Corrosion properties of nickel free austenitic stainless steels in 0·5M H2SO4 and 0·5M H2SO4 plus 0·4M NaCl

Abstract

The corrosion behaviour of four nickel free austenitic stainless steels were investigated in 0·5M H₂SO₄ and 0·5M H₂SO₄ plus 0·4M NaCl solutions by means of potentiodynamic and potentiostatic anodic polarisation testing. The performances of the nickel free alloys are compared to those of an experimental intermediate nickel alloy (4%Ni) and a standard AISI 304 steel grade. Once passivity was reached all alloys displayed similar current densities i _p in 0·5M H₂SO₄, independent from alloying. Mo and Cu were shown to be beneficial in decreasing the active dissolution currents and i _crit values. The commercial AISI 304 steel displayed superior resistance to pit initiation during potentiodynamic testing, and AISI 304 steel displayed the highest E _pit value of all alloys tested. When tested potentiostatically the N and Mo alloyed nickel free alloys showed excellent resistance to pit initiation and growth. The dominant effect of N was associated with repassivation of incipient pits, while Mo appeared to act at an earlier stage, suppressing initiation.     

Electrochemical impedance spectroscopy of anodic processes on cobalt disilicide in sulfuric acid solutions

The frequency dependences of the impedance of a CoSi₂ electrode in solutions of 0.5 M H₂SO₄ and 0.05 M H₂SO₄ + 0.45 M Na₂SO₄ are measured in the range of potentials of the active-passive transition, passive state, and transpassive state. Impedance spectra are described using an equivalent circuit with regard to the presence of an oxide layer on the electrode surface. The dependence of equivalent circuit parameters and time constants on the electrode potential is analyzed. It is found that the high anodic stability of cobalt disilicide in sulfuric acid solutions is due to the barrier properties of the oxide film. The variations in the thickness of the oxide layer with increasing electrode potential in the range of passive dissolution are estimated. It is revealed that the time constant related to the oxide layer increases before the transpassive transition.     

Electrochemical impedance spectroscopy of anodic processes on Co<Subscript>2</Subscript>Si electrode in sulfuric acid solutions

Frequency dependences of the impedance of Co₂Si electrode in 0.5 M H₂SO₄ and 0.05 M H₂SO₄ + 0.45 M Na₂SO₄ solutions are measured in the ranges of the active dissolution, active-passive transition, passive state, and transpassivity. There are potential values in the active range when the impedance can be interpreted as the impedance of the selective cobalt dissolution. The impedance spectra in the passive range and in the beginning of the transpassive potential range are described with the use of an equivalent circuit taking into account the existence of an oxide layer on the electrode surface.     

The aqueous chemistry and electrochemistry of polyacetylene, (CH)x

Polyacetylene, (CH)_x, is p-doped (oxidized) to the metallic regime by (i) gaseous oxygen in aqueous 48% (7.4M) HBF₄, (ii) aqueous HClO₄ and H₂SO₄ and (iii) electrochemical methods to give highly conducting materials that are remarkably stable in the aqueous solutions in which they are synthesized. The chemical doping reactions are consistent with the reduction potentials of polyacetylene in its various oxidation states and with the reduction potentials of the oxidizing agents. Polyacetylene can act as an electrocatalytic, ‘fuel-cell type’ electrode and also as an electrode-active cathode in rechargable battery cells in certain aqueous acid solutions.     

Chemical and phase composition of nanosized oxide and passive films on Ni-Cr alloys. II. XPS analysis of films produced by anodic passivation of alloys in 1 N H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

XPS data of thin (1 to 2 nm) oxide films formed by the anodic passivation of Ni-2 at % Cr and Ni-6 at % Cr alloys in 1 N H₂SO₄ are discussed. Thermodynamic calculations of the solid-phase chemical reaction 3NiO + 2Cr = Cr₂O₃ + 3Ni are carried out taking into account the changes in the surface energy at the alloy-oxide film interface along with the Gibbs energy change in the alloy oxidation reaction.     

