| The service environment of sucker rod was getting worse and worse with an increase in mining life and the sucker rod fracture accidents occurred frequently during their service period, causing serious economic losses. Field investigations found that the fracture of sucker rods was mainly caused by their corrosion fatigue. Therefore, in order to reduce the fracture of sucker rods and find appropriate protection methods, it is necessary to understand the corrosion fatigue behavior of sucker rods.In this paper, in a simulated oilfield water at 25 ℃and 60 ℃, the corrosion behavior of7 kinds of sucker rods(GY-KD, GY-D, GY-HL, FRT-D, FRT-KH, JH, JYL) and two sucker rod joint materials(joint 1, joint 2), galvanic corrosion accelerated mechanism for the localized corrosion of sucker rods and the corrosion fatigue behavior of four kinds of sucker rods(GY-HL, GY-D, GY-KD, JH) were investigated using weight loss method, polarization curves, electrochemical impedance spectroscopy(EIS), galvanic corrosion test, corrosion fatigue testing machine, environmental scanning electron microscope(SEM) and 3D microscopy methods. The corrosion behavior research show that the sucker rods and the joint materials all show the active dissolution behavior in the simulated oilfield water, and their corrosion processes are controlled by the reduction process of dissolved oxygen(DO).In weight loss experiments, the sucker rods and the joint materials have no obvious local corrosion characteristics. In general, the GY-KD has the best corrosion resistance and the corrosion rate of the joint 1 is less than that of the joint 2. The coating durability assessment results indicated that the coating on sucker rods plays a protective role in the early stage of corrosion, but the non-uniform damage of the coating can cause galvanic corrosion resulting in the accelerated corrosion on the coating failure place. Further more, the time required for the coating dissolution is longer, this galvanic corrosion will be more serious, which may cause the localized corrosion of sucker rods. When oil film covers on the damaged place or on the coating of sucker rods, the coating failure galvanic corrosion is significantly inhibited, but it can persist. The galvanic couple can also be formed between the coated sucker rod and its joint material, in which the joint material acts as anode and the sucker rod as cathode. Therefore, the corrosion of the joint material is accelerated and results in its local corrosion. However, the acceleration effect of this galvanic corrosion is less than that of the coating failure galvanic corrosion. These kinds of galvanic corrosion may be the reason for the local corrosion of the sucker rods. The corrosion fatigue behavior of 4 kinds of sucker rods, including GY-KD, GY-D, JH and GY-HL, was evaluated in the simulated oilfield water at 60 ℃. The results show that the order of the corrosion fatigue life from long to short is GY-KD > GY-HL > GY-D > JH, which is consistent with the corrosion resistance of these materials, i.e. the better corrosion resistance of the sucker rod material corresponds to the longer corrosion fatigue life. Apparently, the corrosion behavior of sucker rods has an important influence on their corrosion fatigue behavior. |
