Effect of Ni content on the corrosion behavior of Cu-Ni alloys in neutral chloride solutions

The effect of systematic increase of Ni content on the electrochemical behavior of the Cu-Ni alloys in neutral chloride solutions was investigated. The pitting corrosion behavior of Cu-Ni alloys with different Ni contents, namely, 5, 10, 30 and 65 mass% Ni, in a stagnant 0.6 mol dm⁻³ NaCl solution of pH 7.0 was studied. The effect of chloride ion concentration on the electrochemical behavior of these alloys was also investigated. The results show that the increase in nickel content decreases the corrosion rate of the alloys in the neutral chloride solution. The increase of chloride concentration up to 0.3 mol dm⁻³ increases the corrosion rate. At higher concentrations ([Cl⁻] > 0.3 mol dm⁻³) the corrosion rate decreases due to the hydrolysis of Cu(I) chloride to form the passive Cu(I) oxide film. The breakdown potential depends on the chloride ion concentration and the nickel content of the alloy. For these investigations conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) were used. The impedance measurements have shown that the increase of the Ni content and the immersion time of the alloys in the chloride solution increase the corrosion resistance of the alloys. The experimental impedance data were fitted to theoretical values according to a proposed equivalent circuit model.     

Performance evaluation of some fusion-bonded epoxy coatings under water transmission line conditions

The work presented in this paper deals with the performance evaluation of some fusion-bonded epoxy (FBE) coatings under water transmission line conditions. The work is aimed to evaluate corrosion–erosion behavior of selected FBE coatings in different product waters under simulated water transmission line conditions to find applications in sections of water transmission lines which are subjected to high flow, water hammer or turbulence. Three different FBE coatings, namely, Scotchkote 206 N, NAP-GARD 7–2500, and RESICOAT R4 Blue were considered for the studies. The studies were carried out in treated SWRO (seawater reverse osmosis) permeate and treated and untreated MSF (multistage flash) product water. To generate data about the erosion–corrosion resistance of the coatings under pipelines operating conditions, coated steel panels of fixed dimension were subjected to jet impingement test (JIT). ac impedance tests were carried out on the coated steel samples obtained after subjecting to JIT. The tests were carried out to evaluate qualitatively water uptake by the coatings. Adhesion test was carried out to assess the adhesive strength of the coatings. The monitoring of total organic carbon (TOC) in the test media, before and after subjecting of JIT, was also carried out. The extent of formation of TOC in the test media is indicative of the possible degradation/leaching of coatings under severe JIT conditions.

All the three coatings subjected to JIT did not show any impingement damage, loss of adhesion, blistering damage or color changes, thus reflecting their excellent corrosion–erosion property. The effect of residual chlorine concentration on the corrosion–erosion property of the coatings appears to be insignificant. The results of ac impedance showed very high initial impedance for all the three coatings giving them excellent ratings. However, the performances of Scotchkote 206 N and NAP-GARD 7–2500 in treated SWRO permeate was found to be affected after 10 months immersion. The results of TOC monitoring indicated the presence of some organic compounds in the test media possibly due to the degradation/leaching of coatings under severe JIT conditions. The residual chlorine concentration in the test media appears to influence the formation of TOC. Therefore, further studies are needed to establish the safety of the coating from health point of view.     

Pitting corrosion studies on Al and Al-Zn alloys in SCN solutions

Pitting of Al and Al-6%Zn and Al-12%Zn alloys in KSCN solutions was studied by means of potentiodynamic anodic polarization, cyclic voltammetry, potentiostatic and impedance techniques. Measurements were conducted under different experimental conditions, complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA). The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface (in case of Al) and the formation of ZnO on the Al₂O₃ matrix, in case of the two Al-Zn alloys (as evidenced from EDXA). The passive region is followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of SCN⁻ anions. SEM images confirmed the existence of pits on the electrode surface. Alloyed Zn was found to enhance pitting attack. The pitting potential (E_pit) decreases with an increase in SCN⁻ concentration and temperature, but increases with increasing potential scan rate. The current/time transients show that the incubation time for passivity breakdown decreases with increasing applied positive potential, SCN⁻ concentration, and temperature. Impedance measurements showed that Nyquist plots are characterized by a depressed charge-transfer semicircle, the diameter of which is a function of SCN⁻ concentration, applied potential, solution temperature and sample composition.     

