Scaling investigation of API 5L X65 steel in a 'sweet' followed by a 'sour' environment

摘要

This paper describes the corrosion behaviour of X65 steel in a mixed 1 % H_2S/CO_2 brine after exposure to a pure COrsaturated brine at ambient temperature. 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 continuously increased general corrosion rates, which are mainly attributed to the rapid formation of mackinawite. Mackinawite further promoted the anodic dissolution of iron at the low temperature of 25°C and a pH around 5, which led to the generation and loose deposition of more mackinawite, which was not found to have a protective effect. Cementite remained undissolved as ferrite was preferentially corroded during the first part of the experiment. The formation of mackinawite undermined the cementite by the formation of valleys with mainly uniform appearance and rough surface profiles. These valleys eventually grew further, forming pits. No significant deposition of iron carbonate on the material was observed, due to the combined effect of the presence of H_2S and the low temperature. The estimated corrosion rates indicated that the uniform attack is more extensive and damaging to the steel than the local attack.