Transpassive Potential Research Articles

The stability and breakdown of passive oxide films on metallic anodes

The passive oxide film on metals in aqueous solutions is stable (corrosion-resistive) to the extent that the Fermi level at the film/solution interface remains within the electronic energy band gap of the film (in the state of electronic band edge level pinning). Transpassivation that leads to a potential-dependent dissolution of the passive film occurs in the range of electrode potential in which the Fermi level is pinned in the electronic valence band level at the film/aqueous solution interface. Chloride adsorption introduces the surface state of electronic acceptor levels on the passive oxide film. When the Fermi level of passive metal anodes is pinned at the chloride-induced surface state, the localized transpassive dissolution of the passive film proceeds at the chloride adsorption sites to cause a local film breakdown. The critical potential for the local film breakdown appears less anodic (more negative) than the general transpassivation potential.

Chemical perturbation of the passive–active transition state of Fe in a sulfuric acid solution by adding halide ions. Current oscillations and stability of the iron oxide film

Perturbation of the passive–active transition state of the Fe ∣ 0.75 M H 2SO 4 system was carried out by adding a low concentration of halide ions (Cl −, Br −, I −). The dynamical response of the system to this chemical perturbation is studied by following changes observed in the I– E and I– t curves. The changes observed in I– E curves within the passive–active and passive regions are: (i) an extension of the oscillatory region beyond the Flade potential of the halide-free system, towards the stable passive region; (ii) a shift of the transpassivation potential towards lower values; (iii) an increase of the current in the passive region; (v) deviation of the maximum oscillatory current from the kinetics of the active region. According to theses changes the aggressiveness of halide ions is classified in the order Cl −>Br −>I −. The main changes observed in potentiostatic I– t curves are: (i) existence of an induction period of time before the oscillations start; (ii) the oscillations deviate from the monoperiodicity observed for the halide-free system and they become complex periodic and aperiodic. The dynamical response of the halide-free Fe ∣ 0.75 M H 2SO 4 system is studied as a function of the applied potential, the concentration and nature of halide ions. On the basis of experimental results it seems that Cl − ions influence the stability of the oxide film and in particular the transition between an unstable in acid solutions oxide related to Fe 3O 4 and a more stable one related to Fe 3O 4 ∣ γ-Fe 2O 3. Halide ions trigger local dissolution by an adsorption mechanism, which leads to the destabilization of the film related to γ-Fe 2O 3 and establishment of appropriate electrochemical conditions for the lifting-off of the oxide and activation of the Fe electrode.

Electrochemical Passivation of Iron in NO3-, SO42-, and ClO4- Solutions

Electrochemical passivation of iron was studied in NO3-, SO42-, and ClO4- solutions of different pH values. Passivation potential of iron in NO3- and ClO4- was similar for any pH examined, while in SO42- it was more anodic. Passive current was lowest in NO3- (10 ± 2 μA/cm2), in SO42- was in the range of 40−60 μA/cm2 while in ClO4- passive current of 30−50 μA/cm2 was reached only at 600−800 mV. After passivation, the properties of passivated iron are similar in NO3- and SO42- but differ in ClO4-. In the latter, the passive region is shorter by 200−300 mV. The transpassive potential in ClO4- is 1.2 V vs SCE and independent of pH, but shifts anodically with decrease in ClO4- concentration. The transpassive potential in NO3- and SO42- is higher, O2 evolution starts, and the potential shifts cathodically with increase of pH. At the transpassive potential in ClO4-, a black substance forms which dissipates in acidic solutions and is stable in strong alkaline solutions. Microphotographs of anodically polarized ir...

