Effect of localized corrosion on welded steel joints

Abstract

With steel structures placed in ever increasing depths in the oil industry, inspection of the structures are increasingly more challenging. Loss of coating on a weld exposes the weld to a corrosive environment, where the lack of cathodic protection will result in harmful corrosion on the weld. Localized corrosion like pitting corrosion results in small pits which acts as cracks in the surface. Corrosion in combination with low cycle fatigue can result in catastrophic failure of a structure prior to the designed fatigue life of the structure.

This paper is based on the assumption that corrosion has an effect on the fatigue life of a welded steel joint. To investigate this assumption further an experiment was designed. The experiment was designed to check if localized corrosion had an effect on the fatigue life of a welded steel joint when subjected to loads in the plastic range. In cooperation with Worley Parson Rosenberg AS 15 steel members were prepared. Two different corrosive environments were chosen. The specimens were subjected to low cycle fatigue tests on three different stress levels. Specimen surface and fracture surface were investigated using a scanning electron microscope. The results of the fatigue testing initially showed that the specimens that were subjected to corrosion failed at lower cycles than the non-corroded specimens. This was disproven by the four corroded specimens that were tested later. Though the fatigue test was inconclusive with regards to number of cycles to failure, the investigation of corroded surface with the scanning electron microscope showed promising results with regards to pitting corrosion. The pictures obtained from the microscope showed over 20 individual pits formed over a period of four weeks. This shows the rapid formation and initiation of pits in the surface, and it is reasonable to state that the effects of this type of corrosion will play a significant role to estimated fatigue life on a welded steel joint.