Corrosion–Fatigue Damage Characteristics and Evolution Mechanisms of Failed Sucker Rods

Sucker rods operate under long-term cyclic loading while being exposed to produced fluids, making corrosion–fatigue damage an important integrity concern in rod-pumped wells. This study applied a field-scale multi-source evidence framework to produced-fluid samples from 16 sucker-rod failure wells and 16 corresponding failed rods from a Daqing oil production plant. Ionic-composition analysis, SEM/EDS, bulk chemical analysis, metallography, axial tensile testing, exploratory fatigue testing, and field rod-string load analysis were integrated. The produced fluids exhibited an HCO3−-dominated ionic background, with average HCO3−, Cl−, and SO42− concentrations of 345.7, 0.129, and 2.55 mg/L, respectively. Representative fracture surfaces contained corrosion pits, corrosion products, secondary cracks, and lamellar or step-like features, while local EDS analyses showed substantial O, together with detectable Cl and S. Residual tensile strength varied markedly among rods with different service histories. The exploratory fatigue dataset exhibited a non-monotonic stress–life response and, with only one specimen per source-rod/loading-level combination, was insufficient to establish a conventional S–N relationship or fatigue limit. For well X9-D2-149, the actual fracture location did not coincide with the position of maximum calculated nominal axial stress. Integration of the field, microstructural, fractographic, mechanical, and loading evidence supports a representative damage pathway from localized corrosion and pit formation to crack initiation, cyclic propagation, crack coalescence, and final rapid fracture.