Behaviour of Hastelloy C22 steel in sulphate solutions at pH 3 and low temperatures

The behaviour of Hastelloy C22 steel (N06022 UNS) in solutions at pH 3 and different temperatures, for radioactive water reprocessing plants was studied. The corrosion potential was found to increase with temperature. In the neighbourhood of the active-passive potentials, Hastelloy steel was corroded by means of a transient form. In addition, the potentials of the active and transpassive peaks shifted towards more negative values with increasing temperature. The transpassive peak increased with temperature and its potential shifted towards smaller values. The temperature increase led to a reduction of the passive region and a higher current in the passivity. As shown by current instabilities during the backward scan and a higher current than those in the forward transpassivity scan, all of the oxide layer could not be destroyed at the breakdown transpassive potentials. From electrochemical impedance spectroscopy (EIS), apparently the thickness of the passive oxide decreased with temperature in the prepassive region. In passivity, the EIS showed an enhancement of corrosion for higher potentials or temperatures. At the passivity-transpassivity limit potentials, an inductive loop may be the result of adsorption. It was found that the diameter of the capacitive semi-circle and the inductive loop decreased when the temperature increased, indicating an enhancement of corrosion and a decrease in adsorption. The number of active sites and oxidation rate were determined at different potentials.

Effects of Neutron Irradiation on Transpassive Corrosion Behavior of Austenitic Stainless Steels

Abstract Irradiated and unirradiated austenitic stainless steels (SS) were exposed to a transpassive potential (1,050 mVSCE) in a sulfuric acid (H2SO4) solution, and the corrosion responses were studied. Applicability of the transpassive technique to evaluation of radiation-induced segregation (RIS) of impurities was considered. The charge density during the potentiostatic transpassive test and the corrosion morphology after the test were functions of both the material chemistry and irradiation. The charge density of the unirradiated materials was correlated successfully with the bulk impurity level, as defined in terms of the parameter (wt% silicon + wt% phosphorus × 10). The charge density increased steeply above an impurity level parameter of 0.5, which corresponded to the region where intergranular attack (IGA) became visible. The change in charge density after neutron irradiation also was a function of the impurity level. Irradiation resulted in a considerable increase in charge density and grain-bou...

Corrosion characteristics of Fe2O3Cr2O3 artificial passivation films under potentiostatic control

Fe 2 O 3 Cr 2 O 3 composite oxide thin films, that is, artificial passivation films, with various Cr 2 O 3 content were formed on Pt substrates by a MOCVD method. The rate of change of film thickness as a function of potential was examined in 1 kmol m −3 H 2 SO 4 using in-situ ellipsometry under potentiostatic control. The rate of thinning can be divided into four characteristic ranges according to potential; i.e. the range of the reduction dissolution of Fe 2 O 3 component, the range of no dissolution, the range of the field-assisted dissolution of Fe 2 O 3 component, and the range of the oxidation dissolution of Cr 2 O 3 component. The range of no dissolution of Fe 2 O 3 Cr 2 O 3 composite films was named the intrinsic passivity. The anodic polarization curve for an Fe18Cr alloy in passive and transpassive potentials was also divided into four characteristic ranges which correspond to the ranges for the artificial passivation films. The dissolution behavior of passive and transpassive films can be explained based on the origin of the four ranges. It was presumed that passive films on FeCr alloys exhibit their intrinsic nature in the potential range of the intrinsic passivity.

The behaviour of passive films on carbon steel in sulfuric acid solutions

Abstract A kinetic study of the passive state of carbon steel in H 2 SO 4 solutions was performed based on impedance measurements. Complex impedance diagrams with only two capacitive components were obtained in the potential range 0.6–1.4 V/SCE. The parameters of the passive electrode ( R HF , C HF , α and R m ) were obtained as a function of potential. The product IR m of the passive current I and the transport resistance R m in the passive film was found to be proportional to the film thickness. The thickness of the passive film increased with increasing polarization potential at relatively low potentials, but decreased with increasing degeneration of the outer layer of the film at high potentials. The polarization potential is lower than both trans-passive potential and the oxygen evolution potential.

