海洋大气环境中铝合金亚稳态点蚀形核长大与缓蚀机制

Intergranular corrosion and stress corrosion cracking of aluminum alloy induced by pitting corrosion could severely damage the security of aircrafts. In the oceanic atmosphere, the deposition of salt on the surface of aluminum alloy and the following formation of thin electrolyte layer (TEL) may destroy the protective film, resulting pitting corrosion. However, systematic recognition was inadequate about the initiation and stabilization of metastable pits of aluminum alloy beneath TEL. This project aims at studying the mechanism of initiation, stabilization and rehabilitation of pitting corrosion of AA6063 alloy without external polarization based on the fine analysis of electrochemical noise transients. In addition, we try to clarify the catalytic dissolution of rare earth conversion film and the following rehabilitation of pits by the dissolved rare earth elements. The initiation and growth of metastable pits dependence on the diffusion, migration, and competitive adsorption between Cl- ions and inhibitive species inside pits are also emphasized via micro-Raman and ion-selective fluorescent probe. The time dependence of the initiation rate, lifespan and the integration of current noise of metastable pits are related to the multivariate interactions, such as surface damages, metallurgical components, corrosion products and stress. Eventually, we expect to develop a critical threshold for the stabilization of metastable pits influenced by multiple factors. The accomplishment of this project is expected to illustrate the dynamic mechanism of pitting nucleation and growth of aluminum alloy, but also to provide a new technique for the atmospheric corrosion monitoring and quantitative evaluation of pitting corrosion inhibitors.

由点蚀引发的铝合金晶间腐蚀和应力腐蚀开裂会严重降低航空器的服役安全性。海洋大气中的盐粒沉降与薄液膜的形成，可破坏铝合金表面的保护膜而导致点蚀，然而当前对铝合金亚稳态点蚀萌发与稳态转变仍缺乏系统认识。本课题以AA6063铝合金为基材，借助电化学噪声峰形态分析和丝束电极技术，于自然状态下研究薄液膜下铝合金表面亚稳态点蚀诱导形核、稳定化转变或再钝化过程；弄清铝合金表面稀土转换膜的溶蚀及溶出物抑制蚀点萌生与长大的规律。采用光纤式荧光探针和显微激光拉曼，研究Cl-离子与缓蚀剂粒子在蚀孔内的扩散、迁移和竞争吸附规律；探讨第二相组织、表面损伤、应力和腐蚀产物等因素对亚稳态蚀点的形核速率、平均寿命和积分电流的影响规律。通过软件仿真，建立多因素耦合下铝合金亚稳态向稳态点蚀转变的判别准则。研究成果将有助于阐明铝合金点蚀生长动力学机制，并为铝合金的大气腐蚀监测和局部腐蚀缓蚀剂的定量评测提供新思路.

铝合金广泛应用于建筑、汽车、电子及航空航天工业等领域。而铝合金大气腐蚀尤其是海洋性大气腐蚀极为普遍。此外铝合金结构件还会受到应力和环境的协同腐蚀作用，导致铝合金发生应力腐蚀开裂(SCC)。因此，研究海洋性高湿度环境下铝合金的局部腐蚀以及SCC腐蚀机理和抑制方法变得十分重要。.本项目采用大气腐蚀模拟箱和电化学方法研究了NaCl薄液膜下AA6061铝合金的局部腐蚀行为。通过研究不同湿度条件下阴极极限扩散电流密度随NaCl液膜厚度的变化规律，探索了液膜厚度对氧扩散行为的影响。发现液膜越薄，溶解氧穿过薄液膜的扩散速率越快，因而在阴极极化下表现出更大的氧还原电流。采用铝合金阵列电极研究了薄液膜下Ce3+离子的扩散行为，发现无Ce3+时，随着潮解时间延长，中心阳极区腐蚀产物的堆积形成了较大的IR降，反而降低了该区域的腐蚀倾向。而盐粒外围腐蚀产物较少，腐蚀反而逐渐趋于严重，表明盐粒潮解后期，发生了显著的极性反转。.通过研究AA7A04铝合金恒应变C型环试样在3%NaCl溶液中SCC裂纹的萌生和扩展过程，发现裂纹萌生和发展会导致明显的规律性电流与电位噪声。通过对噪声峰的峰形及寿命分析，发现裂纹的萌生与成长表现出明显的阶段性。试样在弹性变形阶段以点蚀为主；当拉伸应力过了屈服强度后，基线电流出现较大波动，此时试样进入裂纹快速扩展阶段；当拉伸应力过了最高拉伸强度后基线电流波动幅度减小，电流特征峰消失，此时试样进入了快速断裂阶段。.总之，通过本项目的研究，取得了如下重要认识： .1）.含盐薄液膜下铝合金表面比溶液中更容易产生点蚀并堆积了更多的腐蚀产物，而当NaCl溶液或薄液膜中含有Ce3+时，溶液和薄液膜下铝合金的腐蚀规律正好相反, 同样浓度Ce3+下，薄液膜中相比溶液中铝合金表面的亚稳态点蚀更容易受到抑制；.2）.电化学噪声可以灵敏反映SCC裂纹的萌生、发展以及淬灭过程。弹性变形区的噪声发生与亚稳态点蚀生长过程有关，而屈服阶段的噪声则与裂纹扩展过程有关。分子半径越大的缓蚀剂越难扩散到裂纹尖端，无法对抑制裂纹扩展；而半径较小F-可以渗透到裂纹前端而抑制裂纹扩展。