Electrochemical Study On Corrosion Behavior Of Carbon Steel In Acidic NaNO₂ Solution

Carbon steel is a widely used alloy material suitable for various fields of industrial construction in China.Carbon steel has weak self-passivation ability,so it is easily corroded by the environment during practical application,resulting in great economic loss.Previous studies showed that NaN02 can promote the passivation of mild steel in alkaline or neutral medium,and a passivation film can protect the carbon steel matrix from corrosive ion in the environment.But in the acidic medium,the study of NaNO2 is not systematic.In the work,we explored the passivation and corrosion behavior of L80,N80 and X65 carbon steels in acid 0.01mol/L NaNO2 solution.In the 0.01mol/L HN03/CH3COOH-NaN02 solution,when the pH?4,L80,N80 and X65 exhibited the electrochemical characteristics of activation-passivation-transpassivation,when the pH=5,the three carbon steels exhibited electrochemical characteristics of activation-self passivation.In the 0.01 mol/L HCl-NaN02 solution,when the pH=2,all three carbon steels exhibited activated electrochemical characteristic.When the pH=3 or 4,the three carbon steels showed activation-passivation-self passivation,in the pH5 solution,the three carbon steels exhibited electrochemical characteristic of self passivation-pitting.In the three solutions,with the pH value increased,the values of icorr and ipp decreased,and the difference between the activation-passivation transition potential and the self-corrosion potential(Epp-Ecorr)shifted negatively,and the passivation film resistance increased,indicating that with the pH of 0.01mol/L NaN02 solution increase,the corrosion rate decrease.And the passivation effect of NO2-on three carbon steels is stronger in solution with high pH than that in solution with low pH.In the 0.01 mol/L HN03/CH3COOH-NaNO2 solution,the corrosion type of L80,N80 and X65 steels may exhibit uniform corrosion.But when Cl-was present in the solution,the possibility of local corrosion of the three carbon steels in 0.01 mol/L HCl-NaN02 solution increased,and the possibility of local corrosion in the solutions of pH=3 or 4 was greater than that in the pH=2 solution,and the pits occured in pH5 solution,indicating that local corrosion is most likely under these conditions.The presence of Cl-only increases the possibility of local corrosion of carbon steel in solution without changing its electrochemical behavior in solution.Local corrosion mainly occured in the activation-passivation transition zone,while the pits has to occur under the condition that both passivation and aggressive ions were present.The presence of nitrite(NO2-)in three acidic environments could enhance the passivation of mild steel and the pH value has a great influence on the passivation;When the pH?4,the effect of passivation of NO2-on carbon steel surface was mainly attributed to its high reduction potential in acidic environment;when the pH=5,the effect of passivation of NO2-on carbon steel surface was mainly becaused of its strong oxidation.