无外表面的腐蚀缝隙内pH值的分布

THE DISTRIBUTION OF pH WITHIN CORROSION CREVICE WITHOUT AN EXTERNAL SURFACE

用设计的无外表面的缝隙连续测试了LC4铝合金在不受力情况下,在100ppm和3.5％NaCl溶液中,在自腐蚀电位及不同极化电位下,pH值随时间变化和沿缝深的分布。结果表明:自腐状态下,在100ppm NaCl溶液中,缝隙内pH呈碱性。在3.5％NaCl溶液中,腐蚀缝隙内靠近缝隙口的pH呈酸性,而远离缝隙口处的pH呈碱性。在100ppm NaCl溶液中,对缝隙口进行阳极极化时,靠近缝隙口经2h后,pH值由6.8降到约2.5呈酸性;而远离缝隙口pH值由6.8降到约4.5,随后增加到约8呈碱性;阴极极化时缝隙内pH值升高呈强碱。该研究结果与铝的电位-pH图相吻合。

The distribution of pH along the crevice and its change with time for the aluminium alloy LC4 in 100ppm and 3. 5%NaCl solution at the free crrosion potential ad various polarization potentials were measured continuously by using simulative crevice without an external surface. Results showed that the pH value near the mouth of crevice is acidic but that at the remote end of the crevice turns to be mildly alkaline in 3. 5%NaCl solution. The pH value in the crevice is alkaline in 100ppm NaCl solution at free corrosion potential, when the anodic polarization potential is applied to the crevice mouth,the pH value near the crevice mouth decreases from 6. 8 to 2. 5 after about two hours. That at the remote end of the crevice reduces from 6. 5 to about 4. 5 and then increases slowly to about 8. When the cathodic polarization potentials are applied to the crevice mouth,the pH value in crevice is alkali. Result of the study is coincident with the potential-pH diagram of aluminium.