Timo Laitinen

The transpassive dissolution mechanism of highly alloyed stainless steels. I. Experimental results and modelling procedure

The transpassive dissolution of austenitic stainless steels (AISI 316L, AISI 904L, 254SMO and 654SMO) in a 0.5 M sulphate solution with pH 2 was studied by conventional and rotating ring–disc voltammetry, as well as electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI), while small amounts of lower-valency Cr or Mo species are released as well. Secondary passivation readily occurs for AISI 316L, whereas the remaining highly alloyed steels dissolve at high current densities in the whole potential range studied. The dissolution rate was found to increase in the order AISI 904L < 254SMO < 654SMO. Thus it can be correlated to the increase in the Cr and especially Mo content of the steel substrate. The impedance spectra contain contributions from the transpassive dissolution of Cr and secondary passivation, probably due to enrichment of Fe in the outermost layer of the surface film. A kinetic model of the process is proposed, including a two-step transpassive dissolution of Cr via a Cr(VI) intermediate and the dissolution of Fe(III) through the anodic film. The model was found to be in quantitative agreement with steady state current vs. potential curves and electrochemical impedance spectra. The kinetic parameters of transpassive dissolution were determined and the relevance of their values is discussed. � 2002 Elsevier Science Ltd. All rights reserved.

The transpassive dissolution mechanism of highly alloyed stainless steels: II. Effect of pH and solution anion on the kinetics

The effect of pH and solution anion on the kinetics of transpassive dissolution of highly alloyed austenitic stainless steels (AISI 904L, 254SMO and 654SMO) was studied by a combination of electrochemical techniques. The experiments were performed in 0.5 M sulphate and 0.5 M chloride solutions, and in an equimolar mixture of the two. The transpassive dissolution was found to start at higher potentials in solutions with higher pH. The rate of transpassive dissolution was shown to decrease with increasing pH and to be the lowest in chloride solutions and the highest in sulphate electrolytes. The steady-state current vs. potential curves and the impedance spectra of the studied materials in the transpassive potential region were found to be consistent with a proposed kinetic model. The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the anodic oxide film via parallel reaction paths. The kinetic parameters of the model in solutions with different pH values and different anions were determined. The role of pH and solution anion in the transpassive dissolution process is discussed in relation to changes induced by these parameters in the composition of the anodic passive film. The factors determining the efficiency of Fe as a secondary passivating agent are also considered.

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—I. Corrosion processes and anodic dissolution of antimony

The corrosion processes at open circuit conditions and in the active dissolution region of antimony, at low anodic overpotentials, have been studied in sulphuric acid solutions. It was established that antimony dissolution at open circuit is coupled to oxygen reduction, which occurs via the hydrogen peroxide route. The rate of antimony dissolution was found to increase linearly with increasing acid concentration. Based on potentiodynamic, galvanodynamic and steady-state measurements as well as ac impedance data possible mechanisms for the corrosion processes at open circuit as well as for the dissolution of antimony in the active region are proposed and discussed.

Conduction Mechanism of the Passive Film on Iron Based on Contact Electric Impedance and Resistance Measurements

The application of a mixed conduction model and a new contact electric impedance (CEI) technique to predict quantitatively the electronic and ionic transport properties of oxide films on iron in a nearly neutral tetraborate solution is discussed. The mixed-conduction model emphasizes the coupling between the ionic defect structure and the electronic conductivity in an anodic film. Conventional electrochemical techniques have not been sufficient to characterize properly the electronic and ionic properties of anodic films on metals. The CEI technique makes it possible to distinguish between processes taking place at different rates within oxide films. Using this technique together with the contact electric resistance technique, we have found that the diffusion coefficient for the electronic conduction in the anodic film on iron is several orders of magnitude higher than that for the ionic transport. This shows that the passive film on iron is predomiantly an electronic conductor. The fitting of the experimental results to the mixed conduction model gives a good agreement and thus supports the validity of this model in the present case.

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—II. Formation and properties of the primary anodic layer

Abstract The formation of an anodic layer during the oxidation of antimony in sulphuric acid solutions as well as the structure and properties of the layer have been studied using linear sweep voltammetry, potentiostatic oxidation, ac impedance measurements, X-ray diffraction analysis and scanning electron microscopy. The electrochemical investigations as well as the shape of the impedance spectra during electrode passivation indicate that the surface layer probably has a complicated structure. Under the conditions of these experiments the passivating film is mainly amorphous, gel-like, exhibiting good ionic conductivity and a relatively high dissolution rate. Assuming that the anodic layer consists mainly of antimony hydroxide a reaction scheme for the growth and dissolution of this film is presented.

