Initial stages of nickel passivation and dissolution in acidic sulfate solutions

Regularities of the electrochemical dissolution of freshly formed surface (FFS) of iron and nickel are similar as a whole, but exhibit a number of distinctions. At an oxidation stage Me⁰-Me⁺, a water molecule chemisorbs to form a surface complex with charge transfer; however, a mean fraction of transferred charge in the complex of nickel (0.8) is significantly higher than that of iron (0.5). In weakly acid sulfate solutions (pH 1.7–3.2), the iron FFS dissolution is predominantly inhibited by hydrogen atoms formed by the discharge of hydroxonium ions and adsorbed at the dissolution centers. On nickel at pH > 2.7, the inhibition is caused by the formation of adsorbed oxygen corresponding to more positive potentials in a range of active nickel dissolution. Original Russian Text ? A.N. Podobaev, I.I. Reformatskaya, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 1, pp. 73–75.     

Inhibition effect of some surface active agents on dissolution of copper in nitric acid

Abstract

The inhibitive efficiency of some nonionic, amphoteric, cationic, and anionic surface active agents (surfactants) on the dissolution of copper in 4·5M HNO₃ was studied at 25°C by the weight loss method. The percentage inhibition I increases as the concentration of the cationic, nonionic, and amphoteric substances increases and reaches a limiting value. It is suggested that this is due to the formation of a monolayer on the surface of the metal; the Langmuir adsorption isotherm was confirmed. In the case of anionic surface agents, the percentage inhibition values were at their highest at low surfactant concentrations and decreased as the concentration of the surfactant increased.     

Influence of Pb ions on the dissolution and passivation of nickel and Ni–21Cr in acidic solutions

The deleterious effect of dissolved Pb⁺⁺ on passivity of Ni and Ni–21Cr has been explored at 90 °C using two solutions: 0.1 M perchloric acid, and a pH 3.5 acetic acid/sodium acetate buffer. Pb⁺⁺ strongly inhibits true (low-potential) active dissolution of pure Ni; this is consistent with underpotential deposition (UPD) of elemental Pb, apparently a new observation. The range of potentials where this occurs is greater for the more acidic solution. By inoculating Pb⁺⁺ at different potentials, the maximum underpotential can be shown to be at least 300 mV. In the less-aggressive acetic acid environment, a ‘pre-passive’ state of Ni is more noticeable than in the perchloric acid, and in contrast to the true active region, this region is subject to a strong activating effect of Pb. Dissolution of Ni proceeds rapidly on a surface that is trying to form its pre-passive film but is partly covered by Pb. The time-dependence of the passive current in the acetic solution is altered subtly by incorporation of Pb into the metal-film interface, such that the normal decay with time is truncated, leading to a higher long-term dissolution or film thickening rate. The presence of 21% Cr suppresses active dissolution, greatly ennobles the open-circuit potential, and eliminates (for these particular conditions) both the inhibiting and the activating effects of Pb. The relevance of these results to corrosion and SCC caused by lead in nuclear power systems is briefly discussed.     

Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.     

Effects of nitric acid passivation on the pitting resistance of 316 stainless steel

Samples of 316 stainless steel have been subjected to passivation treatments at ambient temperature for 1 h in solutions of up to 50 wt% nitric acid. Pitting potentials of the treated samples were measured in 1 M NaCl at 70°C and were shown to vary with the concentration of the pre-treatment; increasing with concentration up to 25 wt% and then decreasing as the acid concentration was further increased. The corrosion potential reached during the passivation treatment increased with acid concentration, such that the highest measured pitting potential was associated with a final passivation potential of 300–400 mV versus SCE. MnS inclusions were at least partially removed by treatment with any nitric acid concentration, whilst chromium enrichment of the film reached a peak value for an acid concentration of 25 wt%. The rate of metastable pitting was also found to vary with the concentration of the acid used in the passivation treatment, as did the probability of a metastable pit becoming stable. A probabilistic pitting model suggests that acid treatment reduces the number of possible pit initiation sites at low potentials, but the most dangerous sites in corrosion terms are also the most difficult to remove by acid treatment.     

