Performance of API 5L X65 in a 'sweet' followed by 'sour' environment

摘要

This paper reports the corrosion behaviour of X65 steel in a mixed 1% H_2S (in CO_2) brine after exposure to a pure CO_2-saturated brine at 40°C. The objective of the study was to identify the scales formed and understand their effect on the corrosion performance of X65 steel upon transition from pure sweet to sour conditions. Electrochemical testing indicated reduced general corrosion rate during the exposure to pure CO_2 environment, which is mainly attributed to the formation of protective but porous siderite scale. After the introduction of mixed CO_2/H_2S gas, the corrosion rate decreased further for a short period due to precipitated mackinawite above the existing siderite. As H_2S and brine were diffusing through the pore network in siderite, the corrosion rate exhibited an increasing trend due to the accelerating effect of the formed mackinawite within the intermediate layers and on the steel surface. Owing to the possible development of internal stresses in the newly formed scale, the existing siderite layers were ruptured, leading to the precipitation of more mackinawite and promoting further the dissolution of ferrite into the solution. The estimated corrosion rates indicated that the uniform attack is more widespread than the local attack.