When C-Mn steels are in an environment which causes active corrosion it is often found that weldments are more severely affected than parent material. This is known as preferential weld corrosion (PWC). It has notably been observed in sea water injection systems in North Sea oil and gas production systems, although this is by no means the only situation in which preferential corrosion is observed. The problem arises from the fact that weld metal compositions(which are normally optimised for mechanical properties) tend to be slightly anodic to the parent steel. Therefore, the weld metal corrodes at a higher rate than the parent.
The preferential corrosive attack of weldments can occur for a number of reasons:
- Differences in composition between the weld metal and the base metal can generate a potential difference in certain environments, thus setting up a galvanic cell, leading to corrosion.
- Even if the weld metal is close in chemical composition to the base material, differences in as-welded microstructure could make the weld metal sufficiently different from and even less corrosion resistant than the base metal.
- Microstructural differences between the base metal and as-welded heat affected zones can lead to localised attack of the HAZ.
The causes of weldment corrosion are linked to material (both base and filler) composition and welding conditions. It is widely accepted that corrosion occurs more rapidly in a hardened steel in acidic environments than in a fully tempered steel. Therefore it can be taken that as-welded structures are probably more susceptible to preferential weld corrosion than post weld heat treated structures.
The corrosion of weld metals is further complicated by the presence of deoxidation products, and this is largely dependent on the type of flux used. It is accepted that the use of a basic flux can lead to greater corrosion rates in the weld metal than in a weld made using a rutile flux.
Preferential corrosion of welds usually occurs when the environment in contact with the material has a high electrical conductivity, such as seawater, but can also occur in low conductivity CO2 containing environments. Alterations to the environment, such as the addition of a biocide, can change the corrosion characteristics of a system. For example, a joint may be totally resistant to corrosion in a particular environment, but with the addition of a biocide, the joint may become susceptible to preferential corrosion.
In the case of seawater environments, the occurrence of preferential corrosion is largely influenced by material composition, a possible method of prevention would be to employ consumables containing elements which ensure the weldis more noble than the surrounding steel. The addition of 1% Ni is probably the most common, but additions of Cr, Mo, Cu etc also improve the weldment corrosion resistance. A reduction in the Si content is also considered beneficial.
In recent years it has been found that PWC can occur in some wet CO2 containing systems in the oil and gas industry. A study of this type of corrosion behaviour was conducted as a Group Sponsored Project run by TWI together with CAPCIS Ltd and Inst. for Energy Technology (IFE)(12886 Preferential Corrosion of Ferritic Steels in CO2 Containing Production Environments).
The results of this study showed that the use of 1% Ni consumables did not ensure avoidance of preferential, in fact, it showed that welds made from 1% Ni were more susceptible to PWC than welds made with a matching consumable. The preferential attack of carbon steel weldments in CO2 containing systems was also shown be less in welds with low HAZ hardnesses. The mechanism was shown to be related to the inherent difference in corrosion rate between the weld metal, HAZ and parent metal in a wet CO2 containing environment. In addition, if the water present is of low conductivity and/or if thin films of water are present, any galvanic effects will be small and localised, and will not be sufficient to provide protection for all the weld metal as in the case of a seawater environment. Furthermore it has been found that temperature, flow and the formation of carbonate scales can also influence the corrosion behaviour of weldments in these environments.
The use corrosion inhibitors was shown to mitigate this problem; however, it is recommend that selection of corrosion inhibitors should be carried out using welded samples to ensure effective protection from PWC in addition to general corrosion.
For further information refer to the following documents:
Queen D, Lee C-M, Palmer J and Gulbrandsen E, 'Guidelines for the prevention, control and monitoring of preferential weld corrosion in wet hydrocarbon production environments containing CO 2 ', Proc. of First International Oilfield Corrosion Symposium, SPE, 28 May 2004, Hilton Aberdeen Treetops Hotel, Aberdeen, U.K.
Lee C-M, Bond S and Woollin P, 'Preferential weld corrosion: Effects of weldment microstructure and composition', Paper 05277, Proc. of Corrosion 2005, NACE International.
Gulbrandsen E and Dugstad A, 'Corrosion loop studies of preferential weld corrosion and its inhibition in CO2 environments', Paper 05276, Proc. of Corrosion 2005, NACE International.
Turgoose S, Palmer JW and Dicken GE, 'Preferential Weld Corrosion of 1% Ni Welds: Effects of Solution Conductivity and Corrosion Inhibitors', Paper 05275, Proc. of Corrosion 2005, NACE International.