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.     

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。     

氯离子和电偶腐蚀对碳钢在热钾碱溶液中腐蚀的影响

研究了Cl-对碳钢和不锈钢在(K2CO3+KHCO3+ V2O5)溶液中腐蚀的影响,结果表明Cl-对阳极钝化曲线所示的腐蚀电位、钝化区间、过钝化电位和析氧过程无明显影响,未见试样有孔蚀的迹象.但随着Cl-离子浓度增大,其维钝电流密度也增大.90℃时,不锈钢-碳钢在此溶液中构成电偶腐蚀时,测得碳钢腐蚀速率为0.064 mm/a,比未成偶对时碳钢的腐蚀速率大一倍.90℃时,将已钝化的不锈钢-碳钢在(K2CO3+KHCO3)溶液中构成电偶腐蚀时,测得碳钢腐蚀速率为4.0 mm/a,比含有V2O5溶液中碳钢的腐蚀速率大64倍.     

Effects of a magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in weakly alkaline solutions with or without halides

The effects of a 0.4 T horizontal magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in bicarbonate solutions of various concentrations and in dilute bicarbonate solutions with or without halides are investigated by electrochemical polarisation measurements. The applied magnetic field does not affect the activation-controlled anodic current, the steady passive current and the transpassive current, but significantly affects the activation-passivation transition processes for iron in bicarbonate solutions without halides. The effects of magnetic field are strongly dependent on passivation mechanisms that result in different types of surface films and corresponding rate determining steps of film dissolution. There is a synergistic effect between the applied magnetic field and halides, chlorides or bromides, on attacking the passivation of iron in dilute bicarbonate solutions. The effects of the magnetic field are analysed based on the previously proposed electrochemical kinetics equations. The magnetic field affects the anodic polarisation behaviour through its enhancing effects on mass transport processes at the precipitation-dissolution type surface film/solution interfaces. The magnetic field shows little or no effects on continuous and steady passivation films where the oxidation rate is controlled by mass transport processes within surface films. Magnetoelectrochemistry measurements are suggested as a prospective method for researches on corrosion or passivation mechanisms.     

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.     

Über die Ursachen der primären anodischen Passivierung,insbesondere der Eisenmetalle

Origin of primary anodic passivation, in particular or ferrous metals It is shown, that a quantitative interpretation can be given of the characteristic current density decrease which accompanies increasing polarization in acid solutions and which is characteristic of the primary anodic passivation process on ferrous metals; the interpretation is arrived at when two concurring reactions are stipulated to take place with participation of water. One of these reactions yields active dissolution, the other one results in the formation of a passivating MeO layer. This passive layer increases in thickness until an oxide phase is formed; his process occurs with preference at surface defects and can be interpreted in terms of e.g. the theory developed by Mott and Cabrera for the growth of thin oxide layers. With high exchange current densities of the anodic dissolution, e.g. in hydrochloric acid, the direct formation of solid salt on the metal surface may produce the same current/potential characteristics. Since the passage of electric charges through the ion-conducting salt layer is efficiently inhibited up to high potentials, much larger passivity regions are found here. Reference is made to the influence of surface structure and pretreatment on the kinetic parameters of metal dissolution.     

Open circuit potentials of metallic chromium and austenitic 304 stainless steel in aqueous sulphuric acid solution and the influence of chloride ions on them

Open circuit potential measurements and cyclic voltammetry of chromium and 304 stainless steel in deaerated aqueous H₂SO₄ solution of pH 1, without and containing NaCl in the concentration range 1–4 M revealed that chromium exhibits two stable open circuit potentials both having the character of a Wagner–Traud corrosion potential. One, E_corr.1, was established on the passive surface formed by previously exposing Cr to air or by potentiostatic passivation in a controlled manner, and the second one, E_corr.2, at the bare Cr surface obtained by prolonged cathodic activation. There was a small difference in the E_corr.1 values depending on the properties of the passive layer. Addition of NaCl accelerates to some extent, the hydrogen evolution reaction on the passive surface, while the same reaction on the bare surface was somewhat inhibited by NaCl. On the other hand, presence of NaCl accelerates the anodic reaction on the bare surface, and it activates the dissolution of the passive layer so that the passivation currents increase with addition of NaCl. This effect is so large that at concentrations of NaCl larger than 3 M, the destruction of the passive layer was so fast that in a matter of seconds the Cr is activated, and the only stable corrosion potential observed was E_corr.2. No pitting of Cr in the presence of NaCl was observed up to the transpassive potentials. 304 stainless steel could not be activated in sulphuric acid solution, and its open circuit potential, unlike the corrosion potential of the 400 types of stainless steel, was established by the hydrogen evolution reaction on the passive steel surface. The small anodic peak often observed on 304 stainless steel if the metal had been cathodically pre-treated is a pseudo-peak due to the anodic oxidation of hydrogen absorbed inside the metal. This finding should be elaborated more in recommendations (e.g. ASTM standards) for the application of electrochemical corrosion rate measurement to 304 stainless steel corrosion. Addition of NaCl activates the anodic dissolution of steel with the current of the passivation peak being proportional to the NaCl concentration. Unlike chromium, austenitic 304 stainless steel achieves only one corrosion potential in sulphuric acid, both in the presence and absence of NaCl, with the value of ca. −0.200 to −0.350 V (SCE) in the absence and −0.450 V (SCE) in the presence of NaCl, when steel corrodes as the active metal.     

