A field investigation of pitting corrosion of welded pipeline steels

For containment structures, such as pipelines, risers and tanks, the most aggressive type of corrosion attack often is considered to be pitting corrosion since metal perforation can cause leakage of the contents and thus potentially have serious environmental and economic consequences. For welded pipelines there is the complicating factor that the heat-affected zone is known to be more prone to corrosion than either the weld zone or the parent metal. However, quantitative data for this effect are scarce. The present paper reports on new field studies performed at Jervis Bay over a total of 3.5 years of exposure for the welded zone of API X56 Spec 5L grade pipeline steel. The corrosion patterns are reported as well as the increase in corrosion mass loss and maximum pit depth with time. It is shown that the development of pit depth and mass loss with time is consistent with earlier observations for mild steel under immersion conditions. As expected, the data show a considerable difference between the losses and pit depths for the weld metal, the heat affected zone and for the parent metal. The potential reasons for this are discussed.