Transpassive dissolution of mild steel in NaNO3 electrolytes

A study has been made of the transpassive dissolution of mild steel in sodium nitrate solution over a range of current densities from 2 to 100 A cm^–2. The dissolution current efficiency and the anode potential free from the electrolyte IR component were measured in a flow cell; optical and scanning electron microscopy were then used to examine the sample surfaces after the dissolution tests. The results show that during the early stage of transpassive dissolution, the mild steel is covered with a compact, electronically conductive Fe₃O₄ film, and the current is consumed mainly in oxygen generation on the film/electrolyte interface. With increasing anode potential and current density, this film is gradually broken and the underlying metal surface becomes exposed to the electrolyte. At this stage, iron dissolution begins at a high rate. The film rupturing process is strongly dependent on nitrate concentration; the higher this is, the lower is the current density required to rupture the film.     

On the role of mass transport in high rate dissolution of iron and nickel in ECM electrolytes—I. Chloride solutions

High rate anodic dissolution of iron and nickel in 5 M NaCl was studied in a flow channel cell under controlled hydrodynamic conditions. Galvanostatic experiments were aimed at investigating the influence of current density and electrolyte flow rate on anode potential, current efficiency for metal dissolution and surface texture resulting from dissolution. Active dissolution at low current densities leads to surface etching and transpassive dissolution at high current densities leads to surface brightening. Transition from active to transpassive dissolution is mass transport controlled and is accompanied by a change in anode potential, surface microtexture and in case of iron by a change in the valence of metal dissolution.     

Transpassive dissolution of iron in sulphuric acid solution

Anodic oxidation characteristics of iron in 3, 10 and 12 mol dm^–3 sulphuric acid solutions have been studied in the transpassive region. Dissolution current efficiency measurements have been carried out using potentiostatic current-voltage curves and solution analysis techniques. The current-voltage curves were split into metal dissolution and oxygen evolution curves assuming that the iron goes into solution as Fe³⁺. The current density value in the passive region increased whereas the current density in the transpassive region decreased with the increase of sulphuric acid solution concentration. In order to obtain information about the nature of the films present on the surface, potential decay curves from different anodic potentials in the transpassive region have been recorded. It seems that there is no passive film present on the specimen surface in 12 mol dm^–3 sulphuric acid solution and a better surface finish is obtained after the dissolution. Depth profile analysis of oxide films by the AES technique in 3 mol dm^–3 sulphuric solution reveals that the sulphur concentration is maximum at the metal/oxide interface rather than at the oxide/electrolyte solution interface as required by an ion exchange mechanism for film dissolution.     

Coloration of stainless steel at room temperature by triangular current scan method

Interference films with gold, red, blue or green color have been successfully produced on stainless steel at room temperature by applying the triangular current scan method. The anodic current scan over the transpassive region led to the forced dissolution of the steel, and the cathodic current on the reverse scan attributed to the reduction of chromate (VI) to Cr³⁺. The metal ions generated by both reactions were hydrolyzed to form an interference film. The thickness (coloration) of the interference film depended linearly on the electrolysis time and the logarithm of the space between the two triangular waves.     

Influence of the electrolyte composition and temperature on the transpassive dissolution of austenitic stainless steels in simulated bleaching solutions

The transpassive corrosion of highly alloyed austenitic stainless steels—UNS N08904, UNS S31254 and UNS S32654—was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.     

Nature of the passive film on nickel

The passive film formed anodically on nickel in borate buffer solution in both the passive and transpassive regions is found to be p-type in electronic character, corresponding to a preponderance of metal vacancies (over oxygen vacancies and nickel interstitials) in the barrier layer. However, at high anodic potentials, some n-type character was detected by Mott-Schottky analysis, which is probably due to the presence of free charge carriers (electrons) from the evolution of oxygen and/or the oxidative ejection of Ni³⁺ at the barrier layer/outer layer interface. The p-type character of the film is consistent with the diagnostic criteria obtained from the Point Defect Model for a passive film, in which the majority defect in the NiO barrier layer is the metal vacancy. The transpassive state is postulated to comprise a thick, porous oxide film on the surface, with the current probably being due to the oxidative ejection of Ni³⁺ species from the barrier layer and oxygen evolution within the pores, or both.     

The improvement of the corrosion resistance of 309 stainless steel in the transpassive region by nano-crystallization

The effect of nano-crystallization on the corrosion behavior of 309 stainless steel in the transpassive region was investigated in 0.5 M Na₂SO₄ (pH 2) solution. Three parts defined as transpassive dissolution, secondary passivity and oxygen evolution can be observed in the transpassive potential region of the anodic polarization curves. In the whole transpassive region the nano-crystalline coating has a smaller corrosion current density than the bulk steel, which indicates the transpassive dissolution rate is decreased by nano-crystallization. In addition, there is an obvious difference in the surface micrographs of the two materials at transpassive potentials, as scanning electron microscopy (SEM) shows. The electron probe microanalysis (EPMA) reveals that nano-crystallization improves the homogeneity of Cr on the surface. Mott-Schottky plots displays that the carrier density of the oxide film in the transpassive region is decreased by nano-crystallization. Thus, the interfacial reactions are decelerated and the corrosion resistance of the stainless steel in the transpassive region has been greatly improved by nano-crystallization.     

