Erosion-corrosion behavior and failure analysis of offshore steel tubular joint

Abstract

An extensive investigation into the erosion-corrosion behavior and failure analysis of an offshore steel tubular joint has been carried out by using various electrochemical techniques, visual inspection, optical microscope examination, mechanical testing, microstructure examination, chemical analyses, Scanning Electron Microscope (SEM) examination, Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD). The open circuit potential (OCP) was recorded for the T-joint material in 0.6 M NaCl without and with electrolyte erosion. Potentiodynamic polarization scans were used to study and compare the material resistance against synergistic impact of erosion-corrosion and corrosion. An optical microscope was used to compare the morphology of pitting corrosion in the absence and presence of erosion. The change in the pitting potential due to electrolyte impingement has been discussed. Pits are slightly stretched in the direction of erosion because of the impingement and disruption of the passive layer in that direction.Visual inspection resultsshowed that the T-joint is heavily eroded and corrosion products were noted. Hardness variation was noted along one of the inspected directions which indicated non-uniform heat treatment. Accordingly, a slow cooling process during heat treatment or manufacturing process is believed to have occurred based on the fine grains observed. Tensile test and microstructure examination indicated that the material of the T-joint is low carbon steel. Furthermore, the chemical analysis indicated that the material of the T-joint is AISI 1026.The results of the chemical analysis of the corrosion products indicated a sulfur-dominant type of corrosion attack. SEM examination revealed presence of cracks, aluminum and calcium inclusions and sea water is believed to be the working fluid. CFD simulation indicated that the fast flow issuing from the main small inlet created relatively high turbulentflow at the area where erosion took place. The fatigue cracks initiation and growth is linked to turbulence intensity and pressure fluctuations.