Nitrate ion effect on the passive film breakdown and current oscillations at iron surfaces polarized in chloride-containing sulfuric acid solutions

Current oscillatory phenomena have been used to study the effect of nitrates on pitting corrosion of passive iron surfaces in chloride-containing sulfuric acid solutions. From the quasi steady-state current-potential and potentiostatic current-time curves of the Fe | 0.75 M H₂SO₄+10 mM Cl⁻ system it is deduced that at lower potentials nitrates stimulate pitting acting as activators of the oxide dissolution. At higher potentials nitrates act as passivators causing a sudden passivation of Fe during a mass-transport controlled process across a salt film. Current oscillations appear over a wide potential region. The oscillation period as well as the induction period of time occurring before the onset of oscillation, both decrease by increasing the nitrate concentration. The effect of nitrates at lower and higher potentials is discussed in terms of the electrochemical and redox reactions of nitrate ions.     

Localized passivity breakdown of iron in chlorate- and perchlorate-containing sulphuric acid solutions: A study based on current oscillations and a point defect model

Current oscillations are used to study the effect of chlorate and perchlorate ions on the iron passivity in sulphuric acid solutions. Quasi steady-state current-potential and potentiostatic current-time curves show the emergence of complex current oscillations, besides the simple periodic ones attributed to general corrosion occurring across the passive-active transition of the Fe|0.75 M H₂SO₄ electrochemical system. The complex current oscillations arising at the iron passive state are indicative of pitting corrosion. Experimental results support that pitting is due to chlorides produced via the reduction of chlorates and perchlorates by ferrous ions either during the active phase of current oscillations or in the passive phase during the H⁺-catalyzed dissolution of the oxide. Thus, chloride is the aggressive ion that causes pitting corrosion and not chlorates and perchlorates themselves. Chloride production induced via the reduction of perchlorates is much slower than that induced by chlorates. A point defect model (PDM) is employed to explain the oxide growth and its breakdown induced by general and pitting corrosion.     

Effect of sulfur-containing amino acids on the corrosion of mild steel in sulfide-polluted sulfuric acid solutions

The influence of cysteine (RSH) and cystine (RSSR) on the corrosion behavior of mild steel in sulfide-polluted H₂SO₄ solutions was studied by potentiodynamic polarization methods and AC impedance technique. The results show that S²⁻ accelerates the corrosion process markedly, especially the anodic dissolution of iron. Tafel polarization curves show that RSH and RSSR act mainly as anodic-type inhibitors without affecting the mechanism of the hydrogen evolution reaction or iron dissolution. Adsorption of RSH and RSSR in most sulfide-polluted H₂SO₄ solutions obeys Temkin’s isotherm. Impedance studies indicate that in the inhibited and uninhibited solutions, charge transfer controls the corrosion process either at E_corr or at 30 mV vs E_corr. Potentiodynamic anodic polarization curves show that RSSR effectively inhibits the steel dissolution both in the active and passive states and greatly reduces the current oscillations observed in the passive region.     

Investigation into designed current oscillations during anodic dissolution of Al in NaCl + NaNO2 solutions

We previously proposed that current oscillations occurred when there were both an active factor and a passive factor that depended on each other and influenced the anodic dissolution processes of metallic materials alternatively. In this study, an electrochemical system, Al|NaCl + NaNO₂, was designed to verify this proposal. The chloride ions are aggressive ions (active factor), assisting the dissolution of the oxide film, while nitrite ions are inhibitive ones (passive factor), assisting the formation of the oxide film. The dissolution and the formation of the oxide film on the surface of the Al electrode occurred alternatively, and the current oscillations were observed to occur in this system, which proved the above point. The Al|NaCl + NaNO₂ system is a new electrochemical oscillator, in which hydrogen evolves periodically from the surface of the electrode during the current oscillations. The results were discussed in terms of the formation and the dissolution of the oxide film. The current oscillations were induced by pitting corrosion of Al in chloride ions containing solutions. It is expected that more oscillatory systems can be found and further studies into the nonlinear dynamic behaviour of metallic materials can be conducted in the light of the proposal.