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51318-10522-Role of Microstructure in the Susceptibility of X42 ERW Linepipe to CO2 Preferential Weld Corrosion

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.

Product Number: 51318-10522-SG
Author: R. Muggleton / P. Linhardt / G. Zehethofer
Publication Date: 2018
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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.

Key words: preferential weld corrosion, CO2 corrosion, ERW linepipe, zero resistance ammetry

 

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.

Key words: preferential weld corrosion, CO2 corrosion, ERW linepipe, zero resistance ammetry

 

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