Perforated mechanism of a water line outlet tee pipe for an oil well drilling rig

A corroded tee pipe belonging to a 10-in new separator water line outlet installed in an offshore oil well drilling rig was investigated. The configuration of the large corroded pit had the shape of an imperfect horse's hoof with a completely corroded interior and a honeycomb-like cavity. There is a badly corroded pit at the welded seam at the joint connecting the tee pipe and flange. The material strength of the tee pipe meets the requirement of ASTM-A234 Gr. WPR, but its chemical composition does not meet the stipulated requirements. A deformed streamline structure or twin crystal in the ferrite phase can be seen near the surface or sub-surface of the perforated corrosion puncture edge. The micro-hardness is also different from that of the original material. The SEM results show that the puncture appears to have been a mode of quasi-cleavage fractures with secondary cracks along the direction of the crystal grain; thus, the failure has the features of mechanical and chemical corrosion. The corroded surface contains high amounts of O, C, and N, as well as S, Cl, Si, Na, Mg, Al, K, Ti, etc., all of which are corrosion products caused by sea water. The penetrating puncture hole at the turn of the tee pipe is likely the result of cavitation erosion accompanied by chemical/electrochemical corrosion, and the corrosion on the seam connecting the tee pipe and flange is likely the result of electrochemical corrosion.