On the corrosion inhibition of low carbon steel in concentrated sulphuric acid solutions. Part I: Chemical and electrochemical (AC and DC) studies

The influence of the concentration of adenine (AD), as a safe inhibitor, on the corrosion of low carbon steel (LCS) in aerated 4.0 M H₂SO₄ solutions was studied. The investigations involved weight loss, potentiodynamic polarization, impedance and electrochemical frequency modulation (EFM) methods. Variations of open-circuit potential (OCP) as a function of time till steady-state potentials were also studied. Measurements were conducted under the influence of various experimental conditions complemented with ex situ EDX examinations of the electrode surface. By using EFM measurements, corrosion current density was determined without prior knowledge of Tafel slopes. Results obtained revealed that together with iodide ion, AD is an effective corrosion inhibitor for LCS corrosion in H₂SO₄ solutions. Synergism between iodide ion and AD was proposed. Potentiodynamic polarization studies showed that AD alone and the mixture of AD and iodide ions act as mixed-type inhibitors for the corrosion of LCS in 4.0 M H₂SO₄ solution. The inhibition mechanism involves the electrostatic adsorption of protonated AD molecules on the LCS surface charged with a negative layer of chemisorbed I⁻ ions. An adherent layer of inhibitor is postulated to account for the protective effect. EDX examinations of the electrode surface confirmed the existence of such adherent layer on the electrode surface. The inhibition efficiency increases with increase in the concentration of AD and immersion time. The potential of zero charge (PZC) of the LCS electrode was determined in 4.0 M H₂SO₄ solutions in the absence and presence of 0.001 M KI, and the mechanism of adsorption was discussed. The results obtained from chemical and electrochemical measurements were in good agreement.     

Effect of KSCN and its concentration on the reactivation behavior of sensitized alloy 600 in sulfuric acid solution

The effects of KSCN addition and its concentration on the reactivation behavior of sensitized Alloy 600 in an H₂SO₄ solution were investigated. Single-loop electrochemical potentiokinetic reactivation (SL-EPR), potentiostatic and potential decay tests were conducted in H₂SO₄+KSCN solutions, including intermittent adjustment of the electrolyte composition and concentration. The SL-EPR results showed that the maximum current densities of the reactivation peaks increased with an increase in KSCN concentration from 0 to 0.05 M in the 0.5 M H₂SO₄ solutions. A decrease in the peak current density was observed, however, if the KSCN concentration was increased to 0.5 M. The potentiostatic test results at +400 mV_SCE showed that sensitized Alloy 600 could be passivated in plain H₂SO₄ and KSCN solutions as well as in mixed H₂SO₄+KSCN solutions. Potentiostatic tests at +30 mV_SCE revealed that the presence of low KSCN concentrations made the passive film less stable in H₂SO₄ solutions, but it enhanced passivation at 0.5 M. The adsorption and desorption of SCN⁻ and its subsequent redox reaction at different concentrations was discussed in relation to the reactivation of passive film formed on sensitized Alloy 600.     

Photocorrosion inhibition -of n-GaAs in acidified aqueous iodide solution

Abstract

The photocorrosion of n-GaAs electrodes in aqueous solutions represents one of the most important factors limiting their application in solar energy conversion. In continuation of previous studies, the corrosion of n-GaAs has been investigated in 0·5M H₂SO₄ using a pllOtoelectrochemical cell. Illumination of the n-GaAs anode with white light of intensity 20 mW cm^-2 generated a saturation photocurrent of 27 mA cm^-2 at biasing voltages more positive than +0·45 V(SCE). This current corresponded to hydrogen discharge at the platinum counter electrode and to oxidation of the GaAs surface. When 0·01-6·0M LiI was added to the H₂SO₄ solution, 12 was formed at the n-GaAs electrode and dissolved in the iodide solution. The onset potential of the photocurrent (E_p) shifted from -0·36 to -0·73 V(SCE) as the concentration was varied from 0·01 to 6·0M I^-. Addition of I^-₃ shifted E_p to more positive values. The extent of corrosion was studied by comparing the ratios of photogenerated I₂ and H₂. In 0·01, 1·0, and 6·0M I^- solutions, I₂/H₂ was 0·38, 0·87, and ～1 respectively. These ratios imply that, at lower I^- concentrations, the holes photogenerated at the n-GaAs surface were consumed in the simultaneous oxidation of I^- and GaAs. However, in 6·0M I^-, where I₂/H₂ = 1, surface oxidation of GaAs was completely suppressed owing to total consumption of the photogenerated holes by the oxidation of I^-. The results are considered on the basis of the band model of the semiconductor/electrolyte interface.     

