Inhibition Effect of Sodium Nitrite and Silicate on Carbon Steel Corrosion in Chloride-Contaminated Alkaline Solutions

The ability of some corrosion inhibitors to protect carbon steel against corrosion was investigated using the voltammetric and electrochemical impedance spectroscopy (EIS) techniques. The surface analysis of samples was carried out by x-ray photoelectron spectroscopy (XPS). EIS measurements have shown that prolongation of steel exposure to solution containing sodium nitrite (NaNO2) from 0.5 h up to 240 h leads to an approximately tenfold increase in charge-transfer resistance (R-t) values. After 240 h of exposure to the solution with the NaNO2 + sodium silicate (Na2SiO3) mixture, Rt values were about threefold higher as compared to those obtained in the solution containing only one inhibitor, NaNO2. The XPS data revealed that an up to 4 nm thick passive layer was formed after 240 h of carbon steel exposure to the solution with NaNO2. An additional introduction of Na2SiO3 into the same solution increased the thickness of the oxide layer >8 nm. In the outer part of the oxide layer, Fe was detected in the form of ferrous oxide (FeO), while magnetite (Fe3O4) was the main constituent of the inner part of the oxide layer.