Preferential weldment corrosion (PWC) of carbon and low alloy steels used for pipelines and process and piping systems in CO2-containing media has been observed increasingly in recent years. In particular, PWC has been detected on weldments made by the manual metal arc (MMA) process using electrodes containing Ni or Ni plus Cu. With information available and industrial experience to 2000, the mechanisms of this corrosion phenomenon were still unclear. It was not possible to predict the preferential corrosion rate that may be experienced, the exact location of attack (i.e. weld metal or HAZ), or the effectiveness of corrosion inhibitors.

A programme of work sponsored by oil and gas operating companies, together with a UK health and safety regulator was conducted collaboratively by three international research organisations. The aim of this programme was to identify if methods of mitigation and prevention could be found in terms of a better understanding of the corrosion mechanisms, the effects of weldment microstructure and composition, including various small alloying additions, and the role of corrosion inhibitors. The results from this programme have been distilled into guidelines for the prevention or control of PWC, in carbon and low alloy steels systems transporting produced hydrocarbons with entrained water and CO2 but in the absence of H2S. The paper briefly describes the experimental approach and summarises the practical guidance derived from the project. The effects of brine conductivity, liquid film thickness, temperature, corrosion scales, and corrosion inhibition are discussed. Appropriate control of inhibition was established as the most reliable means of avoiding PWC. PWC should be considered during selection of inhibitors, since the inhibitor performance may vary between the sections of the weld, and inadequate inhibition may, in some cases, increase selective corrosion rates. Recommendations on inhibitor testing, selection and deployment to avoid PWC are given.

You can access this article if you purchase or spend a download.