铈离子对高强铝合金应力腐蚀开裂的缓蚀作用

基于慢应变速率拉伸实验(SSRT),采用恒电流极化、电化学噪声(ECN)与电化学阻抗(EIS)等方法,研究7A04铝合金在3.5%(质量分数)NaCl水溶液中的应力腐蚀开裂(SCC)行为以及Ce~(3+)对其SCC的缓蚀作用,探讨Ce~(3+)对裂纹孕育与发展过程的抑制机理。结果表明:无论是阳极还是阴极极化,均会促进7A04的SCC倾向,前者增加了裂尖的阳极溶解,后者则加速了裂尖的氢脆效应。Ce~(3+)的加入能延缓7A04的SCC断裂时间,但其有效性仅限于裂纹的萌生阶段。由于Ce~(3+)能够抑制铝合金表面的亚稳态点蚀发育和长大,因而使裂纹的孕育时间显著延长,降低了SCC的敏感性。不过一旦裂纹进入扩展阶段或者试样表面有预裂纹,则由于Ce~(3+)很难迁移到裂纹尖端或在裂尖区难以成膜,不能对裂纹的生长起到有效抑制作用,因而无法降低7A04的SCC发展速率。SEM分析表明7A04铝合金光滑试样SCC主要源于亚稳态或稳态点蚀的诱导作用。

Inhibition of Ce3+ on Stress Corrosion Crack of High Strength Aluminum Alloy

The stress corrosion cracking (SCC) susceptibility of 7A04 high strength aluminum alloy in 3.5％ (mass fraction) NaCl solution and the Ce3+ inhibition of SCC were investigated by slow stress rate test(SSRT),using constant current polarization,electrochemical noise (ECN) and electrochemical impedance spectroscopy (EIS) techniques.The inhibition mechanism of Ce3+ ions on the initiation and propagation of cracking was also analyzed.The results indicate that both anodic and cathodic galvanostatic polarizations can accelerate the SCC of 7A04,the former increases anodic dissolution but the latter accelerates hydrogen embrittlement of crack tip.SCC susceptibility of 7A04 can be reduced effectively by the addition of cerium ions,the fracture time is delayed and slowed down,but only during the initiation other than the propagation stage of cracking.Ce3+ ions can restrain the initiation of metastable pitting on the surface of 7A04 specimen,which therefore increase the induction time of the cracking since that the micro pits are usually the source of cracking.However,once the crack begins to propagate or the specimen is notched,the addition of cerium ions can rarely inhibit the cracking process.This is possibly attributed to that the radius of Ce3+ ion is too large to diffuse into the crack tip or it is hard to form protective CeO2 layer,Ce3+ ion therefore fails to rehabilitate the active alloy at the crack tip and further reduce the SCC developing rate of 7A04.SEM also indicates that the crack initiation of smooth 7A04 specimens is mainly induced by metastable or stable pits.