The Intergranular Corrosion of Mild Steel in CO2 + NaNO2 Solution

Intergranular corrosion (IGC) is observed on mild steel surface when the steel is polarized to passive potential zone in CO2 + NaNO2 solution. The methods of potentiodynamic polarization, scanning electron microscope (SEM) and scanning tunneling microscope (STM) are applied to study the phenomenon. Intergranular corrosion mainly happens as the steel is polarized into the active–passive transition zone, and the width and depth of the corroded grain boundaries increase with potential in the zone. When the potential enters the passive zone, the depth of IGC shows only very slight change. The IGC is related to the segregation of the impurity elements Mn and Si at the grain boundaries. The occurrence of intergranular corrosion is influenced by the potential scanning rate and CO2 concentration in the solution. When the potential scanning rate is below 0.3 mV s−1 IGC phenomenon is observed. In saturated CO2 + NaNO2 solution no IGC is observed which may be attributed to the formation of FeCO3 layer on steel surface that protects the grain boundaries from corrosion. The observed IGC is due to the combined effect of CO2 and NaNO2 in solution. NO2− promotes passivation on the grain surface but CO2 induces corrosion at the grain boundaries.