Corrosion behaviour and mechanism of S135 drill pipe under high-temperature and high-pressure conditions

To investigate the corrosion behaviour and performance degradation of serviced S135 drill pipes under high-temperature, high-pressure CO 2 conditions, weight-loss and constant-load stress corrosion tests were conducted on used drill pipes from the East China Sea and new pipes of the same batch. Tests were performed in an autoclave at 80 MPa total pressure, 6 MPa CO 2 partial pressure, and 200°C. Corrosion products, micro-morphologies, and pitting characteristics were analysed by x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectroscopy, and white light interferometry. Results show that the used drill pipe exhibits 25.9% higher uniform corrosion rate and 90.7% higher pitting rate than the new pipe. Its yield strength, tensile strength, and elongation after fracture decrease by 12.9%, 10.8%, and 11.4%, respectively. The synergy of high temperature, high pressure, corrosive media, and constant load destroys the passive film on the used pipe surface, inducing pitting and microcracks, leading to simultaneous degradation of corrosion resistance and mechanical properties. The novelty lies in the innovative comparative analysis of new vs. used drill pipes under extreme downhole conditions and the identification of a high-temperature-high-pressure synergistic corrosion mechanism. These findings support the safe reuse of drill pipes as test strings, reducing costs and improving operational safety.