Influence of molybdenum on the conduction mechanism in passive films on iron–chromium alloys in sulphuric acid solution

Abstract This paper describes an extension of the mixed-conduction model to predict quantitatively the electrochemical behaviour and transport properties of anodic films on pure Cr, Fe–Cr alloys and Fe–Cr–Mo alloys in 1 M sulphuric acid solution. The anodic films on Fe–Cr–Mo alloys (12 and 25% Cr; 0, 5 and 10% Mo) were studied using rotating ring-disk voltammetry, impedance spectroscopy and resistance measurements. The addition of Mo to the Fe–Cr alloys was found to decrease the resistance of the film both in the passive and transpassive region. During the re-activation of the Fe–12%Cr–x%Mo alloys at negative potentials, soluble products were found to be released at a higher potential than during the re-activation of the Fe–12%Cr alloy. Re-activation proceeds to a lesser extent for the Fe–25%Cr alloy, and was not observed for the Fe–25%Cr–x%Mo alloys. The addition of Mo was also found to lead to a marked increase of the transpassive dissolution rate of the alloys. The impedance spectroscopic results indicated that the addition of Mo increases the rate of the interfacial generation of positive defects and especially annihilation of negative defects. The present extension of the mixed-conduction model for anodic passive films was found to describe quantitatively the resistance and impedance spectroscopic data for Fe–25%Cr and Fe–25%Cr–10%Mo alloy, as well as earlier data on pure Cr. The same kinetic model can be used to describe the behaviour of the films on all these materials. It is thus most likely that a Cr-oxide-based film is formed which determines the behaviour of the Fe–25%Cr and Fe–25%Cr–10%Mo alloys as well.

The effect of antimony on the anodic behaviour of lead in H2SO4 solution: Part II. Dependence of the phase composition of the anodic layer on the oxidation potential

Abstract The behaviour of a Pb+12% Sb electrode (eutectic composition) in the lead oxide and the lead dioxide potential regions has been investigated. For this purpose the electrode was first oxidized at a constant potential for 0.5 h and then subjected to negative potential sweeps at a scan rate of 10 mV/s. The electrode was illuminated and its photocurrent was measured. It was established that parallel to the formation of PbO, PbO n and PbO 2 in the different potential regions, other phases are also formed in which some of the lead ions (Pb 2+ and Pb 4+ in the above oxides) are replaced by Sb 3+ (and probably Sb 5+ ) ions. At potentials up to 0.80 V, substituted Pb (1−x) (Sb) x O oxide and PbO are formed; between 0.80 and 1.20 V, non-stoichiometric Pb (1−x) (Sb) x O n ; and in the highly positive range, Pb (1−x) (Sb) x O 2 is distinguished. Lead dioxide and Pb (1−x) (Sb) x O 2 may be formed simultaneously. The substituted lead oxides are amorphous or/and their X-ray diffraction patterns overlap those of the pure lead oxides. The first two substituted lead + antimony oxides are photoactive, while the last one exhibits high electron conductivity. It is reduced at potentials more negative than that of PbO 2 reduction, which may extend the discharge processes of the PbO 2 active mass and hence increase its capacity.

THE EFFECT OF ANTIMONY ON THE ANODIC BEHAVIOUR OF LEAD IN SULPHURIC ACID SOLUTIONS. I, VOLTAMMETRIC MEASUREMENTS

Abstract The influence of Sb on the formation of the corrosion layer on Pb has been studied with a Pb—12%Sb alloy electrode in sulphuric acid solution using the ring-disk technique decay measurement and galvanostatic oxidation. It was found that in voltammograms of the PbSb electrode a new cathodic peak appears, compared to pure Pb, after oxidation at constant potentials > 0.8V, showing that a new oxidation product is formed. The stoichiometry of this product changes gradually with potential approaching that PbO 2 at high anodic potentials. This product also improves the electronic conductivity of the corrosion layer. The effect of Sb(III) or Sb(V) dissolved in the electrolyte on the formation of this Sb containing product on pure Pb was similar to that of Sb as an alloying agent. It is suggested that this new product is an oxide of Pb (most probably nonstoichiometric) doped with Sb or a mixed PbSb oxide.

Ring-disk electrode studies of soluble intermediates formed during the polarization of Pb in H2SO4

Abstract The use of a rotating ring-disk electrode to detect soluble Pb(1V) and Pb(11) species released into H 2 SO 4 has made it possible to give complementary interpretations for the processes occuring during the anodization and successive reduction of Pb electrodes. It has been shown that soluble Pb(1V) species act as intermediates during the formation tetravalent oxides of Pb, and that the generation of tetravalent products is always preceded by the information of divalent products even at high positive potentials. During the reduction of PbO 2 , species are released. Part of the tetravalent products formed at Pb are reduced only at high negative potentials.

The anodic behaviour of Sb and PbSb eutectic in sulphuric acid solutions

Abstract Ring—disc voltammetric experiments on antimony in H 2 SO 4 solutions show that a partly passivating oxide layer is formed on the metal surface preventing oxygen evolution. Due to the ionic conductivity and/or porosity of the layer the dissolution of the metal proceeds through the layer, and its rate is controlled by diffusion in the layer. Antimony is dissolved mainly in the trivalent state. Based on the determinations of the Tafel coefficient the mechanism of dissolution is dependent on the acid concentration. In the case of the lead—antimony eutectic the formation of a semipermeable lead sulphate membrane retards the rate of dissolution. ac Impedance measurements on antimony show features of a simple passivation process, eg a negative polarization resistance after the maximum in the current. The effect of mass transfer through the oxide layer is also observed. Thus the ac impedance measurements confirm the interpretation given above.