Passive and transpassive behaviour of CoCrMo in simulated biological solutions

In this work, the behaviour of a CoCrMo alloy under simulated body conditions was investigated. More specifically, the electrochemical properties of the alloy and the relevant mechanisms in the passive and transpassive states were studied in detail. Electrochemical techniques such as potentiodynamic and potentiostatic polarisation, cyclic voltammetry, rotating disc electrode and electrochemical impedance spectroscopy were employed. Further, ex situ X-ray photoelectron spectroscopy analysis of the passive films was carried out. A good correlation between the results obtained from all the experimental techniques was achieved. Overall, it was found that the passive film on CoCrMo changed in composition and thickness with both potential and time. The passive behaviour of the CrCrMo alloy is due to a formation an oxide film highly enriched with Cr (≈90% Cr oxides) on the alloy surface. The passive and transpassive behaviour of the alloy is hence dominated by the alloying element Cr. In the transpassive region, strong thickening of the oxide film takes place, combined with a change in the composition of the film, and strongly increased dissolution rate. In the transpassive region, all alloying elements dissolve according to the composition of the alloy. The metal ion release is also very strongly enhanced by cyclic variation of the potential between reducing and oxidizing conditions. In this case, during activation/repassivation cycles, cobalt dissolution is greater than expected from the composition of the alloy. Therefore, active dissolution behaviour is mainly dominated by the alloying element Co.     

Effect of ferrous and hydrogen ions on the anodic dissolution of iron in nitrate solutions

The effect of Fe(II) and H⁺ ions on the anodic dissolution of iron (adr) in weak acid nitrate solutions is studied by means of rapid polarization plots and anodic galvanostatic charge and decay transients. The parameters of the kinetic equation of the adr are determined. The inhibition effect of Fe(II) and OH^? as revealed by the displacements of the anodic Tafel lines is ascribed to the formation of an adsorption film in the presence of the nitrate ions. The rate of the film formation and its dissolution (desorption) appears to be fast so that the diffusion of the film forming species becomes rate determining. The diffusion control is confirmed quantitatively by the analysis of the anodic transients. The properties of the adsorption film are discussed.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Cyclic oxidation processes during anodizing of Al-Cu alloys

The influence of copper on the morphologies of porous anodic alumina has been investigated under current and voltage control using a sputtering-deposited Al-2.7 at.% Cu alloy and a commercial AA 2024-T3 aluminium alloy anodized in either sulphuric acid electrolyte or the same electrolyte but with addition of tartaric acid. The findings indicate that film development involves repeated formation of embryo cells of anodic alumina at the metal/film interface. During the initial stages of anodizing at constant voltage, cell formation is accompanied by current peaks in the current-time response. The porosity of the resultant films has a lateral aspect due to the layering of embryo cells. The thickness of individual layers is proportional to the formation voltage, with a ratio of the order 1 nm V⁻¹. The cell formation is accompanied by enrichment of copper in the alloy, incorporation of copper species into the anodic film, in low amounts relative to the alloy, and evolution of oxygen. These processes disrupt the formation of the classical pore morphology, characteristic of high purity aluminium, due to continuous formation of fresh embryo cells and re-direction of pores. The main effect of the tartaric acid addition to the sulphuric acid was to reduce the rate of anodizing of the alloys at constant voltage by about 10-20%.     

Electropolishing of chromium in phosphoric acid-sulphuric acid electrolytes

The anodic dissolution of chromium in phosphoric acid-sulphuric acid electropolishing electrolytes has been investigated using a rotating disc electrode. The influence of electrolyte composition on mass transport and the resulting surface finish has been investigated. It was found that electropolishing of chromium occurs in the transpassive potential region under mass transport control. The transport limiting species is the anodically generated hexavalent chromium ion. The rate of nucleation of the anodic film present is potential dependent, and film instabilities occur in a certain potential region of the limiting current plateau.     

Zur kinetik der anodischen eisenauflösung bei hohen stromdichten unter stationären bedingungen

During the transpassive electrochemical dissolution of iron in nitrate solutions there appear oxide layers, of which the outward view changes with the current density. The oxide layers mainly consist of Fe₃O₄. The dissolution of iron at high current densities occurs through this layer. From measurements of the current efficiency kinetic parameters are deduced. According this, the reaction charge number is +2 and the reaction order +1 concerning nitrate ions. The Tafel slopes in nitrate solutions are 40 mV and in chlorate solutions 30 mV. Active, passive and transpassive dissolutions of iron are compared with each other and characteristic differences are pointed out. In the transpassive region there exists no closed oxide layer of γ-Fe₂O₃, which responds for passivity. For this reason the high rate of dissolution and the reaction charge number of +2 may be explained.     