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.     

The effect of cathode material on nitric acid reduction kinetics

The combined effect of potential and temperature on the mechanism of autocatalytic reduction of nitric acid is analyzed. The change of solubility of nitrogen oxides with temperature allows separation of potential intervals with predominant contribution of homogenous autocatalysis. Heterogeneous autocatalysis dominates in concentrated media, while homogenous catalysis is more real in totally disassociated solutions. The presence of nickel in steels catalyzes the formation of ammonia ions.     

Three-dimensional modelling of selective dissolution and passivation of iron-chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity.Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.     

Anodic dissolution and passivation of lead in sodium sulfate solutions

The anodic behavior of lead depending on the sulfate solution concentration is studied. The relation between oxide-hydroxide passivation and salt repassivation is found. The main characteristics of the forward and reverse branches of the polarization curves and cyclic voltammograms, as well as anodic chronoammograms and cathodic chronopotentiograms, are determined. The time dependence of the phase-formation current is constructed and analyzed.     

添加剂氧化钙对镍渣强化还原的影响

针对铁以铁橄榄石形式存在而难以直接还原磁选回收的问题,采用在镍渣中配加CaO提升镍渣碳热还原速率及铁金属化率的技术思路。将添加剂与镍渣按比例混合并配加适量还原剂后,在高温炉中进行还原,结合热力学计算及产物特征分析结果,研究添加剂强化还原的机理及还原效果。结果表明:CaO的加入有效促进了硅酸铁的解离,强化了还原过程,还原速率加快,金属相在渣相中的形态结构发生改变,金属铁的聚集长大明显,有利于后续分离回收。随着CaO添加量由0增加到10%(质量分数),还原产物的金属化率和Fe颗粒平均粒径呈先增大后减小的趋势,FeO含量和残碳量变化规律相反。在CaO添加量为8%时,还原产物中铁的金属化率和粒径达到峰值,分别为87.25%和41.3μm。     

The kinetics of iron dissolution and passivation depending on temperature and ionic strength

The anodic behaviour of pure, recrystallized iron in weak acid sulphate solutions in the potential range between the active and the passive state of the metal was investigated. The influences of the temperature and the ionic strength on the kinetic data were studied. The results establish the iron dissolution and passivation mechanism previously proposed from measurements in sulphate and perchlorate solutions of high ionic strength and at room temperature. This mechanism is characterized by the formation of different intermediates [Fe(OH)_n]_ads with n = 1, 2, 3 depending on the potential.     

Effect of the reaction series of hydroxyazomethine and chloride anions of variable concentrations on the passivation currents of nickel

Nickel passivation in 1 M H₂SO₄ in the presence of hydroxyazomethine derivatives (C = const) and chloride anions in variable concentrations was studied. Plots of the Ni passivation current vs. showed symbatic and antibatic changes, depending on the chloride concentration range. The results obtained were interpreted in terms of the Linear Gibbs Energy Relation (LGER). Original Russian Text ? S.P. Shpan’ko, V.P. Grigor’ev, O.V. Dymnikova, A.S. Burlov, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 589–593.     

Study of the dissolution of lead in nitric acid by the thermometric technique

The thermometric technique was used to study the dissolution of lead in HNO₃. It was found that ΔT and the reaction number increase almost linearly with an increase in the acid concentration to a certain limit, whereafter the reaction is inhibited by a further increase owing to the formation of a protective layer of PbO₂.The mechanism of dissolution is confirmed by the effects of some additives, e.g. NO₂^_ ion, NO₃^_ ion as KNO₃ and NH₄NO₃, Cl^_ ion as HCl and NaCl, as well as urea. These additives were found to increase or decrease the dissolution process depending on whether they or their reaction products were involved in the dissolution reaction.     