The effect of potential bias on the formation of ionic liquid generated surface films on Mg alloys

An electrochemical approach to the formation of a protective surface film on Mg alloys immersed in the ionic liquid (IL), trihexyl(tetradecyl)phosphonium-bis 2,4,4-trimethylpentylphosphinate, was investigated in this work. Initially, cyclic voltammetry was used with the Mg alloy being cycled from OCP to more anodic potentials. EIS data indicate that, under these circumstances, an optimum level of protection was achieved at intermediate potentials (e.g., 0 or 0.25 V versus Ag/AgCl). In the second part of this paper, a small constant bias was applied to the Mg alloy immersed in the IL for extended periods using a novel cell design. This electrochemical cell allowed us to monitor in situ surface film formation on the metal surface as well as the subsequent corrosion behaviour of the metal in a corrosive medium. This apparatus was used to investigate the evolution of the surface film on an AZ31 magnesium alloy under a potential bias (between ±100 mV versus open circuit) applied for over 24 h, and the film evolution was monitored using electrochemical impedance spectroscopy (EIS). A film resistance was determined from the EIS data and it was shown that this increased substantially during the first few hours (independent of the bias potential used) with a subsequent decrease upon longer exposure of the surface to the IL. Preliminary characterization of the film formed on the Mg alloy surface using ToF-SIMS indicates that a multilayer surface exists with a phosphorous rich outer layer and a native oxide/hydroxide film underlying this. The corrosion performance of a treated AZ31 specimen when exposed to 0.1 M NaCl aqueous solution showed considerable improvement, consistent with electrochemical data.     

The corrosion behaviour of rare-earth containing magnesium alloys in borate buffer solution

In this work, the corrosion behaviour of magnesium alloys ZK31, EZ33 and WE54 was studied in sodium borate buffer solution at pH 9.2.The electrochemical processes were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The composition and morphology of the alloys and corrosion products formed were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).The experimental findings highlighted the differences in the corrosion mechanisms of the different alloys tested. The results showed that the presence of rare-earth elements (RE) only increases the corrosion resistance when present in solid solution, as is the case of the WE54 alloy. At pH 9.2, an amorphous yttrium oxide/hydroxide thick film was formed, which possesses greater stability when compared to magnesium oxide/hydroxide. The role of RE in the corrosion mechanism was discussed.     

Influence of microstructure and composition on the corrosion behaviour of Mg/Al alloys in chloride media

The influence of the microstructure and aluminium content of commercial AZ31, AZ80 and AZ91D magnesium alloys was evaluated in terms of their corrosion behaviour in an aerated 3.5 wt.% NaCl solution at 25 °C. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). The surface was characterized by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). The extent of corrosion damage was strongly dependent on the aluminium content and alloy microstructure. Two key factors were observed for the lowest corrosion rates, which occurred for the AZ80 and AZ91D two-phase alloys: the aluminium enrichment on the corroded surface for the AZ80 alloy, and the β-phase (Mg₁₇Al₁₂), which acted as a barrier for the corrosion progress for the AZ80 and AZ91D alloys. Surface potential maps suggested that, between the β-phase and the α-matrix, the galvanic coupling was not significant.     

Corrosion control of Cu-Ni alloys in neutral chloride solutions by amino acids

The corrosion inhibition of Cu-Ni alloys was investigated in aqueous chloride solutions using amino acids as environmentally safe materials. The corrosion rate was calculated in absence and presence of the corrosion inhibitor using polarization and impedance techniques. The inhibition efficiency of the different amino acids was also calculated.The experimental results have shown that a simple amino acid like glycine can be used as efficient corrosion inhibitor for the Cu-Ni alloys in neutral chloride solutions. An inhibition efficiency of about 85% could be achieved at very low concentrations of the amino acid (0.1 mM). For low Ni content alloy (Cu-5Ni), 2.0 mM cysteine shows a remarkable high (∼96%) corrosion inhibition efficiency. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model for the electrode/electrolyte interface, and the mechanism of the corrosion inhibition process was suggested. Different adsorption isotherms were tested and the corrosion inhibition process was found to depend on the adsorption of the amino acid molecules and/or the deposition of corrosion products on the alloy surface. The adsorption free energy of cysteine on Cu-5Ni (−37.81 kJ mol⁻¹) reveals a strong physical adsorption of the inhibitor on the alloy surface.     

