Investigation on Corrosion of Welded Pipeline Steel Under Disbonded Coating Protected by Cathodic Protection

If tape coating gets disbonded from pipeline surface, an appropriate environment for corrosion can be formed under the disbonded coating. Disbonded coatings are usually tape (Poly-ethylene) coatings and can get disbond in different morphologies. Despite extensive researches on corrosion under disbonded coatings, there are limited studies about the pipeline weld line corrosion and electrochemical condition under disbonded coatings. Therfore in this study, the effect of cathodic protection (CP) on electrochemical and corrosion of weld and base metal of pipeline steel under disbonded coating were investigated. Results showed that due to increase in corrosion rate and production of Fe2+, there was increase in H+ concentraion and drop in pH with increase in distnace respective to the open mouth (OM) of the simulated coating disbondment. Also, significant pitting corrosion was observed at 8-12 cm from the OM where it had CP protection of -750 mVSCE to -810 mVSCE. For welded samples, due to formation of localized galvanic cells despite the insignificant difference in pH and potential distrbution under the simulated coating disbondment, higher corrosion rates in comparsion to base alloy was obsrved. Also change in welding current of similar and dismilar welds only affected the corrosion rate of pipeline steel, and did not affect the corrosion morphology of pipline steel under coating disbondment.