Investigation of the Corrosion and Passive Behavior of HP-13Cr Stainless Steel in Formate Annulus Protection Fluid

The corrosion and passive behavior of HP-13Cr stainless steel (HP-13Cr SS) in formate annulus protection fluid was investigated. HP-13Cr SS exhibited good passive behavior in clean formate annulus protection fluid, which was attributed to a thinner and more dense passive film mainly composed of Cr₂O₃. In the formation water solution, the passive film was composed of metastable Cr(OH)₃, which was explained by the isoelectric point theory, resulting in the deterioration of the passive behavior of HP-13Cr SS. When the formation water penetrated the formate annulus protection fluid, a large number of loose FeCO₃ particles formed in the corrosion scales, thus HP-13Cr SS suffered severe corrosion. Therefore, avoiding formation water penetrating the formate annulus protection fluid is conducive to improving the service life of HP-13Cr SS oil tubes in extremely aggressive environment.