Effect of carbonate in slightly alkaline medium on the corrosion of maraging steel

Abstract This study was carried out to ascertain the behavior of maraging steel identified as AMS. 65-12 and used in the tanks of the French plant for the reprocessing of radioactive water. This water has a pH of 9 and contains carbonate ions. The rest or corrosion potentials can be in the transpassive or active region due to radiolytic species. Therefore, the behavior of maraging steel was studied in these two regions. In the neighborhood of the active-passive transition potentials, maraging steel is corroded by means of intermediates. The corrosion potential does not change with carbonate at a constant pH. With carbonate and a low scan rate, the breakdown transpassive potentials are displaced towards more positive potentials and the passive potential region is broader, while at a high scan rate these potentials are displaced towards more negative values. After the beginning of the reverse scan, the current is again positive. These suggest that the corrosion kinetics are not the same. The diffusion coefficient of species inside the oxide layer was determined.

Study of the anodic oxidation of stainless steel alloys, and their constituent elements, in acidic electrolytes

The chronoamperometric behaviour of certain stainless steel alloys and their constituent metals in various electrolytes indicates metal dissolution followed by nucleation and growth of anodic surface films at transpassive potentials. These processes resulted in the appearance of previously unreported rising current transients whose characteristic form is sensitive to the prevailing nucleation and growth mechanism. Rising current transients have been observed for Fe, Mo, Cr and FexCr (x= 16, 18, 20, 23), 444, 4732, 4733, 304L and 316L stainless steels. The temperature, alloy and electrolyte composition were shown to have a significant influence on the form of the rising current transients. A phenomenological model is proposed to describe the nucleation process on the metals under investigation.

Effect of laser surface melting on the corrosion resistance of chromium-plated 9Cr-1Mo ferritic steel in an acidic medium

9Cr-1 Mo ferritic steel forms an integral part of some of the nuclear power generating industries where it is used as the steam generating material. Its corrosion resistance could be further improved by employing a chromium coating over it. However, this chromium coating has been found to be unsatisfactory owing to the microcracks present in the coating. Laser surface melting (LSM) could be effectively used not only to remove these microcracks but also to form a better corrosion-resistant modified surface without affecting the bulk properties of the material. Studies were carried out on the laser surface melted chromium-plated 9Cr-1Mo steel. The specimens with chromium deposit thicknesses ranging from 30–70 μm were prepared and then laser irradiated. Optical microscopic studies on the cross-sections of these specimens revealed an average laser-melted thickness of around 0.1–0.5 mm, depending upon the irradiation parameter used. Aqueous corrosion behaviour of these specimens was studied by anodic polarization in 1N H2SO4 medium. Anodic polarization experiments were carried out for specimens after repolishing the same specimen until the 9Cr-1Mo base metal was reached. The passive and peak current density values, range of passivity, peak and transpassive potentials, were determined at each stage of polishing, and these were compared with those of pure chromium metal as well as 9Cr-1Mo alloy in the same medium. Observation of these data indicates that the laser surface melting could be beneficial in raising the aqueous corrosion resistance of such chromium-coated steels, to a level comparable with that of the pure chromium metal.

高温硝酸中における純ZrおよびZr-15Ti合金の歪電極挙動と耐SCC性

The repassivation behaviors of newly created surfaces of pure Zr, Zr-15Ti alloy, pure Ti and 25Cr-20Ni-0.25Nb (310Nb) stainless steel were investigated at both passive and transpassive potentials in 17%HNO3 solution by a rapid straining electrode test.(1) The current density (i) measured by the strain electrode test decayed according to the following formula: i∝exp(−tn) in stage I for first 5 ms after a straining stopped and i∝−t−n (t: time, n: constant) in stage II over a time range from about 5 ms to at least 100 ms.(2) The ratio of the amount of charge in stage II and stage I (Q2⁄Q1), drastically changed at the transition potential from the passive to transpassive potential. It is thought that both parameters of this potential dependence of Q2⁄Q1 and the amount of Q1 can evaluate whether the localized corrosion occurs or not.(3) Stress corrosion cracking (SCC) of pure Zr in the high potential region was suppressed by Ti addition due to the formation of a passive film containing Ti.

