Effect of HCO_3- on the passiviation and pitting behavior of super 13Cr martensitic stainless steel

摘要

Super 13Cr martensitic stainless steel is widely used in oil field downhole corrosion environmentdue to its good mechanical properties, moderate price and excellent carbon dioxide corrosionresistance. In the downhole service environment of oil and gas wells, high salinity formation wateris the main liquid phase corrosive medium. High salinity formation water contains a lot of anionsand cations such as CO_3~2-、HCO_3~-、OH~-、Cl~-、SO_4~(2-)、Ca~(2+)、Mg~(2+)、K~+、Na~+. Theexistence of Cl~-can easily induce pitting corrosion of stainless steel. However, when other anionscoexist in the solution, especially HCO_3~-, the corrosion behavior of Super 13Cr martensiticstainless steel will be greatly affected.The effect of HCO_3~- on the passiviation and pitting behavior of super 13Cr martensitic stainlesssteel in a Cl~- containing borate acid buffer solution was investigated using potentiodynamic andpotentiostatic polarization tests, electrochemical impedance spectroscopy measurements, Mott-Schottky analysis. In addition, scanning electron microscopy was used to characterize the pittingmorphology. The results showed that the passive region of super 13Cr was enlarged withincreasing HCO_3~- concentration, contributing to more positive pitting potential, which indicatedhigher pitting resistance in the presence of HCO_3~-.. The nucleation frequency and growth rate ofpits were inhibited in both the metastable and stable states, and such inhibition effect was strongerwith the increase of HCO_3~- concentration. Furthermore, with the addition of HCO_3~-, thesemiconductivity of the passive film changed from n-type to n+p type, and the density of pointdefects in the passive film decreased, which enhanced the protective effect of the passive film onthe steel matrix.