Pitting corrosion in pipeline steel weld zones

Steel pipelines externally exposed to seawater pit more severely in heat-affected weld zones but longer-term quantitative data are scarce. Maximum pit depths and variability are reported for the longitudinal weld in API X56 Spec 5 L grade steel pipeline samples exposed continuously to natural Pacific Ocean seawater for 3.5 years. For the first year maximum pit depths and pit depth variability for the different weld zones were very similar. Both became much greater subsequently, with the greatest increases for the heat affected zone. This is compared with steel composition and grain size and potential reasons for the observations discussed.

► Maximum pit depth for longitudinal welds on pipeline steels is greater in the HAZ. ► Maximum Pitting trend with time does not follow conventional power law relationship. ► Pit extreme value trends are not consistent with the conventional Gumbel distribution. ► Numerous maximum pit depth for the HAZ results from localized anoxic corrosion niches.