Transpassive Region Research Articles

Role of surface reactions in the transpassive dissolution of ferrous alloys in concentrated H 3PO 4

The transpassive dissolution of a range of ferrous alloys (Fe–12% Cr, Fe–12% Cr–5% Mo, Fe–25% Cr, Fe–25% Cr–10% Mo) in concentrated H 3PO 4 was studied within the frames of an investigation of electropolishing mechanisms. Measurements by the contact electric resistance (CER) technique have demonstrated that in the transpassive region, the resistance of the anodic film first decreases with increasing potential due to enhanced conductivity of that film and then increases at higher potentials, indicating transpassive film growth. The release of soluble Cr(VI) as detected by rotating ring–disc measurements is the higher, the higher the Cr content in the alloy, and further increases upon addition of 10% Mo to the Fe–Cr alloys. Impedance spectra in the transpassive region have been found to include contributions both from the anodic film and a multistep transpassive dissolution reaction at the film/solution interface. The results were interpreted using a generalised model of transpassive dissolution. The kinetic parameters of the process were determined and the influence of alloy composition on their values is discussed in relation to the processes of anodic surface treatment.

Electrochemical Behavior of Nickel-Chromium Alloys in a High-Temperature Aqueous Electrolyte

Abstract This paper describes the electrochemical behavior of oxide films formed on pure Ni, pure Cr, and Ni-Cr alloys at elevated temperatures, 200°C and 300°C. The applicability of the mixed-conduction model (MCM) to explain the features observed on Ni-based materials and Cr was studied. The MCM emphasizes the coupling between the spatial and potential distribution of ionic point defects in the film and the electronic conductivity of the film. The experimental part of this work includes measurements with pure Ni, pure Cr, and three Ni-Cr alloys (10, 15, and 20 wt% Cr) in 0.1 M sodium tetraborate (Na2B4O7) at 200°C (pH200 ≈ 8.7) and 300°C (pH300 ≈ 9.1) using cyclic voltammetry, contact electric resistance (CER), and electrochemical impedance spectroscopic (EIS) techniques. The measurement results showed that the electrochemical behavior of Cr dominated the electrochemical response of the alloys in the passive region at 200°C, but in the transpassive region Ni dominated. The passive region was quite narro...

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 Ni 3+ 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 Ni 3+ species from the barrier layer and oxygen evolution within the pores, or both.

Transpassive Dissolution Mechanism of Ni-Based Alloys in a Simulated Bleaching Solution : Effect of Alloying Elements

The transpassive dissolution of several nickel-based alloys has been studied in a simulated bleaching solution (0.014 M 0.017 M and 0.0025 M diethylenetriaminopentaacetic acid, DTPA, pH 2.8) by electrochemical techniques. The alloys have been prepared by welding superaustenitic stainless steels (UNS S31254 and S32654) with nickel-based filler metals (UNS N06625 and N06059). According to the rotating ring-disk voltammetric studies, the release of soluble high-valency products from the alloys plays a significant role in the transpassive potential region. Electrochemical impedance spectroscopic data point to a competition between transpassive dissolution and secondary passivation reactions. The experimental data have been compared to a previously proposed model for the transpassive dissolution of nickel-based alloys. The model treats the transpassive dissolution of Cr as a two-step reaction featuring a Cr(VI) intermediate. It assumes that the dissolution of Ni follows two parallel reaction paths, one of which involves a self-catalytic step. The kinetic parameters of the process for the studied alloys have been determined, and a discussion of the role of alloying elements such as Fe and Mo on the transpassive dissolution mechanism is given. © 2002 The Electrochemical Society. All rights reserved.

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.

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 and XPS studies of sputter-deposited ternary Mn–Ta–Cr alloys in chloride-free and chloride-containing sulphuric acid solutions

Amorphous manganese-rich (67–80 at.%) Mn–Ta–Cr alloy films were prepared by DC magnetron sputtering method. The corrosion behavior and stability of the Mn–Ta–Cr alloy films were examined in chloride-free and -containing 1 M H 2SO 4. AC and DC electrochemical techniques in combination with XPS analysis were used. Mn–Ta–Cr alloys exhibited very high corrosion resistance and stability up to transpassive dissolution region of chromium. The high corrosion resistance of Mn–Ta–Cr sputtered films is based on the formation of double oxyhydroxide passive film composed mainly of chromium and tantalum. The partial substitution of tantalum with chromium improves the corrosion resistance of the sputter-deposited alloys via accelerating the preferential dissolution of manganese and stabilizing alloy/passive film interface. A change in the passive film structure was observed when the alloys were anodically polarized at potentials higher than 0.6 V (SCE).

