Corrosion Behavior of 20 Steel and 13Cr Steel in Different Content Formic Acid-CO₂ Environment under High Pressure and High Temperature

The corrosion rate and corrosion type of 20 steel and 13Cr steel in simulated well pipe environment (foric acid coexisting with CO₂, 100℃, 1 MPa) were studied by the weight method and analyzation of corrosion product. The effect of formic acid content on corrosion behavior was discussed. The results indicate that the corrosion rate of two steels increased with increase of formic acid content, and the corrosion rate of 13Cr steel was lower than that of 20 steel. The corrosion type of 20 steel was uniform corrosion, and corrosion type of 13Cr steel was pitting. There were FeCO₃ production and carbonate deposition on the surface of 20 steel. The substrate was dissolved continuously in the position covered by FeCO₃ production, and the product was deposited continuously in the position deposited by carbonate. The position with lower corrosion rate was bulged on the surface of substrate. The dissolution of Cr occurred in 13Cr steel and passive film formed on the surface. With increase of formic acid content, the stability and density of passive film were reduced and H⁺ and Cl^- contacted with substrate through passive film which promote the occurrence of pitting.