Development of EPR test technique for alloy 800

The degree of sensitization (DOS) of austenitic stainless steels and some nickel-based alloys (e.g., alloy 600) is evaluated by the electrochemical potentiokinetic reactivation (EPR) test. In this study a number of test solutions based on H₂SO₄ + KSCN composition have been evaluated to establish a reliable EPR test method for alloy 800. Different passivation (vertex) potentials are also tested. It has been shown that dilute test solutions with lower vertex potentials resulted in single loop (SL) and double loop (DL) EPR test methods that distinguished between different sensitized samples and also between sensitized and desensitized samples. It has been shown that an SL-EPR test in 1 M H₂SO₄ + 0.002 M KSCN (de-aerated) at 26 °C at a scan rate of 3 mV/s from a vertex potential of 700 mV_SCE (180 s hold time) gave results that matched with the DOS indicated by microstructures and the Huey test results. Similarly, the DL-EPR test in 1 M H₂SO₄ + 0.002 M KSCN (de-aerated) at 26 °C, forward scanning from the OCP to + 700 mV_SCE and then backward scanning from there to the OCP at a scan rate of 2 mV/s produced a good measure of DOS as indicated by the Huey test results. The effectiveness of the EPR test was ascertained by testing on alloy 800 containing Ti and Al (alloy 800 HT) and Nb (alloy 800 Nb).     

Cr元素对Fe基块体非晶合金形成能及腐蚀性能影响的研究

利用微合金化技术,制备了Fe_68.4-xCo_7.6Si₇B₁₀P₅C₂Cr_x (x=0, 1, 2, 3)非晶合金,并分别使用单辊急冷甩带法和铜模铸造法制备了带状和棒状样品。借助XRD、DSC、DTA表征该非晶合金系的热力学性能与非晶形成能;并进一步采用电化学动电位极化曲线法研究了该非晶合金系在硫酸溶液中的的耐腐蚀性能。实验结果表明,通过微量添加Cr元素的方法,使该合金系的非晶形成能普遍提高,当Cr元素添加量为2%时,获得了该系列非晶合金中的最大过冷液体区间(ΔTx=57K),并且成功制备了直径为5mm的圆棒状样品;同时,由于Cr元素的添加,在1N浓度的硫酸溶液中,材料表面上形成富含Cr元素的保护层,可以有效阻止材料内部的进一步腐蚀,耐腐蚀性能明显得到改善。     

The protective effect of polypyrrole coating on the corrosion of steel electrode in acidic media

In this study, the corrosion parameters of stainless steel containing 12% Cr, have been determined by Tafel extrapolation method in 1 M HCl, H₂SO₄ and H₃PO₄ media. Later, steel was coated with polypyrrole in 0.1 M Pyrrole + 0.3 M Oxalic acid solution by cyclic voltametric method. The corrosion parameters and percentage inhibition efficiencies of coated electrodes were investigated according to immersion times in the same media. In all acidic media studied, increases in immersion time, produced increased corrosion densities and a decrease in percentage inhibition efficiencies were determined.     