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。     

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.     

High-speed anodic dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: II. Nitrate solutions

Anodic dissolution of two heat-resistant chrome-nickel alloys containing 12% (weight) tungsten and 8% tungsten with 6% rhenium in a 2 M NaNO₃ solution was investigated using the rotating disk electrode method at current densities up to 30 A/cm². It is shown that anodic dissolution of these alloys occurs in the transpassive region of potentials with transition of the components in solution to forms with the highest oxidation level. Various mechanisms of alloy dissolution determining the processing speed (together with electrochemical dissolution) are proposed, including disintegration of the hardening phase, chemical oxidation of low-valence intermediates by solution components, and electrochemical formation of surface oxide layers. The results of change in the chemical composition of surfaces depending on the processing regimes are presented. Some variants of control by regimes of electrochemical dimensional processing (ECDP) of details from these alloys to achieve the optimal parameters of ECDP are proposed.     

冷轧带钢电解清洗极板腐蚀机理和原因

采用线性电位扫描法研究了碳钢在不同脱脂剂浓度下的阳极极化过程,进一步研究了碳钢在不同不同脱脂剂浓度条件下、不同电位下的恒电位、恒电流电解行为,在此基础上对生产现场进行了考察。电化学实验表明：碳钢阳极极化过程依次经历活化区、钝化区、过钝化区及极限区;活化区基体活化溶解电流小,不影响电极寿命;过钝化区电极表面开始发生析氧反应;极限区析氧反应达到极限,使基体腐蚀反应大量发生,电极寿命降低;脱脂剂浓度越低、阳极电位越高,电极表面基体溶解反应速率越大,相应电极使用寿命越短。实际生产中,极板表面由于电流分布不均使其局部电流超过极限电流导致局部腐蚀发生,而电解槽内极板安装精度、电极表面气泡无法逸出、极板表面污泥淤积等都均会影响电流分布。     

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

用XPS研究了1Cr18Ni9不锈钢表面氧化膜的组分与结构。实验表明膜中的Cr/Fe约为体内的3倍,并在组分和价态上具有不均匀的三层结构;表面层的主要成分为Cr,Fe的含氧氢氧化物;中间层的主要成分为Cr,Fe的氧化物;而过渡层则主要由金属状的Fe和少量Fe,Cr的氧化物组成。当阳极氧化膜用Na_2MoO_4溶液处理后,发现在整个氧化膜中都有Mo存在,其含置约为2％。表面层与中间层统称为阻挡层,它是不锈钢抗腐蚀的主要壁垒。过渡层介于阻挡层和基体之间,在这区域中有透过阻挡层进来的少量的氧存在。根据氧化膜中间层中Cr,Fe的氧化程度和过渡层中氧的含量与透入深度,讨论了阳极氧化膜与其它钝化膜(硝酸钝化膜和自然钝化膜)在抗腐蚀性能上差异的原因。     

Corrosion Behaviour of Manganese-Containing Stainless Steels. II. Potentiodynamic measurements in H2SO4 solutions

The behaviour of steels with (%) 17.3 Cr, 5.3 Ni, 0.3–13.9 Mn during potentiodynamic polarization k tests is comparable to that of 18 8 CrNi steels. Sharp peaks are found in the polarization curves; they correspond to the dissolution of the iron base: This region is followed by a passive zone characterized by low and potential-independent current flow. At a certain potential, however, a steep current rise sets in. In diluted acid there is even a transpassivity zone corresponding to the dissolution of Cr and Mn. The beginning of this zone corresponds to the beginning of the steep current rise in more concentrated solution. Cyclic cathodic reduction and anodic oxidation results in a considerable reduction of the height of the peaks in the active region, while this treatment has no effect on the transpassive region. The optimum composition of a steel would be (%) 17.3 Cr, 5.3 Ni, 5.6 Mn; higher Mn contents deteriorate the corrosion behaviour.     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

Passivity and passivity breakdown of zinc anode in alkaline medium

The electrochemical behaviour of zinc in NaOH solutions has been investigated by using potentiodynamic technique and complemented by X-ray analysis. The E/i curves exhibit active, passive and transpassive regions prior to oxygen evolution. The active region displays two anodic peaks. The passivity is due to the Formation of a compact Zn(OH)₂ film on the anode surface. The transpassive region is assigned to the electroformation of ZnO₂. The reverse sweep shows an activation anodic peak and one catholic peak prior to hydrogen evolution. The influence of increasing additives of NaCl, NaBr and Nal on the anodic behaviour of zinc in NaOH solutions has been studied. The halides stimulate the active dissolution of zinc and tend to break down the passive film, leading to pitting corrosion. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: I^? > Br^? > Cl^?. The susceptibility of zinc anode to pitting corrosion enhances with increasing the halide ion concentration but decreases with increasing both the alkali concentration and the sweep rate.     

