Preferential Weld Corrosion: Effects of Weldment Microstructure and Composition

Abstract

Abstract Preferential weldment corrosion (PWC) of carbon and low alloy steels used for pipelines and process piping systems in CO2-containing media has been observed increasingly in recent years. In particular, this has been on weldments made by the manual metal arc (MMA) process using electrodes containing Ni or Ni plus Cu. This paper presents the results of a joint industry research programme which was conducted collaboratively by three research organisations to investigate this corrosion mechanism and to seek practical solutions. The effect of composition and microstructure on PWC in CO2-containing media was investigated on 12 weldments produced in X52 and X65 grade pipe materials using TIG and MMA processes. Corrosion tests were conducted in a re-circulating vessel on segmented weld electrodes in CO2-containing media, with two levels of chloride content. The addition of increased amounts of nickel and silicon was detrimental, whilst additions of molybdenum and chromium (of up to 0.7wt%) did not give improvements in PWC behaviour. Autogenous weldments, made without filler additions, and weldments made with matching composition consumables gave the best PWC resistance. It is also shown that empirical relationships exist between PWC and hardness levels and microstructure, with unrefined microstructures, having high hardness, being detrimental. The implications of the data for design of welding procedures to minimise PWC are considered.