Cerium-tannic acid passivation treatment on galvanized steel

A novel cerium-tannic acid passivation treatment was performed on galvanized steel. The corrosion resistance of cerium-tannic passivated samples was tested by dropping test with 0.5 wt.% CuSO₄ aqueous solution. The mass loss per unit area of passivated samples was measured after the corrosion in 0.5 mol/L NaCl + 0.005 mol/L H₂SO₄ at room temperature for 96 h. The electrochemical behaviors of cerium, tannic acid, and cerium-tannic acid passivated samples on galvanized steel in 0.5 mol/L NaCl solution were investigated by polarization curves and electrochemical impendence spectra. The corrosion equivalent circuit was established according to the impedance characteristics. The results show that cerium-tannic acid treated samples exhibit better corrosion resistance than the sole cerium or tannic acid treated samples under the same condition. The mechanism of synergistic effect for cerium-tannic acid passivation on galvanized steel was discussed.     

Thermodynamics of chemisorption of sulfur from thiocyanate on nickel during electrochemical passivation

The effect of potassium thiocyanate content in an aqueous H₂SO₄ solution on the activation potential of a passive film formed on the nickel surface is studied. Based on the results, a thermodynamic model of chemisorption in electrochemical passivation of nickel in an aqueous H₂SO₄ solution containing surface-active KSCN thiocyanate additive is proposed. In the model, the formation of a NiO-NiS film is considered as a result of the equilibrium processes at the interfaces of the Ni|NiO-NiS|H₂O, SCN^? system. The total Gibbs energy of the chemisorptive phase formation of NiO-NiS solid solution is determined by partial chemisorptive formation energies of NiO and NiS. In turn, partial formation energy of either component is estimated as a sum of the Gibbs energy of the chemical formation of the component and the surface Gibbs energy of nickel, which depends on the KSCN concentration in the solution in the case of NiS formation. The system can be controlled due to the relation between the Gibbs energy and Flade electrode potential. As was experimentally found, the latter value determines the quantitative NiO: NiS ratio in the passive film.     

Cerium-tannic acid passivation treatment on galvamzed steel

A novel cerium-tannic acid passivation treatment was performed on galvanized steel. The corrosion resistance of cerium-tannic passivated samples was tested by dropping test with 0.5 wt.% CuSO4 aqueous solution. The mass loss per unit area of passivated samples was measured after the corrosion in 0.5 mol/L NaCl + 0.005 mol/L H2SO4 at room temperature for 96 h. The electrochemical behaviors of cerium, tannic acid, and cerium-tannic acid passivated samples on galvanized steel in 0.5 mol/L NaCI solution were investigated by polarization curves and electrochemical impendence spectra. The corrosion equivalent circuit was established according to the impedance characteristics. The results show that cerium-tannic acid treated samples exhibit better corrosion resistance than the sole cerium or tannic acid treated samples under the same condition. The mechanism of synergistic effect for cerium-tannic acid passivation on galvanized steel was discussed.     

Impedance of NiSi electrode in sulfuric acid in the active anodic dissolution range

Anodic behavior of NiSi electrode in 0.5 M H₂SO₄ in a potential range of active dissolution is studied. A conclusion about the selective dissolution of nickel from nickel silicide at a small anodic polarization (up to 0.15 V) is drawn based on the impedance data. The solid-phase diffusivity of nickel and the thickness of the diffusion zone are estimated.     

Active,passive and transpassive dissolution of a nickel base super alloy in concentrated acid mixture solution

The electrochemical and corrosion behaviour of a nickel base super alloy (C-263) has been investigated in the deaerated binary and ternary solution mixture of concentrated phosphoric acid, acetic acid, sulphuric acid, nitric acid or water using potentiostatic technique at 35°C. The possibilities of electropolishing of this alloy in these solution mixtures have been also explored. The alloy showed distinct active, passive and transpassive behaviour in the experimental solutions. The alloy remained active and turned passive in the negative potential region. Transpassive dissolution of the alloy is observed and electropolishing is achieved in this region. The best electropolishing is obtained in 50% H₃PO₄ + 40% CH₃COOH + 10% H₂SO₄. Higher content of water in the electrolytic solution is not useful for electropolishing of the alloy The experimental results also suggest that a current plateau in the transpassive potential region is not a sufficient condition to achieve electropolishing.