Preferential weld corrosion (PWC) is a well-documented phenomenon. The composition of the weld consumable or the post welding microstructure of the carbon steel parent pipe can contribute towards the attack. PWC can be prevented or mitigated by selection of a suitable corrosion inhibitor. However, uniform inhibitor filming cannot be guaranteed and sensitivity to weld metal composition is possible with some chemicals. Weld metal sensitivity can induce or exacerbate the problem.
The work conducted produced conditions where preferential weld corrosion initiated in uninhibited conditions and was sustained throughout inhibition. Two types of weld metallurgies were evaluated; the principle difference being that one weld consumable contained nominally 1% nickel whilst the second was welded with a parent pipe matching consumable.
The performance of two generic corrosion inhibitor formulations was assessed; a sulphur-containing chemical - a quaternary amine with thioglycolic acid synergist, and a non-sulphur containing chemical, consisting of imidazoline and a phosphate ester. The influence of the functionality of the compounds versus PWC mitigation was examined. In addition, the effect of the precorrosion period on the concentration of corrosion inhibitor required to mitigate corrosion was investigated.
Keywords: Preferential weldment corrosion, matching consumable weld, nominal 1% nickel weld, galvanic current, precorrosion, corrosion inhibition, carbon dioxide.