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51314-3847-Chemical Inhibition of Preferential Weld Corrosion in Sea Water Pipelines

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Product Number: 51314-3847-SG
ISBN: 3847 2014 CP
Author: Ray Case
Publication Date: 2014
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In-line inspection followed by Radiographic Testing (RT) of a carbon steel pipeline transporting seawater from an onshore treatment plant to processing facilities revealed weld attack after 27 years of service. Corrosion inhibitor injection was initiated to control the weld corrosion. The effectiveness of corrosion inhibitor to mitigate Preferential Weld Corrosion (PWC) susceptibility was evaluated experimentally taking both the metallurgical condition of the welds and the inhibitor content as independent variables into consideration. The experimental design adopted for the study was focused on simulating the environmental conditions associated with the seawater treatment system that utilizes de-aeration by sparging natural gas containing 1.5% CO2 and 150 ppm of H2S as well as introducing oxidants from hypochlorite treatments for bacterial or biofouling control.The testing was performed by utilizing electrochemical methods (Tafel polarization and electrochemical noise) implemented in multi electrode specimens fabricated from sections of : Parent metal Heat affected Zone (HAZ) and Weld metal taken from each weld tested. This technique allows the study of each component separately as well as evaluates the effect of the galvanic coupling.The experimental results obtained indicate that in welds susceptible to PWC an adequate corrosion inhibitor concentration is required to reduce the incidence of galvanic corrosion caused by shifts in the relative corrosion potentials. However the effect is sensitive to the applied dosage of the corrosion inhibitor where under dosage is shown to actually increase the corrosion rate exacerbating the effect of the galvanic coupling. 
In-line inspection followed by Radiographic Testing (RT) of a carbon steel pipeline transporting seawater from an onshore treatment plant to processing facilities revealed weld attack after 27 years of service. Corrosion inhibitor injection was initiated to control the weld corrosion. The effectiveness of corrosion inhibitor to mitigate Preferential Weld Corrosion (PWC) susceptibility was evaluated experimentally taking both the metallurgical condition of the welds and the inhibitor content as independent variables into consideration. The experimental design adopted for the study was focused on simulating the environmental conditions associated with the seawater treatment system that utilizes de-aeration by sparging natural gas containing 1.5% CO2 and 150 ppm of H2S as well as introducing oxidants from hypochlorite treatments for bacterial or biofouling control.The testing was performed by utilizing electrochemical methods (Tafel polarization and electrochemical noise) implemented in multi electrode specimens fabricated from sections of : Parent metal Heat affected Zone (HAZ) and Weld metal taken from each weld tested. This technique allows the study of each component separately as well as evaluates the effect of the galvanic coupling.The experimental results obtained indicate that in welds susceptible to PWC an adequate corrosion inhibitor concentration is required to reduce the incidence of galvanic corrosion caused by shifts in the relative corrosion potentials. However the effect is sensitive to the applied dosage of the corrosion inhibitor where under dosage is shown to actually increase the corrosion rate exacerbating the effect of the galvanic coupling. 
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The work conducted produced conditions where preferential weld corrosion initiated in uninhibited conditions and was sustained throughout inhibition. Two types of weld metallurgies; one weld consumable with 1% nickel, the second matched the parent pipe.
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