ABSTRACT

Longitudinally welded linepipe can be more susceptible to localized corrosion than seamless linepipe due to chemical composition gradients between the weld and base metal, resulting in galvanic attack. While electric resistance welding (ERW) is an autogenous process that eliminates these chemical gradients, some ERW linepipes still experience preferential weld corrosion (PWC) in CO2 environments. The ERW seam heat treatment causes several chemically similar yet microstructurally different zones to be present in the weld region. Multi-channel zero resistance ammetry performed inside a stirring autoclave showed that while initial galvanic interactions between these different microstructural zones were low, strong galvanic couples developed as corrosion product layers grew. Both of the commercially produced X42 ERW linepipes investigated were galvanically attacked at a region where seam heat treatment had locally modified cementite morphology. Localized attack was more severe at higher temperatures and CO2 partial pressures. Scanning electron microscopy revealed these different microstructural zones formed morphologically different corrosion product layers. This new approach to assessing PWC susceptibility showed seamless linepipe forms uniform corrosion product layers, with galvanic interactions 1 to 2 orders of magnitude lower than ERW.

INTRODUCTION

The presence of dissolved carbon dioxide in production fluids poses a serious risk to oil and gas asset integrity. Literature suggests approximately 46% of material failures in oil and gas are related to CO2 or preferential weld corrosion (PWC) 1-3.

It is widely accepted that PWC in saline CO2 environments occurs from galvanic effects 4-11. Differences in microstructure and chemical composition between the weld metal (WM), heat affected zones (HAZs), and base metal (BM), can establish galvanic couples that lead to localized attack and premature failure. The use of 1% nickel welding consumables to make the WM cathodic relative to the HAZ and BM is a common way of increasing resistance to PWC 7, 10-13. Such an effort affords the WM a form of cathodic protection, providing the fluid is sufficiently conductive. Over alloying the WM with nickel, or using a nickel welding consumable when the base metal contains Cu and Ni (>0.1 wt%), can make the HAZ anodic leading to preferential attack 13.

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