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH^–]9 M), the dissolution process appeared to follow zero-order kinetics.     

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10-30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l⁻¹ KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.     

Transpassive behaviour of titaniummolybdenum alloys in 1 N H2SO4

The electrochemical behaviour of titaniummolybdenum alloys (10, 20,30 wt.%Mo) in boiling, deaerated 1 N H₂SO₄ was investigated. Attention was focused on the potential region of transpassive dissolution of molybdenum. Potentiostatic polarization curves in conjunction with Auger electron spectroscopy were used to correlate the current-potential relationship in this region with the composition of the oxide films. It was shown that at open-circuit potential, for Ti—20 and 30Mo an accumulation of molybdenum in the oxide film results in alloy passivity. However, in the region of the transpassivation with respect to the molybdenum, the presence of Mo in the oxide led to a higher dissolution rate than that exhibited by pure titanium. In the transpassivation region, the steady state currents increased with increasing potential up to a characteristic anodic potential. At potentials more positive than this point, the anodic currents declined. This decrease in current was correlated with a depletion of Mo from the outer layers of the oxide with the consequent formation of a more protective titaniumoxide film structure.     

Studies of the interface between indium and beta/beta″ alumina tubes

The anodic degradation of a beta/beta″ alumina electrolyte in contact with liquid indium was studied by voltammetric techniques at 35°C. The electrolyte tubes were found to crack when the anodic current reaches about 4 mA cm⁻², referred to the apparent surface area. Simultaneously, a passivation occurred which is attributed to film formation on indium in contact with the molten nitrate through the cracks. Transmission electron micrographs taken before and after the anodic cycle yielded information on how the degradation may proceed. At the cathodic side of the electroactive domain of the solid electrolyte, the process of sodium dissolution in the indium at lower voltages and the formation of indium-sodium compounds at larger voltages can be distinguished.     

Passivation of iron in borate buffer solution

The mechanisms of formation of the passivating film and of anodic dissolution in the presence of this film have been studied at different potentials. In the low potential region, E ? −0·4 V(sce), the data fit the preferential-dissolution/-adsorption mechanism: anodic dissolution proceeds by the direct, activated transfer of ions from the metal surface into solution. At potentials above −0·3 V(sce), the preferential-dissolution/-adsorption mechanism may be operative at short times, but the steady state is achieved only in the presence of an oxide film more than a unit cell in thickness, and anodic dissolution proceeds by ionic transport through the oxide. The passivation reaction is not reversible and it is not expected that the passivation potential corresponds to a reversible potential.     

High throughput electrochemical screening and dissolution monitoring of Mg–Zn material libraries

A combinatorial study on Mg–Zn material libraries obtained by thermal evaporation is performed in order to investigate the effect of alloying magnesium on the electrochemical behaviour and dissolution rate of zinc in borate buffer of pH 7.4. The surface morphology of the graded samples is complex and subject to a detailed discussion, whereas the crystal composition revealed Mg, MgZn₂ and Zn exclusively.Open circuit potential measurements and potential sweeps along the graded samples are combined with downstream zinc detection and revealed several strongly non linear dependencies between electrochemical features and magnesium content. While the chemical dissolution rate of zinc by the electrolyte was found to reflect the film stoichiometry except in the regions of high surface roughness, the open circuit potential revealed a local minimum around 20 at.% magnesium accompanied by a maximum in the current plateau during the anodic sweep and a high thickness of the native oxides present prior to electrochemical experiments. All compositions showed passive like behaviour during anodic sweeps with high plateau current densities (200–350 μA cm⁻²) originating from slow but constant oxide dissolution as supported by XPS analysis of the surface before and after contact with the electrolyte.     

Anodic dissolution of nickel in concentrated sulfuric acidic solutions

The anodic dissolution of nickel in concentrated sulfuric acidic solutions is characterized by two diffusion plateaux related to the active and transpassive regions. For the same speed of rotation of a rotating disc electrode, the two plateaux currents are almost identical. However, the less anodic one gives rise to a rough electrode surface while a polished surface is observed at the more anodic one. For nickel as-received, mass transport influences the current over the whole potential range. After heat treatment, only the current for the two plateaux were mass transport controlled. Electrochemical impedance diagrams show that the lowest frequency capacitive loop is influenced by mass transport. Electrohydrodynamic (EHD) impedance diagrams show two different regions. In the low frequency range, the results follow the theoretical curve corresponding to a uniformly accessible electrode with a very high Schmidt number around 10⁷. At high frequency, the EHD impedance may correspond to an interface covered by a gel layer formed from the products of the anodic dissolution.