Inhibition effect of silicate and molybdate on the corrosion of SS 316 in neutral corrosive solution at high temperature

The inhibition effects of silicate and molybdate on the corrosion of stainless steel 316 (SS 316) in neutral corrosive solution are reported. The neutral solution contains chloride, sulfate and bicarbonate ions with pH = 7.86. The inhibition effect of molybdate, silicate and silicate—molybdate composite on the corrosion of SS 316 was tested comparatively as a heuristic guidance, then a simulated investigation in an autoclave at 200 °C was conducted. The surface morphology of corroded samples is analyzed by optical microscope and macroscopic views, and XPS test is employed to detect the surface scales of corroded samples. The results show that molybdate, silicate and their composite show good inhibition effect on SS 316 in the neutral environment, the corrosion current density (CCD) in the electrochemical test (ECT) is reduced to I ₀ = −7.6047 E-08 as 250 mg l⁻¹ silicate is added, while the CCD in the ECT decreased to I ₀ = −1.6915 E-07 Amp cm⁻² as the composite of 200 mg l⁻¹ silicate and 50 mg l⁻¹ molybdate is added, which is much less than the CCD in the ECT I ₀ = −5.0235 E-07 Amp cm⁻² without any inhibitor added; XPS analysis indicates that iron oxide formed on the surface of the sample in the neutral corrosive solution without any inhibitor and pitting was observed clearly; While the XPS analysis indicates that there forms iron molybdate and iron oxide on the surface of the sample in the neutral corrosive solution with addition of molybdate 200 mg l⁻¹ and less pitting was observed; Moreover XPS analysis indicates that there were iron molybdate and silicate deposition on the surface of the sample in the neutral corrosive solution with adding silicate 200 mg l⁻¹ and molybdate 50 mg l⁻¹ without pitting trace. Comprehensive consideration, the proper amount of inhibitors could be addition of 250 mg l⁻¹ silicate or combination of 200 mg l⁻¹ silicate and 50 mg l⁻¹ molybdate composite. (paper)