Critical Pitting Potentials of Tungsten in lithium chloride-methanol-water solutions

The critical pitting potentials of tungsten were determined in 0.5 M LiCl - X M H₂O - methanol solutions where X ranged from 0 to 55. They were obtained from potentiostatic current time curves measured by two different methods: (1) The potential of the electrode in solution was brought to the desired value at a definite rate of potential change (2) Dry electrodes were immersed into the solution under applied potential. At water concentrations up to 11. M the cpp's obtained by (1) and (2) are less than 2 V vs Ag/AgCl/0.5 M LiCl(aq). The cpp's obtained by method (1) are 52 V, 62 V (maximum value) and 33 V for solutions having 14. M, 28. M and 55. M H₂O, respectively. The cpp's obtained by method (2) are about as half as large as those of (1) with the exception of the values for 14. M and 17. M H₂O, which are about 4/5 as large. The shape of the cpp-water concentration curve is similar to that of the viscosity-concentration curve of 1 M LiCl–H₂O-methanol solutions given in the literature. A mechanism for chloride ion attack of the film involving the formation of WO₂Cl₂ is proposed.     

Investigation of passivating film on iron by proton diffusion method

Measurements of the hydrogen transfer kinetics through the iron membrane electrode, coated with a passivating oxide film in 0.5M H₂SO₄, have shown that this process is considerably influenced by the electrode potential. This effect is attributed to the influence of the film electric field on the proton transfer velocity. A theory of hydrogen ion transfer has been evolved, and with its help an estimation of film field strength φ = (1.0–4.6) × 10⁵ V cm⁻¹ has been obtained.     

Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H₂SO₄ under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H₂SO₄ concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H₂SO₄ concentration the size of the pit clusters increased. There is an optimal H₂SO₄ concentration, which is 0.01 M H₂SO₄ in this study, to form a deep etched layer of uniform thickness with high surface area. At H₂SO₄ concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.     

Copper corrosion inhibition in O2-saturated H2SO4 solutions

Corrosion inhibition of copper in O₂-saturated 0.50 M H₂SO₄ solutions by four selected amino acids, namely glycine (Gly), alanine (Ala), valine (Val), or tyrosine (Tyr), was studied using Tafel polarization, linear polarization, impedance, and electrochemical frequency modulation (EFM) at 30 °C. Protection efficiencies of almost 98% and 91% were obtained with 50 mM Tyr and Gly, respectively. On the other hand, Ala and Val reached only about 75%. Corrosion rates determined by the Tafel extrapolation method were in good agreement with those obtained by EFM and an independent chemical (i.e., non-electrochemical) method. The chemical method of confirmation of the corrosion rates involved determination of the dissolved Cu²⁺, using ICP-AES (inductively coupled plasma atomic emission spectrometry) method of chemical analysis. Nyquist plots exhibited a high frequency depressed semicircle followed by a straight line portion (Warburg diffusion tail) in the low-frequency region. The impedance data were interpreted according to two suitable equivalent circuits. The kinetics of dissolved O₂ reduction and hydrogen evolution reactions on copper surface were also studied in O₂-saturated 0.50 M H₂SO₄ solutions using polarization measurements combined with the rotating disc electrode (RDE). The Koutecky-Levich plot indicated that the dissolved O₂ reduction at the copper electrode was an apparent 4-electron process.     

The behaviour of copper-zinc alloys in alkaline solutions upon alternate anodic and cathodic polarization

Galvanostatic cyclic anodic and cathodic polarization curves for four CuZn alloys are traced in alkaline solutions of different concentrations. The anodic behaviour of the first three alloys, with 15.9, 46.8 and 50.5 wt% Zn, resembles that of pure Cu. Oxidation arrests, corresponding to the formation of Cu₂O, Cu(OH)₂, HCuO₂? and Cu₂O₃, are recorded before the evolution of O₂ on the passive electrode. The Zn of the electrodes does not develop its oxidation arrest. It affects, however, the behaviour of the alloys in a number of ways. The results are explained on the basis of kinetic interactions, and in relation to the phase diagram of the Cu-Zn system.The fourth alloy, with 85.9 wt% Zn (ε + η phases), yields upon oxidation in concentrated alkali solutions a series of five or six arrests. The first two of these represent the oxidation of the Zn of the η- and ε-phases, respectively. Calculations have shown that the activity of the Zn in the ε-phase is ～ 4.6 × 10^?10 times that of the free η-phase. The other oxidation steps correspond to the oxidation of the Cu of the alloy. In 0.1N NaOH the same alloy behaves as if it was pure Zn.Critical current densities for the passivation of Cu, Zn and the four alloys are determined in 0.1M Na₂SO₄. The ability of the tested materials to withstand electrochemical dissolution decreases in the succession: Alloy I > Alloy III > Cu > Alloy II > Alloy IV > Zn.     