In vitro corrosion of some Co-Cr and Ni-Cr alloys used for removable partial dentures: influence of heat treatments

The influence of heat treatments on the corrosion resistance of some base-metal alloys currently used in dentistry as a base for porcelain substrate systems has been investigated. Four Co-Cr alloys (Wironit, Wironit extra hard, Wirobond and Wirolloy) and two Ni-Cr alloys (Wiron 88 and Wirolloy) were submitted to heat treatments for 30 min at 880, 980, 1080 °C for 30 min, and slowly cooled in air. The electrochemical characterization was performed by means of anodic polarization curves in Ringer's solution and artificial saliva at 37 °C. In both solutions it was found that ageing at 880 °C detrimentally affects the corrosion resistance of Co-Cr alloys; the passive zone is narrow and shows rather high current densities. On the contrary, the samples annealed at 1080 °C display a superior corrosion resistance. With Wiron 88 the effect is opposite, but the electrochemical behaviour of Wirolloy, is not affected by heat treatments; although it does have a more negative transpassive potential in artificial saliva and cannot be passivated in Ringer's solution. The SEM examination of the alloys evinces the microstructural modifications induced by heat treatments responsible for changes in the corrosion behaviour of the base-metal alloys.

ＮａＯＨ水溶液中での電解着色ステンレス鋼の表面組成と耐食性

A coloring treatment has been applied to SUS 430 and SUS 304 stainless steels by anodizing in 5kmol m-3 NaOH solution. XPS and AES analysis were used to measure the chemical state of the constituents of the color film and composition-depth profiles of the film. Corrosion resistances were estimated by the interfacial impedance method for the stainless steels colored at 298K in 0.1kmol m-3 H2SO4, and at 353K in 20kmol m-3 NaOH. SEM photographs showed some porous surface structures in SUS 430 and SUS 304 polarized at -0.05V for 1h. XPS analysis showed that Cr on the film surface was depleted with increases in polarization time or in applied potential. Films formed on SUS 430 at the transpassive potential (about -0.25V) were Cr-rich and the film became Cr-deficient at more nobler potentials. It can be explained by an increase in the transpassive reaction rate for the Cr component on the surface with increasing applied potential. Since Ni dose not dissolve readily in alkaline solution, its concentration in films on SUS 304 was high. The time during which high interfacial impedance occurred in 298K, 0.1kmol m-3 H2SO4 a and 353K, 20kmol m-3 NaOH solutions increased with increases in polarization time or potential. This is thought to be during to the thicker surface films, giving rise to a higher corrosion resistance for SUS 430 and SUS 304 stainless steels.

Corrosion-resistant ion-plated Al–Zn films

Microcrystalline, thin, passivated, metal films can surpass similar bulk material in corrosion resistance. If these films form a spinel oxide in situ, corrosion protection might be further enhanced over simple oxides. We extend our previous work on Al–Mg and Al–Cu films to Al1−xZnx, 0<x<1. We deposited Al–Zn films, 1–2 μm thick, onto liquid-nitrogen-cooled substrates at ∼0.1 μm/min by dc ion plating. Films were smooth, fine grained, and columnar. Potentiodynamic data were taken at 20 mV/min in aerated 3.5% NaCl solutions at room temperature. Corrosion (rest) potentials were ∼−1.05 V (saturated calomel electrode) for all x. Anodically scanned specimens had a short passive region, then one to three active–passive peaks followed by a long passive region that ended in transpassive behavior due to crevice corrosion. The total protection range (rest to transpassive potential) was ∼2.2 V for all x. Galvalume (55Al–Zn) and Galfan (5Al–Zn), commercial hot-dipped alloys, have ranges of 0.4 V. Thus, ion-plated Al–Zn alloys seem markedly superior to hot-dipped coatings. Auger electron analysis of corroded films indicated the presence of aluminum and oxygen with only small amounts of zinc. No evidence of spinel oxide was found by reflection electron diffraction (RED). RED results best fit aluminum oxides. The role of zinc is unclear. It may have formed a very thin spinel layer, or it may have facilitated the formation of the aluminum oxides that are good barriers.

