The Potential - pH Diagrams of AISI 316L Stainless SteelImmersed in the Simulated Underground Waters in ThailandConstructed by the Cyclic Polarization Method

摘要

The present work aims to construct the potential – pH diagrams of AISI 316L stainless steelimmersed in the simulated underground waters in Thailand by the cyclic polarization method.According to the field report, the simulated underground waters were prepared by the additionof 3550-ppm sodium chloride (NaCl), 300-ppm sodium sulfate (Na2SO4), and 3550-ppm NaClmixed with 300-ppm Na2SO4, to the de-ionized water. The solutions were kept constant at 27℃and saturated by laboratory air. Comparing to the potential - pH diagram of this alloy immersedin pure water, the addition of 3550-ppm NaCl to pure water tended to lower the transpassive-potentialline and the protection-potential line down to the less noble direction. The imperfectpassivation regime was observed over the entire range of the tested pH. The addition of 300-ppm Na2SO4 to pure water widened the perfect passivation regime by leveling the protection-potentialline up to the positive direction. Furthermore, the area of imperfect passivation did notexist. When 300-ppm Na2SO4 was mixed to the 3550-ppm NaCl solution, the inhibitive roledue to the addition of Na2SO4 could not be observed. It was further found that the addition ofNa2SO4 tended to lower the transpassive-potential line when the solution pH was basic.Nitrogen gas was bubbled instead of laboratory air in the (3550-ppm NaCl + 300-ppm Na2SO4)solution to investigate the effect of the dissolved oxygen. It appeared that the dissolved oxygentended to lower the protection-potential line and the transpassive-potential line when thesolution was alkaline.