High-speed anode dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: I. Chloride solutions

The results of a study on the anode dissolution of two heat-resistant chrome-nickel alloys containing tungsten (12 wt %) as well as tungsten and rhenium (8 wt % of W and 6 wt % of Re) are described. The experiments took place in 2 M NaCl at a current density of up to 40 μ/cm² using a rotating disk electrode. It is shown that the alloy with the greater tungsten content dissolves at a lower rate (due to the formation and accumulation of insoluble oxides layers on the surface) and at a current density lower than the maximum anode current density for the basic component (nickel, and, probably, cobalt) of the anodic dissolution. Transpassive dissolution takes place under conditions of thermokinetic instability of the electrode process. In this case, a decreasing dependence of the current efficiency on the current density is observed, and the dissolution rate is independent of the tungsten concentration in the alloy. In the region of the maximum anode currents and the transition from one area of dissolution to another, abnormal anode dissolution takes place due to chemical oxidation of intermediate products by oxidizers—anode-dissolution products or solution components. The results of varying the chemical composition on surfaces depending on the treatment mode are presented.     

Technical note: Transpassive anodic dissolution of nickel in sulphuric acid

Abstract

Anodic dissolution behaviour of nickel in 3 and 10M H₂SO₄ solutions in the transpassive region has been investigated. Nickel dissolution current efficiency has been determined by splitting potentiostatic polarisation curves into oxygen evolution and nickel dissolution curves by carrying out solution analysis. Nickel dissolution current efficiency is minimum at 1800 mV(SCE) in 3M H₂SO₄ and primary passivation is not observed in 10M H₂SO₄, To study the stability fo the passive film formed on the surface of nickel in 3 and 10M H₂SO₄ solutions, potential decay curves have been recorded from various anodic potentials.     

Anodic Behaviour of 17–7 Precipitation Hardening Stainless Steel in Acid Media

Abstract

The anodic behaviour of 17–7 precipitation hardening stainless steel in 0.5 M H₂SO₄ and 0.5 M H₂SO₄ + Cl^? solutions was investigated in detail. The coulometric areas of the two anodic peaks in the potentiodynamic curve in the chloride-free solution are dependent upon the duration of the free corrosion between the preliminary cathodic activation and the starting of the potentiodynamic scan. When the free corrosion time exceeds 6 minutes the peak at the more active potential disappears completely.

The influence of the free corrosion time was much less evident in the presence of chloride ions; in these conditions anodic currents were greater.

It was also established that the two anodic peaks correspond to the active-passive transitions relative to the two phases composing the steel: austenite and δ-ferrite. This was seen by potentiostatically etching the steel in the active dissolution ranges of both anodic peaks.

Finally it was shown, both micrographically and by comparing the anodic curves of pickled and polished specimens, that ordinary nitric–hydrofluoric pickling solutions selectively attack the austenite.     

The electrochemical behavior of austenitic stainless steel with different degrees of sensitization in the transpassive potential region in 1 M H2SO4 containing chloride

This paper investigates electrochemical methods to estimate the degree of sensitization (DOS) for as-rolled and solution-treated austenitic stainless steels (AISI 304). The change in DOS was introduced by heat treatment of various time periods at 700 °C, and the electrochemical experiments were conducted at 27 °C in 1 M H₂SO₄ + 0.2 wt.% NaCl. The results show that there is no obvious difference in the anodic polarization curves of the specimens with the differences in DOS. However, the DOS of specimens can be clearly differentiated with AC impedance response in a particular transpassive potential region. In the transpassive potential region of the anodic polarization curve, as overpotential is increased beyond the breakdown potential, three types of anodic dissolution, passive dissolution, grain-boundary attack, and pitting can be characterized. The DOS can only be correctly evaluated with AC impedance response in the middle of the transpassive potential region, where anodic dissolution is of grain-boundary attack type, from 1.05 to 1.1 V. The chromium-depleted zone was preferentially attacked and an intergranular corrosion was found for the serious sensitized specimen after AC impedance test. The healing effect was observed in as-rolled specimens as the specimens were heated more than 72 h, but the effect was not found in the solution-treated specimens sensitized for up to 480 h at 700 °C.