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

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Abstract
Sucker rods operate under long-term cyclic loading while exposed to corrosive produced fluids, making corrosion–fatigue damage a major integrity concern. This study examined produced-fluid samples from 16 wells with sucker-rod failures and 16 corresponding failed rods from a Daqing oil production plant using ionic-composition analysis, SEM/EDS, bulk chemical analysis, metallography, axial tensile tests, exploratory fatigue tests, and field load analysis. The fluids were HCO₃⁻-dominated, with average HCO₃⁻, Cl⁻, and SO₄²⁻ concentrations of 345.7, 0.129, and 2.55 mg/L, respectively. Representative fracture surfaces showed corrosion pits, corrosion products, secondary cracks, and lamellar or step-like morphologies, while corroded microregions were enriched in O and contained detectable Cl and S. Local discontinuities, including decarburization, inclusions, and scratches, were also observed. Residual tensile strength varied substantially with service history, and exploratory fatigue results showed considerable scatter. The actual field fracture location did not coincide with the maximum nominal axial stress. The combined evidence supports a representative sequence of localized corrosion and pit formation, crack-susceptible region development, microcrack initiation, cyclic crack propagation, crack coalescence, loss of effective load-bearing area, and final rapid fracture.

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