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 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.     

Anodic oxidation and kinetics of titanium in 1 M chloride solutions

The anodic behaviour of titanium was investigated in 1 M chloride solutions of pH ranging from 0.0 to 12.0 by using electrochemical techniques combined with chemical analysis. No active region appears in solutions of pH above 2.0. In the passive region, the amount of titanium dissolved into the solution during 1 h oxidation decreases with increasing pH, and no dissolution occurs at pH 10.5. The thickness of passive films estimated from the electric charge accumulated on the surface increases linearly with the potential. The kinetic analysis between the anodic current and the film charge indicates that the growth of the passive film obeys the inverse logarithmic rate law, and the kinetic parameters are calculated.     

High rate anodic dissolution in chloride solutions of steel after electrothermochemical treatment

High rate (up to 15 A/cm²) anodic dissolution of St3 steel in chloride solutions (1–6 M NaCl) was investigated after its electrothermochemical al treatment (nitriding and thermal treatment). Various potential ranges (or current density ranges) can be emphasized: the region of anodic anion activation (sublimiting currents), the diffusion limitation area (caused by salt passivation), and the transpassive dissolution region. It is shown that the heat treatment influence is the most substantial in the region of salt passivation (or the limiting current region). The double role of the hydrodynamics is revealed during dissolution under these conditions: the influence of the ionic mass transfer rate in the solution and of the removal of the hard carbonaceous products produced as a result of the selective dissolution of the components of the nitrided steel. It is shown that, in sublimiting currents and transpassive dissolution ranges, the specific dissolution rate depends on the nitrogen content of the studied steel.     

The influence of sulphate,chloride and nitrate anions on the cyclic strain-enhanced dissolution behaviour of mild steel

The transient electrochemical behaviour of mild steel during cyclic plastic deformation in 0.5 M aqueous solutions of sulphate, chloride and nitrate ions was studied in the pH range 3–14. In all solutions dissolution current transients were observed during both tensile and compressive strain cycles. The magnitude of the dissolution effects, however, depended upon the solution composition, pH, potential and the extent of strain cycling. Analysis of the current transient characteristics revealed that for the anion concentration employed in this study, sulphate appeared to exhibit more aggressiveness than chloride and both these exerted a greater disruptive influence on surface films than nitrate ions. Tests in 0.25 M Na₂SO₄ revealed that the effect of sulphate could not be attributed simply to the increased ionic strength of the solution.     

氢对304不锈钢阳极行为的影响

<正> 虽然奥氏体不锈钢在42％MgCl_2沸腾溶液中的应力腐蚀并不是由氢致开裂引起的,但在应力腐蚀过程中生成的氢可以进入试样并可在裂纹前沿富集。为澄清进入试样的氢是否对阳极溶解过程产生促进作用,曾有人用失重法、晶间腐蚀和极化曲线变化等研究氢对阳极行为的影响,并得出氢促进阳极溶解的结论。应当指出,充氢试样的阳极电流包括了氢的氧化电流,因此充氢后阳极电流的增大并不证明氢促进了阳极溶解过程。虽然失重法能反映氢的促进作用,但只适用于腐蚀较为严重的情况。     

XPS investigations of electrochemically formed passive layers on Fe/Cr-alloys in 0.5 M H2SO4

Passive layers on Fe20Cr and Fe15Cr were prepared by anodic oxidation in 0.5 M H₂SO₄ and were investigated by X-ray photoelectron spectroscopy. The preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed under protection of purified argon without any contact to the laboratory atmosphere. The prepassive layer consists of a homogeneous film with Cr(III)hydroxide and Cr(III)-sulphate with a low content of Fe(II). In the passive potential range a bilayer structure with enrichment of Cr(III)-oxides with an outer hydroxide and an inner oxide part is formed. Iron exists only in a bivalent oxidation state. In the transpassive potential range a pronounced change of the layer composition was observed: The outer part of the transpassive layer is formed predominantly by Fe(III) species whereas the inner part still contains a strong enrichment of Cr₂O₃.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

Transpassive Behaviour of Iron–Chromium Alloys in Acidic Sulphate Solutions

Abstract

The mechanism of the transpassive behaviour of Fe–Cr alloys containing 5–30% Cr has been investigated using stationary and rotating disc electrodes in acidic sulphate solutions. At potentials > 1000 mV (NHE) the alloys undergo transpassive dissolution terminating in a plateau, at a c.d. the logarithm of which increases linearly with Cr content. This relationship, and its apparent independence of potential, lead to the assumption of limiting current behaviour, which should be proporiional to the rates of diffusion of the substances reacting at the electrode surface. On the basis of this assumption and the independence of the plateau c.d. for the 15% Cr alloy on rotation, it appears that surface processes are rate determining and that solution soluble intermediates do not participate. Restriciion by diffusion occurs only within the passivaiing fllm on the electrode.     

