ℃, indicating the chan-ge of corrosion mechanism in the temperature range from 60 to 70 ℃. In the medium of0.1 mol/L Na_2SO_4＋1 mmol/L Na Cl, the free corrosion potential decreased, while the free corrosion current density increased firstly and then decreased with the increase of temperature due to less dissolved oxygen at higher temperatures. Differences between potential parameters such as ΔE1（EpitEcorr）, ΔE2（Ecorr-Eptp） and ΔE3（Ecorr-Erep） were determined as criteria to assess localized corrosion. The value of ΔE3（Ecorr- Erep） increased linearly with the increase of Cl^-concentration. However, ΔE3showed a turning point at 70 ℃ because uniform corrosion occurred at 80 ℃ as deduced from corrosion morphology. It can be concluded that ΔE3（Ecorr-Erep） only increases linearly with the corrosion propagation at the first stage of localized corrosion.