Abstract
The influence of 0.01 mol L-1 NaNO2 addition on the corrosion and passivation of X65 carbon steel in pH 1 to 5 HNO3 solutions was investigated and compared by electrochemical methods and microstructural techniques. In the pH 1 to 5 solutions without NaNO2, the X65 steel presented the electrochemical characteristic of active dissolution, and the corrosion rate of X65 carbon steel decreased gradually with the raise of pH value. By contrast, with the addition of 0.01 mol L-1 NaNO2 in pH 1 to 5 HNO3 solutions, the electrochemical characteristic of X65 carbon steel transferred from the active dissolution in the pH 1 to 5 solutions without NaNO2 to the anodic passivation in the corresponding pH solutions with NaNO2. For the X65 steel in the pH 1 to 5 solutions with NaNO2, with the raise of pH value, the corrosion rate also decreased gradually but the passivation capability strengthened obviously. The corrosion and passivation of X65 carbon steel in pH 1 to 5 HNO3 solutions without and with 0.01 mol L-1 NaNO2 were related to the cathodic reactions of H+ reduction, O2 reduction and NO2-/HNO2 reduction.
Highlights
The application of carbon steels in production and living fields is wide and universal;[1,2,3] the inevitability of corrosion and failure usually leads to the untimely damage of steel structure.[4,5,6] The addition of inhibitors into environmental media is the one of main and important methods to decrease the corrosion rate of carbon steels in their service condition,[7] which is attributed to the oxidation or the adsorption mechanism of inhibitors on the surface of carbon steels.[8]
In the following electrochemical tests of polarization and electrochemical impedance spectroscopy (EIS), all the X65 samples were immersed in the corresponding solutions for 10 min before the polarization and EIS tests were begun
The above influences of pH values on open circuit potential (OCP) and Ecorr suggest that the dominated cathodic reaction occurred on the X65 surface in the pH 1 to 3 solutions without NaNO2 is the H+ reduction with the standard potential (Est) of 0 VSHE (SHE: standard hydrogen electrode): 2H+ + 2e → H2 (Est = 0 VSHE, pH 0)
Summary
The application of carbon steels in production and living fields is wide and universal;[1,2,3] the inevitability of corrosion and failure usually leads to the untimely damage of steel structure.[4,5,6] The addition of inhibitors into environmental media is the one of main and important methods to decrease the corrosion rate of carbon steels in their service condition,[7] which is attributed to the oxidation or the adsorption mechanism of inhibitors on the surface of carbon steels.[8]. With the addition of 0.01 mol L–1 NaNO2 in pH 1 to 5 HNO3 solutions, the obvious variation of electrochemical characteristic indicates the effectiveness of NO2– on the surface passivation of X65 carbon steel.
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