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.
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. Attempts to control PWC have previously involved making minor additions of noble metals (e.g. Ni, Cr, Mo, Cu) to the weld consumable in order to make the weld metal cathodic with respect to the adjacent parent pipe and HAZ. This approach has proved successful in preventing preferential weld attack in seawater injection systems where additions of either Ni or Ni and Cu can be used to prevent preferential corrosion providing care is taken to avoid over-alloying, which can induce attack of the HAZ 1
. The use of nickel-containing weld consumables has also been widely adopted in production systems. However, while some companies have satisfactory experience of using nickel-containing consumables, there have been examples of severe preferential attack of the weld metal in sweet environments2.
Studies have shown that PWC in CO2-containing media is influenced by a complex interaction of several parameters including the environment, flow conditions, scaling effects, parent steel composition and welding procedure3-5. No general agreement exists on the role of alloying elements and microstructure in preferential corrosion of welds3.
A recently completed joint industry project investigated the PWC of ferritic steels in CO2-containing environments. This paper reports the findings on the effects of weld microstructure and composition on PWC from the research programme. Two other papers, published in this conference, will focus on corrosion flow loop studies of PWC and its inhibition in CO2 environments6 and PWC of 1%Ni welds as a function of solution conductivity and inhibition7. Based on the findings of the project a set of guidelines for the prediction, control and monitoring of PWC of ferritic steels has recently been published8.