A study of the effects of inhibitive and aggressive ions on oxide-coated aluminium using secondary ion mass spectrometry

A Cameca Ion Analyser has been used to examine aluminium surfaces, supporting barrier-type anodic films ca. 720Å thick, after natural immersion for 1–25 h in 1M chloride and chromate/dichromate solutions of varying pH, and neutral 1M phosphate solution. In chloride solutions, chloride is present on the film surface, but not within the film material per se, within the limits of measurement. The degree of film thinning is governed by pH, being more pronounced in alkaline than acid solutions. In chromate/dichromate solutions, chromium uptake occurs by two distinct mechanisms. The first is associated with relatively rapid chromate/dichromate reduction to Cr₂O₃, probably hydrated, within flaws in the anodic film. The second is associated with a more gradual direct chromate/dichromate anion entry into the bulk material probably through the intercrystalline regions in the film, ultimately resulting in film thinning, even in neutral solutions. In phosphate solutions, phosphate anions penetrate and thin bulk anodic alumina in a similar manner to chromate/dichromate anions. The results are interpreted on the basis of the differing surface chemical properties of the various anions in relation to bulk alumina.     

Temperature effect in the corrosion resistance of Ni-Fe-Cr alloy in chloride medium

The corrosion susceptibility of alloy 33 in 0.5 mol/L sodium sulphate solutions containing or not 0.1 mol/L sodium chloride was tested at three different temperatures: 22 °C, 40 °C and 60 °C. Electrochemical studies were performed using corrosion potential measurements (E_corr) as well as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion potential measurements showed that alloy 33 was passivated by a previously air formed film which was not destroyed during immersion in both solutions. No corrosion was observed during these tests although the temperature affected the film. Potentiodynamic polarization experiments showed that at high anodic potentials the previous film was broken up, and localized corrosion occurred in both solutions and at the three temperatures tested. Electrochemical impedance spectroscopy tests confirmed the presence of a stable passive film on the alloy surface at open circuit potential. Mott-Schottky analysis indicated that the passive film is an n-type semiconductor due to the presence of point defects of donor species, such as oxygen vacancies and interstitial metallic cations. As the potential increases the Cr(III) present in the barrier layer oxidizes producing Cr(VI) soluble species. The dissolution creates metallic cation vacancies that are acceptor species and the film changes from n-type to p-type semiconductor. The passive film rupture and the following localized attack are related to the drastic oxidative dissolution of the film at high anodic potentials, independent of its p-nature, chloride presence or increased temperature.     

In-depth profiles of anodic oxide films on FeNi alloy in boric acid-sodium borate solutions

The in-depth profiling of anodic oxide films formed for 1 h on a 55Fe-45Ni alloy in boric acid-sodium borate solutions was made by the simultaneous use of AES (Auger Electron Spectroscopy) and sputter-etching with Ar⁺ ion to determine the thickness and composition of anodic oxide films and to elucidate the experimental variables which affect them. The anodic oxide film was thicker and richer in the Fe component in pH 6.48 solution than in pH 8.45 solution. It was estimated from the in-depth profile of the anodic oxide film that a significant part of the Fe component in the anodic oxide film was in the di-valent state. Enrichment of the Ni component at the oxide/substrate interface and depletion of Ni component in the film were measured as a function of pH and anodic potential. These results are explained in terms of the combination of the following factors: (i) preferential dissolution of the Fe component, (ii) preferential oxidation of the Fe component, and (iii) preferential anodic deposition of the Fe or Ni component.     

Prevention of transpassive dissolution of austenitic stainless steel counter electrodes in nickel based electrolytic coloring solutions for anodized aluminum

In low nickel ion containing, weak acidic, electrolytic coloring solutions for anodized aluminium, austenitic stainless steel counter electrodes dissolve transpassively. The role of nickel and cobalt ions in preventing the transpassive dissolution of AISI Type 304 and 316 austenitic stainless steel counter electrodes was investigated. Potantiodynamic polarization method was used. The test solutions were a buffer (29.66 g/lt boric acid), a buffered supporting electrolyte (32.6 g/lt boric acid + 26.18 g/lt magnesium sulphate heptahydrate + 14,95 g/lt ammonium sulfate) and the coloring solutions (supporting electrolyte with different nickel (27.6–101 g/lt as heptahydrated sulfate) and cobalt (33–166 ppm) content). The oxidation of bivalent nickel ions starts 200 mV lower than the transpassive dissolution of stainless steel. However, even a small amount of cobaltous ions (165 ppm) in the solution produces a much steeper oxidation peak on the top of the nickel oxidation current. In order to transpassively dissolve a stainless steel electrode in cobalt containing nickel solutions, anodic current must be greater than the combined oxidation peak of nickel and cobalt ions, otherwise the working potential of the stainless steel electrode is always kept at the passive region hence the transpassive dissolution is prevented.     