Growth of anodic films on Fe18Cr stainless steel alloys

The chronoamperometric behaviour of a Fe18Cr stainless steel alloy in 0.1M H 2SO 4 indicates nucleation and growth of an anodic surface film at transpassive potentials. Existing nucleation theory has been found to be inadequate for prediction of the data.

ChemInform Abstract: Anodic Behavior of Carbon Steel in Concentrated NaOH Solutions.

AbstractThe anodic behavior of a carbon steel rotating disk electrode in deaerated 1‐9 N NaOH solutions at 25‐80 °C is examined over a potential range from active to transpassive potentials.

ＮａＯＨ水溶液中におけるＳＵＳ４３０鋼過不働態域の着色とインピーダンス特性

The anodization of SUS 430 stainless steel was studied in 5kmolm-3 NaOH solution at 298K and 353K, and a color film was found to be formed on the stainless steel in the transpassive potential region. Impedance was measured at frequencies ranging from 10KHz to 1mHz to evaluate the kinetic mechanism of film formation at several potentials. It was found that impedance characteristics changed with anodic potential in the steady state. The impedance diagram showed a capacitive loop in the high-frequency region and an inductive loop in the low-frequency region. A capacitive loop or a straight line response similar to Warburg impedance was seen at lower frequencies when the anodic potential became to more positive than the transpassive potential. It was considered that the high-frequency response was caused by charge transfer and the low-frequency response by the diffusion of FeO2-in a diffusion layer or the adsorption of some intermediate species. It was observed that the film formed on the surface of stainless steel had excellent coloration, and it was concluded that the transpassive reaction had an importance on the formation of the color film.

Evidence of Chemical Passivity in Amorphous Ni‐20P Alloy

not Available.

Pitting Corrosion Behavior of Alloy 800 in Chloride-Sulfate Media

Abstract Potentiodynamic polarization techniques were used to determine characteristic pitting potentials for Alloy 800 in chloride and chloride-sulfate media. The effect of chloride concentration in the range of 0.001 to 1 M was investigated. Decreasing chloride concentrations resulted in noble shifts of the critical nucleation potential. Immunity to pitting corrosion was evident at chloride levels below 0.01 M. Increasing sulfate concentrations resulted in improved chloride pitting resistance, although detrimental effects associated with pitting at transpassive potentials were evident with low-level sulfate additions to dilute chloride media. Potentiodynamic and potentiostatic polarization results for Alloy 800 in a lake water environment were found to be consistent with that predicted on the basis of lake water chloride and sulfate contents.

Dissolution of Glassy Ni‐P Alloys in H 2 SO 4 and HCl Electrolytes

The effect of alloyed P on the dissolution of Ni was investigated with glassy Ni‐P alloys. The anodic behavior of Ni‐P in and differed substantially from that of Ni. In , glassy Ni‐P passivated at potentials up to several hundred mV anodic to , whereas Ni dissolved in the active state at comparable anodic overpotentials. At higher overpotentials, Ni‐P underwent transpassive dissolution, but Ni passivated. The behavior of Ni‐P in was virtually identical to that in , but Ni pitted severely in at small anodic overpotentials. The kinetics of passivation of Ni‐P did not vary over a range of chloride concentration from 0.2 to 3.0N, and the alloy completely resisted chloride‐induced pitting at potentials below about 800 mV(SCE). Coulometric and gravimetric measurements indicated that, during dissolution of Ni‐P at passive potentials, the P was not oxidized as completely as it was during polarization at transpassive potentials.