Susceptibility to Preferential Corrosion of Pipeline Welded Joints in Condensed and Dragged Water Droplet - Part 1

Abstract

Abstract Preferential weld corrosion (PWC) is due to the formation of galvanic cells between the weld metal (WM), the parent metal (PM) and the heat affected zone (HAZ). This work has studied PWC susceptibility in longitudinal and circumferential welded joints of submarine systems and the applicability of corrosion inhibitors to mitigate the corrosive process. DNVGL SAW 450, DNVGL SMLS 450 and low alloy forged, with different nickel, copper and silicon content were tested. Several factors influence PWC susceptibility in welded structures and those selected to be studied were weld joint geometry, PM fabrication process, welding process and welding consumable chemical content. For each welding processes, welded coupons were made with similar heat input. Pipe SAW seam welds of two different plate suppliers and different nickel and copper content were tested. Tests in circumferential weld joints were predicted for 79 different combination of chemical composition: PM chemical composition and manufacturing process; welding processes; chemical composition of welding consumables. Selected welding processes are the most used by offshore industry (SAW, mechanized GMAW and manual GTAW). The root configuration of respective welding procedure specifications produce a desired variation in width geometry. The corrosion tests started with the longitudinal joints and will be further reproduced for circumferential joints. A test procedure has been developed for corrosion evaluation through immersion test, galvanic current measurement (GCM) through zero resistance ammeter (ZRA) and localized electrochemical test through SVET. Two test solutions were considered, simulating condensed and dragged water droplets in order to verify the susceptibility or occurrence of the preferential welding corrosion in the welded joints. The results for longitudinal joints indicated a greater susceptibility to PWC in dragged water than in condensed water droplets and a greater susceptibility of joints with greater anodic potential due to a higher nickel and copper content in the parent metal. A correlation between the corrosion rates obtained in both medium and the moisture contents of gas pipelines will be performed to determine the need for the addition of corrosion inhibitors and to establish the minimum required dosage. A future work will involve circumferential joints and the evaluation of the optimal dosage of corrosion inhibitors.