Choice of buffer solution for stainless steel OCTG materials at laboratory corrosion test to carry out SSC and SCC resistance

摘要

Corrosion environment in oil and gas well is very stable and can be regarded as equilibrium state. Therefore, pH is stable at each depth from wellhead to bottomhole. Buffer solution should be used for laboratory test to appropriately evaluate corrosion resistance for OCTG materials, and the stability of buffer solution, which corresponds to the capability to keep targeting pH throughout test duration, is important to carry out laboratory corrosion tests. From the viewpoint of buffer stability, this paper reviews a theoretical background of buffer solution for stainless steel OCTG materials at laboratory corrosion test, and then the concept is applied to NACE-TM0177-based solution and their modified solutions: (1)Solution-A-based buffer solution (0.5%CH_3COOH with added CH_3COONa), (2)Solution-B-based buffer solution (2.5%CH_3COOH + 0.41%CH_3COONa with added CH_3COOH or CH_3COONa), (3) Solution-C-based buffer solution with CH_3COOH (0.4g/L CH_3COONa with added CH_3COOH and (4) genuine Solution-C buffer solution (0.4g/L CH_3COONa with added HCl or NaOH), in addition to 1bar-CCVsatutated NaHCO_3 buffer solution and high-pressurized CO_2-satutated solution. It is theoretically shown that the pH in the above buffer solution is based on molarity ratio of [CH_3C00H]/[CH_3C00], and the stability of each buffer depends on magnitude of molar concentration of acetic acid and/or acetate. In other words, buffer solution is more stable in rich-rich pair of acetic acid and acetate than in lean-lean pair. Among investigated conditions, richer-richer pair of Solution-B-based buffer solution is most stable, and genuine Solution-C (recommended in NACE MR0175/ IS015156) is less stable than high-pressurized CO_2-saturated condition which is likely to be expected in oil/gas well. When using genuine Solution-C, stainless steel is evaluated too severely (too conservatively) in some cases, because the buffering stability is too weak at a certain pH to mitigate pH decrease at bottom of pitting corrosion. Buffered solution for laboratory test should simulate high-pressurized CO_2-saturated environment occurring in real oil and gas well, or rather, should be slightly severe condition. At higher pH, the gap in buffering stability is likely to expand between genuine Solution-C and high-pressurized COrsaturated environment. For accurate SSC evaluation, a careful attention should be paid to appropriate choice of buffer solution for stainless steel in terms of simulation for high-pressurized CO_2-saturated environment occurring in oil and gas well.