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.
Background to PWC
The location and morphology of PWC is influenced by a complex interaction of many parameters including the environment, the parent steel composition, the weld metal composition and the welding procedure. Changes in any one of these parameters may cause a significant difference in the weldment corrosion behaviour.
Preferential corrosion of carbon steel weldments can be caused by galvanic effects due to compositional differences between the weld metal, the parent steel and the heat affected zone (HAZ) caused by the welding process1.
Studies have shown that weld metals with harder microstructures; largely unrefined ferrite with aligned second phase root microstructures, tended to suffer corrosion.2 The welds most resistant to corrosion were found to be consumables without significant alloying additions, whilst the addition of 1% Ni and 1% Si to the weld metal increased corrosion rates, 0.5% Mo or 0.6-0.7% Cr offered no benefit with regard to preventing corrosion.3