Mechanism of transpassive dissolution of nickel-based alloys studied by impedance spectroscopy and rotating ring-disc voltammetry

A generalised kinetic model of transpassive dissolution of industrial nickel-based alloys is presented in this paper. It is based on electrochemical impedance spectroscopic (EIS) and rotating ring-disk (RRDE) measurements on several nickel-based alloys in 0.5 M sulphate and chloride solutions with pH ranging from 2 to 7. The model assumes that the dissolution of Cr from the transpassive film occurs via a Cr(VI) surface intermediate species. The dissolution of Ni as Ni(II) is proposed to occur via two parallel paths featuring two reaction intermediates. The role of Fe as a secondary passivating agent and Mo as an accelerator of transpassive dissolution is also taken into account. The kinetic model has been found to reproduce quantitatively the current vs. potential curves and the impedance spectra for the studied nickel-based alloys in the transpassive region. The proposed mechanism is sensitive to the impact of alloy and electrolyte composition on the dissolution process. Thus it can serve as a starting point to describe the transpassive behaviour of engineering alloys.

Photoelectrochemical analysis on the passive film formed on Fe–20Cr in pH 8.5 buffer solution

The electronic properties of passive film formed on Fe–20Cr stainless steel in pH 8.5 buffer solution were examined by the photocurrent measurements of the film. The passive film on Fe–20Cr exhibited an n-type semiconductor, which was confirmed by anodic photocurrent. The photocurrent spectrum for the passive films formed on Fe–20Cr was almost same in shape to that for the passive film on Fe except for the large difference in photocurrent intensity, which demonstrated that the passive film on Fe–20Cr is composed of Cr-substituted γ-Fe 2O 3 involving the d-d and p-d electron transitions. However, the large difference in photocurrent intensity for passive film between Fe and Fe–20Cr was due presumably to the fact that Cr 3+ ions in passive film act as effective recombination sites of electron-hole pairs. The abrupt increase in the photocurrent intensity for the passive film formed on Fe–20Cr at a transpassive potential is due presumably to the decrease in the recombination site resulting from the reduction of Cr 3+ content accompanying the oxidation of Cr 3+ to Cr 6+ in transpassive region.

Observation of Species of Intermediate Oxidation Degrees, Formed during Anodic Dissolution of Chromium, by the Rotating Ring–Disk Electrode Method

The possibilities and limitations of the RRDE method in recording and identifying intermediate and target products of chromium dissolution in sulfuric acid solutions of pH 0.3–2 are analyzed on the basis of own and literature data. From the experimental data, partial currents of chromium dissolution to Cr(II) ions in the region of active dissolution of chromium and its transition into a passive state are computed. The Cr(III) ions can be discovered through the reaction of their reduction on an Hg(Au) ring electrode only at pH ≥ 1.8. The current efficiency of reactions that lead to the formation of Cr(II) and Cr(III) ions near the anodic-dissolution peak is other than 100%. Assumptions as to the causes of this effect are made. In the solutions studied, Cr(VI) ions are recorded with a 100-% current efficiency on a gold ring electrode in a transpassive region of the chromium disk potentials.

Growth and properties of Cr-rich thick and porous oxide films on Type 304 stainless steel formed by square wave potential pulse polarisation

Extraordinarily thick and porous oxide films are obtained on Type 304 stainless steel. The material was polarised in 5 kmol m−3 H2SO4 solution at 50–80 °C with applied potential modulated as square wave. The films formed are classified into two types according to applied potentials. For the anodic type, Fe and Cr actively dissolve at the lower potential of the square wave, EL, into Fe2+ and Cr2+, then re-oxidise at the higher potential, EH, to be mainly Cr3+ oxide. Alternatively, for the cathodic type, Fe and Cr dissolve at EH, which is in the transpassive region, then the dissolved Cr6+ ions are reduced into Cr3+ oxide at EL. Transmission electron microscopy showed that the porous film consists of nanocrystalline grains of around 20–30 nm in diameter, and has many pathways linking each other to be penetrated by ions and water molecular easily. The porous layer is basically composed of Cr2FeO4 spinel oxide with excess Cr. The chemical composition, particle size, and also lattice parameter could be controlled by changing EH, EL, and the width of the potential pulse, tH and tL. The porous oxide films formed by square wave potential pulse polarisation might be applied for various surface functional materials, i.e. a carrier of catalyst, oxide electrode, and biocompatible materials.

