Stability of buffer solutions for SSC tests of stainless steel OCTG material by experimental approach

摘要

As an extended report on buffer stability following the previous paper from a theoretical approach in 2016, this paper focuses on the buffering capability of various test buffer solutions from an experimental viewpoint, and presents experimental data supporting the theoretical results, based on the scratch repassivation behavior in electrochemical measurements. In this paper, the stability of buffer solutions for martensitic stainless steel OCTG material, which corresponds to the ability of the solution to maintain the targeted pH throughout the test duration, was experimentally evaluated by the scratch repassivation technique in electrochemical measurements. The measurements were carried out by using a specially-designed electrochemical apparatus with a sharp-pointed crooked dental scaler to make scratches on the surface of stainless steel OCTG material samples in 1 bar CO_2-saturated buffer solutions. While rest potential is being measured, a short linear scratch corresponding to artificial pitting corrosion is intentionally applied. The potential drops at the moment of scratching, and then returns to the original rest potential. The stability of the buffer solution is evaluated by the time required for the potential to return to the original level, that is, the time required to repassivate the scratched area. A series of scratch-based electrochemical tests were carried out at pH 3.0, pH 3.5 and pH 4.0 to determine the stability of four kinds of NACE-TM0177-based buffer solutions. The experimental results support the phenomena of buffer stability from the theoretical viewpoint. A buffer solution with a shorter time for repassivation is consistent with the one with a more stable buffering effect from the theoretical approach. Precisely, a rich-rich-pair-based buffer solution, which is composed of a highly-concentrated weak acid and highly-concentrated conjugated base, has greater buffering stability than a lean-lean-pair-based one, which is composed of a dilute weak acid and a dilute conjugated base.