Synergistic Effect of Thiourea and Mannich Base as Corrosion Inhibitors for 13Cr Stainless Steel in High Temperature and High Concentration Hydrochloric Acid

The Mannich base corrosion inhibitor could efficiently inhibit the uniform corrosion of 13Cr stainless steel in the conditions of high‐temperature acidizing and fracturing in deep carbonate reservoirs, but it only has a limited ability to inhibit the pitting corrosion. To further enhance the pitting corrosion inhibition effect, thiourea was compounded with Mannich base, and the inhibition effect and release mechanism were systematically studied. The inhibition ability of the mixed inhibitor was evaluated by weight loss, electrochemical experiments, and 3D microscopy. The adsorption tendency of the thiourea‐compounded Mannich base inhibitor on the surface of 13Cr stainless steel was studied by density functional theory analysis and molecular dynamics simulation. X‐ray photoelectron spectroscopy (XPS) was used to detect and analyze the adsorption and distribution of the inhibitors. When the addition amount of thiourea was 4 g/L, the corrosion inhibition efficiency of 1% Mannich base inhibitor reached the highest. In hydrochloric acid solutions with concentrations of 10%, 15%, and 20%, the inhibition efficiencies were 99.96%, 99.65%, and 99.28% respectively, and the pitting depths were the shallowest. Results of electrochemical studies indicated that the Mannich base corrosion inhibitor can simultaneously inhibit the cathode and anode reactions, and it was a mixed‐type corrosion inhibitor mainly controlling the anode process. Based on the theoretical analysis of molecular frontier orbitals and the outcomes of molecular dynamics simulations, the Mannich base inhibitor molecules were spontaneously adsorbed on the surface of 13Cr stainless steel, with a tendency towards parallel adsorption.