An SECM observation of dissolution distribution of ferrous or ferric ion from a polycrystalline iron electrode

The dissolution of iron as ferrous or ferric ion from a polycrystalline iron electrode during anodic polarization in pH 2.3 sulfate solution was evaluated by using scanning electrochemical microscopy (SECM). A graphite reinforcement carbon (GRC) microelectrode was employed as a probe electrode of SECM to detect ferrous or ferric ions dissolved from the iron electrode in the active-dissolution, passive or trans-passive region. The probe current above the iron electrode surface subjected to active-dissolution showed the dissolution distribution of ferrous ion, depending on the substrate grains. It was found that the active-dissolution rate of iron as ferrous ions from the grain on which the thicker film was formed in the passive region, was lower than that from the grain on which the thinner film was formed in the passive region.

Passive film breakdown during anodic oxidation of zirconium in pH 8 buffer containing chloride and sulfate

Metal passivation and passive film breakdown in aqueous systems are complex processes. Zirconium passive films grown in basic solutions containing buffer; chloride and buffer; and chloride, sulfate and buffer have been examined with particular attention to the passive and transpassive regions using cyclic voltammetry (CV). Mechanically polished zirconium electrodes have been oxidized anodically in pH 8 buffer solutions containing 0.021 M chloride, with varying sulfate concentration. The CV curves show that zirconium forms passive layers under these conditions, which are more stable with increasing sulfate concentration. A model is proposed to explain the passive to transpassive transition taking oxygen evolution, crystallization and ohmic behavior into account.

An Alternative Explanation for the Apparent Passivation of Molybdenum in 1 mol/L Hydrochloric Acid

A key question arising from studies on the corrosion behavior of molybdenum has been whether the passivation of molybdenum originates from a three-dimensional oxide film or from chemisorbed oxygen. In the present paper an alternative explanation for the apparent passivation behavior of molybdenum is given based on XPS measurements in combination with electrochemical techniques. Below the transpassive potential region, anodic current is obtained only if the sample has been cathodically activated and not after activation by ion etching. The experiments also show that the commonly used cathodic activation is insufficient to reduce the air-formed film. Thus, for such samples the anodic current in the apparent passive region could be due to oxidation of low valent oxides on the surface. However, a transient anodic current is observed also for ion-etched samples that have been subjected to cathodic polarization. In addition, impedance measurements show an unexpectedly large capacitance in the apparent passive region, indicating some kind of adsorption equilibrium at the metal-solution interface. This leads to the suggestion that hydrogen formed in the reduction process can diffuse into the molybdenum metal and cause a limiting oxidation of hydrogen upon positive polarization. Thus, in the apparent passive region the current is mainly due to oxidation of hydrogen and an extension of the transpassive corrosion yields a very low corrosion current for molybdenum. This is in agreement with the XPS experiments, where no more than a monolayer of oxidized molybdenum can be detected in the apparent passive region. © 2001 The Electrochemical Society. All rights reserved.

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

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.

Dissolution and Passivation of Nickel-Free Austenitic Stainless Steel in Concentrated Acids and Their Mixtures

Electrochemical polarization behavior of nickel-free austenitic stainless steel in concentrated phosphoric (H3PO4), sulfuric (H2SO4), acetic (CH3COOH), and formic (HCOOH) acids and binary and ternary solution mixtures was investigated by a potentiostatic polarization technique at 30 ± 1°C. The cathodic polarization curves were almost linear and similar in all binary and ternary solution mixtures and resembled that of the hydrogen evolution reaction. However, in ternary solution mixtures of H3PO4, H2SO4, and HCOOH, the shape of the curves indicated composite reactions of hydrogen and oxygen reduction. Anodic polarization curves showed active, passive, and transpassive behaviors in all the solution mixtures. When H2SO4, CH3COOH, or HCOOH was added to H3PO4, the aggressive nature of the binary solution mixtures toward the alloy was intensified in the same order. Thus, maximum corrosion was observed in the case of binary solutions of H3PO4 with HCOOH. However, in the ternary solution mixtures, the corrosion of the alloy decreased. In the transpassive region, a secondary current peak and current plateau were observed in most cases. However, electropolishing was not achieved in these solution mixtures. The mass-transfer-controlled condition is probably not achieved in such cases even when a current plateau is observed. Resulting surface morphologies were characterized with scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smoothing action was observed in the transpassive region.

