Corrosion And Passivation Behavior Of Q235 Carbon Steel In Acidic Environment Containing Nitrite Anions

Carbon steels were applied in the different fields of production and living widely,which was limited greatly by the weak capability of spontaneous passivation.In alkaline and neutral environments,the presence of nitrite anions?NO₂^-?repaired the natural and air-formed passive film on the steel surface,further promoting the repassivation of carbon steels.However,in acidic environments,the related investigations about the effect and mechanism of NO₂^-on the corrosion and passivation behaviors of carbon steels were relatively few.In this paper,the corrosion and passivation behaviors of Q235 carbon steel in nitric acid?HNO₃?,acetic acid?HAc?and hydrochloric acid?HCl?solutions containing NO₂^-were studied,and the related mechanisms were also discussed.The corrosion and passivation behaviors of Q235 carbon steel in pure NaNO₂ solutions containing the different concentrations of NO₂^-were studied.The Q235 steel showed the electrochemical characteristic of spontaneous passivation in NaNO₂ solutions,which was very closely associated with the formation of Fe₂O₃ passive film on the steel surface under the action of NO₂^-.In pure NaNO₂ solutions,the critical NO₂^-concentration of stable passivation for the Q235 steel was 0.01 mol/L,and the passivation parameters,including the resistance,defect and microstructure of passive film,exhibited the slight variation when the NO₂^-concentration was up to the critical concentration.The electrochemical behaviors of Q235 carbon steel in 0.01 mol/L NaNO₂-HNO₃,0.01mol/L NaNO₂-HAc and 0.01 mol/L NaNO₂-HCl solutions were studied.The effects of acid type and pH value on the electrochemical characteristics were very prominent:the electrochemical characteristics of activation-passivation-transpassivation and self passivation-transpassivation were present orderly with the increase of pH value in NaNO₂-HNO₃ and NaNO₂-HAc solutions;however,the electrochemical characteristics of activation,activation-passivation-pitting and self passivation-pitting were present orderly with the increase of pH value in NaNO₂-HAc solutions.The critical pH values for the occurrence of passivation in NaNO₂-HNO₃,NaNO₂-HAc and NaNO₂-HCl solutions were pH 1,pH 3 and pH 3,which was affected significantly by the composition,acidity and oxidability of acids.The effects of pH values on the corrosion and passivation behaviors of Q235 carbon steel in 0.01 mol/L NaNO₂-HNO₃,0.01 mol/L NaNO₂-HAc and 0.01 mol/L NaNO₂-HCl solutions were studied.In the whole acidic pH range,with the increase of pH value,the positive direction of corrosion potential and the decrease of corrosion current density were present in the three solutions,indicating that the corrosion of Q235 carbon steel was inhibited with the increasing pH value.At the same time,in the passivation pH range,with the increase of pH value,the negative direction for the difference between activation-passivation transition potential and corrosion potential and the decrease of critical passivation current density were present in the three solutions,indicating the passivation of Q235 carbon steel was strengthened with the increasing pH value.