CORROSION AND HYDROGEN INSERTION IN UNS G41300 STEELS IN BRINE ACID SOLUTIONS SATURATED WITH HYDROGEN SULFIDE

摘要

Characterization of iron sulfides, determination of corrosion rate and hydrogen absorption were performed in UNS G41300 steel for different regions of Figure 1 of ISO 15156/MR0175 (Environmental severity with respect to SSC of carbon and low alloy steels) at 24°C in high chloride acid solutions (pH<6) with H_2S partial pressures between 0.03 to 1 bar. Weigh loss experiments were performed using coupons in stagnant conditions and hydrogen insertion was evaluated using Devanathan Stachursky double cell. SEM/EDS and X-Ray Diffraction were used to characterize the corrosion products. The initial contact of the steel with an acid brine solution generated an amorphous layer initially rich in oxygen that converted to sulfide by replacement of O~(2-) by S~(2-) once the hydrogen sulfide was introduced in the testing vessel; sometimes leading to a crystalline iron sulfide, M(S,O). The corrosion product was semi protective since hydrogen insertion and therefore steel oxidation were not eliminated after the film formation. Cubic FeS and mackinawite were detected when partial pressures of hydrogen sulfide were higher than 0.1 bar. Cubic iron sulfide was unstable and converted to mackinawite by solid state reaction once the product was removed from the testing vessel. Transformation kinetics cubic FeS was dependent upon the presence of acetate, pH and O_2. Based on the obtained results, a mechanism of sulfide formation was proposed. Corrosion rate and hydrogen insertion were found to be dependent on the product of the dissolved hydrogen sulfide and proton concentrations; suggesting that adsorption of reactants (particularly OH" and S~(2-)) on the surface and stability of the amorphous layer plays a key role in both phenomena.