In situ ESTM and ESTS studies of the nanostructure and energetic properties of Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloy surfaces

Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloys are studied in air and in 0.01 N HCl and 0.1 N H₂SO₄ solutions at the controlled potentials of the specimen and needle using in situ techniques of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS). The changes in the potential of Fe-20% Cr-40% Ni alloy specimen fall in ranges of −0.3 to 1.1 V and 0–0.3 V (N.H.E.) in 0.01 N HCl and 0.1 N H₂SO₄, respectively, and the changes in the potential of Fe-20% Cr-70% Ni specimen, in ranges of 0.09–0.94 V and −0.1 to 0.5 V in 0.01 N HCl and 0.1 N H₂SO₄, respectively. Local spectral dependences of the tunnel current on the tunnel voltage are obtained and processed and the coefficients that characterize the local electrophysical properties of the surfaces are calculated on their basis. It is found that, in the studied acid solutions at the controlled potential, the surfaces are more energetically homogeneous than in air, where a substantially larger dispersion of the properties is observed. It is concluded that the energetic characteristics of separate surface sites of these alloys are determined on the atomic scale by the mutual effects of the neighboring atoms involved in its composition, i.e., by the matrix effect, which was discovered previously in the case of Kh18N10T stainless steel [1].     

Potentiodynamic polarization behaviour of two 17Cr13Ni2·5 Mo austenitic steels with different N contents

Potentiodynamic polarization measurements have been made on two 17Cr13Ni2·5Mo austenitic steels differing from each other in the content of N. The electrolyte solutions have been 1·0M H₂SO₄, mixtures of HCl and H₂SO₄ and 1·0M HCl at 20°C. In 0·9 and 1·0M HCl the polarization curves of the two austenitic steels possess two peaks which are difficult to explain. Experiments indicate that the steel containing N is less corrosion resistant than the steel without N.     

Electrochemical properties and surface analysis of Cu–Zr–Ag–Al–Nb bulk metallic glasses

The electrochemical properties and surface characteristics of Cu–Zr–Ag–Al–Nb bulk metallic glasses (BMGs) were investigated. The alloys exhibit excellent corrosion resistance after immersion in 1N H₂SO₄ and 1N NaOH. The corrosion rates of the alloys in chloride-ion-containing solutions significantly decrease by alloying with Nb element. The formation of Zr- and Nb-enriched surface films could be responsible for the high corrosion resistance.     

ESTM study of nanostructure and surface energetic properties of individual chromium and nickel

Individual chromium and nickel in air and at interfaces to 0.01 N HCl and 0.1 N H₂SO₄ solutions are studied with the use of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS) at the controlled potentials of the specimen and tip. The potential range of chromium specimens was 0.1 to 0.9 V in 0.01 N HCl and 0.3 to 1.1 V in 0.1 N H₂SO₄; the potential range of nickel was −0.01 to 0.64 V in 0.01 N HCl and 0.184 to 0.834 V (N.H.E.) in 0.1 N H₂SO₄. At the controlled potential, the surfaces in the studied acid solutions are smoother than in air. The potential of the specimen affects the electrophysical properties, which determine the electron tunneling transfer, more noticeably than the nanorelief of the metals. The obtained changes in the I _t,dZ spectra dependence on the distance in air on a field of 1 nm² at a step of 0.04 nm enabled us to estimate the effective barrier of tunneling K _ef between neighbor atoms, which is in good agreement with the known dependence of K _ef on the work function.