Influence of some anions on the stress corrosion of low alloy steels in aqueous solutions

Polarization and stress corrosion behaviour of two types of low alloy steels, AISI 4130X and AISI 4145, were studied in agitated 0.05 M sodium acetate and 0.05 M trisodium phosphate solutions saturated with natural gas at room temperature. The magnitude of currents, shown in potentiokinetic polarization curves in the range of —0.8 V(SCE) to +0.8 V(SCE) in these media, is very small compared with earlier results obtained in sulphate and chloride solutions.Stress corrosion experiments conducted using a slow strain rate technique under cathodic conditions indicate the onset of hydrogen embrittlement on both types of steel in sulphate, chloride, acetate and phosphate solutions. Under anodic conditions, the results in 0.05 M sodium acetate solutions indicate strong susceptibility to stress corrosion compared with no susceptibility in 0.05 M trisodium phosphate solutions if there is no onset of pitting. These results are in contrast with results showing strong anodic dissolution and consequent absence of stress corrosion under anodic conditions in sulphate and chloride media.Results of Auger and IR analyses of anodic surface films formed in acetate and phosphate solutions are presented and discussed in relation to their observed mechanical behaviour.     

pH值和氯离子浓度对铁在酸溶液中的腐蚀电化学行为的影响

发展了一种根据弱极化区的单支极化曲线确定腐蚀电流密度Ⅰ_(corr)和阳极反应与阴极反应的Tafel斜率b_a与b_c的方法。利用这种方法研究了溶液pH值与Cl~-浓度对铁在酸溶液中的腐蚀电化学行为的影响,并用强极化区的实验数据进行了校验。结果表明,OH~-和Cl~-都参与了铁的阳极溶解过程,反应级数分别为1和0.5;b_a和b_c分别为40mV和120mV。提出了一个铁在含Cl~-的酸中阳极溶解反应的动力学模型。实验结果还证明,在pH值与Cl~-浓度两者对E_(corr)和Ⅰ_(corr)的影响之间并无交互效应。     

The dissolution and passivation of Fe and Fe-Cr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH values (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.     

Electronic properties of oxide layers on Fe20Cr in comparison with Fe

The electronic properties of passive layers on Fe20Cr were examined at pH 8.4 by C(U)- and ETR-measurements and compared with the experiments on Fe. The behaviour of both systems can be explained with the same models. This means, that electronic models which were developed for passive layers on Fe can be transferred to the passive layers on Fe20Cr. The band structure must be modified by surface charge due to the transpassive dissolution of Cr. The cathodic ET-reaction is determined by the tunnelling through the space charge layer at potentials close to U_fb. The anodic electron transfer reaction is controlled by a Poole-Frenkel effect for both systems, where the rate limiting step is the transport between the resonance centres, which are given by the donor concentration N_D.     

The polarization behaviour of chromium in acidic sulphate solutions

The polarization behaviour of chromium was studied in solutions of various pH, with particular regard to the hydrogen evolution reaction and chromium dissolution. Hydrogen evolution occurred on active and passive chromium and the exchange current density was ca. 10^?6 A/cm² at pH 1.0 on both electrodes. The rate of active dissolution of chromium did not depend on pH, an observation that indicated the absence of a hydroxyl-containing intermediate in the rate determining step. The transpassive dissolution of chromium was strongly dependent on pH, and it was suggested that the rate was controlled by a base-accelerated anodic dissolution of an oxide film.     

AN AES STUDY OF ANODIC OXIDE FILM ON 1Cr18Ni9 STAINLESS STEEL

用Auger电子能谱仪(AES)研究1Cr18Ni9不锈钢表面阳极氧化膜的组分分布。用Ar~+溅射剥层所作的剖面分析发现过钝化阳极氧化膜的表面层中含有大量的Mo与Cr的离子,中间层基本上是一个富Cr的氧化层,Fe处于中性态;然后逐渐过渡到基体内,在过渡层中氧逐渐减少到零。与硝酸钝化膜和自然钝化膜中元素的剖面分布相比,过钝化阳极氧化膜的富Cr区要比后二种膜宽得多,膜中Cr的氧化程度也比较完全。根据这些特点讨论了过钝化阳极氧化膜抗腐蚀性能比硝酸钝化膜优良的原因。     

The dissolution and passivation of Fe and FeCr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH value (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.