【文章內(nèi)容簡介】 case, although design improvements were made, the rod failed in service. A socket was tightened around the pin to reduce the stress concentrations applied on the pin end of the rod. However, the rod broke right at the edge of the socket after a few months of operationin sour service (Fig. 8). The surface of the rupture was examined after acid cleaning. The fracture surface contained the same features as previously discussed: three distinct zones corresponding to the initiation, propagation, and final fracture portion of the failure (Fig. 9). Cracks and corrosion deposits were observed on the initiation zone. Striations and hammering marks were observed in the propagation stage zone. Dimples covered the final rupture zone. The sucker rod failed by corrosionfatigue mode. The corrosion particles were determined to be iron sulfide (xray diffraction) indicating the aggressive nature of the service environment. Microscopy. A transverse cross section through the diameter of the rod showed that crack initiation was followed by the propagation of a nearly straight crack. Small curves were observed as the crack passed through the coating due to the presence of voids in the coating layer (Fig. 10). Microcracks were observed near the surface and a large secondary crack was observed at the end due to the high deformation (Fig. 11). The rod material was of type 8610 and spray coated with a NiCrB eutectic alloy. Since the material was operating in sour service, it should conform to the requirements of NACE Standard MR0176[1].The position of the coating was slightly different than the minimum NACE remendation. [1] Both chrome and boron contents were just over the remended maximum. The hardness of the coating was 63 HRC, while the minimum required[1] is 55 HRC, and the coating thickness was mm while the minimum required[1] is mm.These results lead to the conclusion that the sprayed metal coating conformed to the reference requirements. The main cause of the failure was the stress concentration at the end of the socket where the rod failed. This stress concentration along with the accumulation of abrasive corrosion byproducts, iron sulfide, led to the creation of a physical notch at the surface of the rod. The cyclic load on the rod caused the initiation and propagation of the crack and resulted in a fatigue failure.The protective coating on the surface of the rod increased the corrosion resistance of the rod material but did not prevent the failure. This failure case confirms the fact that both chemical protection of the surface and design must be considered to prevent failure.Fig. 8 Sucker rod broken at the edge of the socket Fig. 9 Surface of the rupture of the sucker rodFig. 10 Initiation stageFig. 11 Propagation and final stage of the rupture3. Environmental Induced FailuresAlthough the sucker rods failed mainly in corrosion fatigue mode, other types of failure of sucker rods are also encountered. The following case describes a sucker rod downhole tubing failure where a long rod string posed of rods of varying diameters was operating in sour conditions with the presence of CO2. ObservationsThe rod string consisted of the rod diameters mm, mm, and mm with the largest diameter rod sections being near the well surface. Two distinct types of surface damage were observed on the rod sections immediately after removal from service:1. Rod sections 1 and 3 (Fig. 12): Large corrosion imperfections aligned along the length of the rod on the same side with no apparent corrosion attack adjacent to the corroded regions.2. Rod section 2 (Fig. 13): Total erosion along the length of the rod. The production tubing revealed severe corrosion attack with pinholes aligned along one side of the length of the tubing. After acid cleaning, the . surface revealed large corroded areas with a flat bottom and stepped ed