Anodic polarization behaviour of cast iron alloys in deaerated concentrated H3PO4 solutions containing Cl− and F− ions

The anodic behaviour of four cast iron alloys containing up to 16.7% Ni, in deaerated 60 wt% H3PO4 with and without 5 × 10−3 M F−, Cl− ions and 1:1 Cl−/F− mixture was studied by the potentiostatic technique. Values of E corr of the alloys are markedly influenced by their composition. The anodic behaviour in the active region is controlled by Fe in the alloys and the dissolution reaction is characterized by Tafel slopes, b a, between 64 and 88 mV (decade)−1. A two-electron transfer mechanism for the anodic dissolution is proposed. Passivation of the alloys is due to the formation of oxide layers including Fe2O3 and/or Fe3O4. Both critical and passive c.d. (I cc and I P) are markedly increased in the presence of Cl− ions, but the presence of F− ions inhibit the active dissolution of the alloys. The Tafel slope for oxygen evolution reaction (o.e.r.) in the transpassive region, $$b_{{\text{O}}_{\text{2}} } $$ is 240 ± 25 mV. In the proposed mechanism for the o.e.r., the rate determining step is an electron transfer reaction and possible interpretation of the high Tafel slopes is given based on the dual barrier model.

Corrosion of chromium plated rotor in drilling fluid

The causes of hard chromium layer spalling in the rotor of a downhole hydraulic motor were analysed. The rotor rotates within a stator in contact with saline drilling mud. Zones with lost coating and base metal attack were observed along the whole rotor. Microscopic examination showed atypical macrocracks that run through the whole chromium layer. These fissures started at base steel heterogeneities or pores in the coating itself. The microprobe analysis of the metal–coating interface showed no metallic deposit between the steel and the chromium layer. Chromium plated steel and base steel showed similar electrochemical behaviour in the mud aqueous phase, whereas pure chromium remained passive up to the transpassive region. The failure was attributed to the salt solution penetrating through macrocracks to the substrate. As the steel substrate was corroded, chromium coating lost adherence and became detached under the influence of frictional forces.

Impedance and photoelectrochemical properties of porous oxide film on Type304 stainless steel formed by square wave potential pulse polarisation

Abstract An extremely thick and porous oxide film can be obtained on Type304 stainless steel. The material was polarised in 5 M H 2 SO 4 solution at 50–80°C with applied potential modulated as a square wave. The porous oxide and/or hydroxide films were formed when the lower potential of the square wave, E L , is in the active region of Fe and Cr, and the higher potential, E H , in the passive region of Cr; this is noted as an anodic type, or when E H is in the transpassive region of Cr and E L in the passive region of Cr, that is, the cathodic type. The electrochemical impedance and photo-electrochemical response were examined for both films. For the anodic type film, the capacitance is larger than that of the substrate steel, and is nearly proportional to the total thickness of the film. In addition, the larger capacitance was observed for the film with the finer porous structure. Therefore, the film ∣ electrolyte interface, at which the electrochemical capacitance arises, is distributed across the whole cross section of the porous film. On the other hand, the capacitance of the electrode with the cathodic type film was almost equivalent to that of the substrate steel. Therefore, the capacitance arises only at the substrate ∣ solution interface. The photo current of both anodic and cathodic porous films exhibited p-type semiconductor responses. The photocurrent for the anodic porous film increased with increasing film thickness, then passed through a maximum at a thickness of about 10 nm, and finally decreased. On the other hand, the photocurrent for the cathodic film remains approximately constant with increasing thickness. The electrochemical impedance and the photoelectrochemical response indicated that the thick porous film has various electrochemical properties depending on the structure, which can be ordered under electrolysis conditions.

Transpassivity mechanism of iron–chromium–molybdenum alloys studied by AC impedance, DC resistance and RRDE measurements

The transpassive dissolution of Fe-25%Cr, Fe-25%Cr-5%Mo and Fe-25%Cr-10%Mo alloys in 1 M H 2SO 4 was studied with a combination of electrochemical techniques—conventional and rotating ring-disk voltammetry, impedance spectroscopy and the d.c. resistance measurements with the contact electric resistance (CER) technique. Rotating ring-disk studies indicated that the amount of soluble Cr VI released from the alloys in the transpassive region increases significantly with the Mo addition in the substrate. The electronic resistance of the anodic film was found to decrease as Cr VI is released and to stabilise subsequently due to the formation of a secondary film depleted in Cr. Impedance spectra of the Fe-25%Cr alloy were found to include contributions from both the film growth and transpassive dissolution reactions. Mo addition was shown to have a catalytic effect on the transpassive dissolution of Cr. The experimental results were compared to a generalized model of the transpassivity of Fe–Cr alloys. The model represents the anodic film as a highly doped n-type semiconductor—insulator—p-type semiconductor (n-i-p) structure. Injection of negative defects at the film/solution interface results in their accumulation as a negative surface charge. The transpassive dissolution reaction is assumed to be a two-stage process featuring a Cr IV intermediate. The relaxation of the Fe fraction in the outermost cation layer of the film is taken into account as well. Fitting of the experimental data on the basis of equations derived for the steady state and impedance response enable the determination of the kinetic parameters of transpassive dissolution as depending on the Mo content in the alloy.