Corrosion Mechanisms of s13Cr Martensitic Stainless steel in Potassium Pyrophosphate-Based Completion Fluids

Potassium pyrophosphate (K4P2O7) completion fluid exhibits excellent chemical stability in high-temperature environments (up to 150°C or above), making it suitable for deep-well and high-temperature reservoir operations. With its inherently alkaline nature (pH ≈ 9 to 12), the fluid presents a corrosion mechanism distinct from the widely studied sweet and sour corrosion. Phosphate species () act as strong complexing agents, forming soluble complexes with Fe2+/Cr3+ and thereby accelerating metal dissolution, an effect that becomes more pronounced with increasing temperature. To investigate this issue, a series of corrosion tests was performed on s13Cr martensitic stainless steel (s13Cr SS, UNS S41500) in simulated completion fluid environments. In situ electrochemical methods and surface characterization techniques revealed that both increased porosity and chemical composition variation contribute to the deterioration of the passive film. Competitive adsorption of phosphate species weakens the protective capability of the external film, while elevated temperatures promote the formation of Fe3(PO4)2 owing to higher porosity. These processes further intensify -induced porosity and ultimately lead to the degradation of corrosion resistance in s13Cr stainless steel, and subsequently cause the localized corrosion and induced high risk of SCC. The addition of potassium chromate (K2CrO4) effectively mitigates corrosion by enhancing passive film integrity, reducing corrosion rates, and preventing phosphate deposition.