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Corrosion and passivation of the 304 stainless steel in formic acid solutions



Anodic and cathodic potentiostatic polarization curves for 18Cr–10Ni stainless steel electrodes in aqueous formic acid solutions of various concentrations (2.65; 5.3; 10.6; 15.9; and 21.2 M) have been recorded at room temperature in presence and in absence of the dissolved oxygen. The stainless steel is in active state in all tested solutions and passes in the passive state by anodic polarization, presenting an extended region of stable passivity, which shifts to positive potentials domain with the increase of the formic acid concentration. The corrosion and passivation electrochemical parameters were evaluated as function of formic acid concentration. The solution containing 10.6 M HCOOH was the most aggressive for the stainless steel in the active and in the passive state, presenting the maximum values of the corrosion current, critical current of passivation and passive current densities. The corrosion potentials remain in the active state even after 72 h of immersion in the tested formic acid solution and in the presence of dissolved oxygen. The formic acid oxidation takes place indirectly, in the transpasive region of the anodic curves, presenting the high rate in solutions of mean concentrations.