Electrochemical corrosion properties of metal alloys used in orthopaedic implants

This study concerns an investigation of the corrosion behavior of 316 stainless steel, CoCrMo and Ti6Al4V alloys in simulated body conditions (ringer lactate) at 37 °C by the use of Tafel plots, mixed potential and electrochemical impedance spectroscopy (EIS). Ti6Al4V alloy has the highest corrosion resistance followed by CoCr alloy. Ti6Al4V–CoCrMO was the best couple for galvanic corrosion with the minimum galvanic potential and current values according to mixed potential theory and Tafel method. It was concluded that Ti6Al4V was the most suitable material for implant applications in the human body.     

Photoelectrochemical study of nickel base alloys oxide films formed at high temperature and high pressure water

The oxide film formed on nickel base alloys at high temperature and high pressure water exhibits semi-conducting properties evidenced by photocurrent generation when exposed to monochromatic light. The use of macro- and micro-photoelectrochemical techniques (PEC and MPEC) aims to identify the different semiconductor phases and their distribution in the oxide film.Three different nickel base alloys were corroded in recirculation loop at 325 °C in pressurised water reactor primary coolant conditions for different exposition durations.PEC experiments on these materials enable to obtain macroscopic energy spectra showing three contributions. The first one, with a band gap around 2.2 eV, was attributed to the presence of nickel hydroxide and/or nickel ferrite. The second one, with a band gap around 3.5 eV, was attributed to Cr₂O₃. The last contribution, with a band gap in the range of 4.1-4.5 eV, was attributed to the spinel phase Ni_1−xFe_xCr₂O₄. In addition, macroscopic potential spectra recorded at different energies highlight n-type semi-conduction behaviours for both oxides, Cr₂O₃ and Ni_1−xFe_xCr₂O₄.Moreover, MPEC images recorded at different energies exhibit contrasted regions in photocurrent, describing the distribution of nickel hydroxide and/or nickel ferrite and Cr₂O₃ in the oxide film at a micron scale.It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale.     

Surface and electrochemical characterization of pitting corrosion behaviour of 304 stainless steel in ground water media

The effectiveness of aminotrimethylidene phosphonic acid (ATMP) as a corrosion inhibitor in association with a bivalent cation like Zn²⁺ and non-ionic surfactant like polyoxyethylene sorbitan monooleate (Tween 80) were investigated by measuring corrosion losses using electrochemical techniques. The corrosion of 304 stainless steel in the ground water medium was inhibited by complexation of the inhibitor. A combined inhibition effect was achieved by adding both ATMP and Zn²⁺ along with Tween 80. The formulation functioned as a mixed type inhibitor. The synergistic effect of the inhibitor compound is calculated. Luminescence spectra, FTIR spectra, XRD, XPS and scanning electron microscopic studies were carried out to understand the mode of corrosion inhibition and also the morphological changes on the metal surface.     

Electrochemical characterisation of passive films on Ti alloys under simulated biological conditions

An electrochemical characterisation of the passive film on Ti, and its alloys Ti6Al4V and Ti6Al7Nb under simulated biological conditions is described. Electrochemical impedance spectroscopy and photoelectrochemistry were employed as in-situ techniques to monitor the evolution of the passive film on exposure to simulated body fluid environments. Of particular interest were the selective interactions of phosphate and calcium ions present in simulated body fluid with the oxide films, immediately after exposure. The effects of these ions on the TiO₂-electrolyte interface could be detected already within the first hour of exposure at 37 °C and were found to be the only ions present in simulated body fluid to interact with the titanium oxide surface. Photocurrent spectra showed no change in the semi-conductive properties of the passive films on Ti and the two alloys, but large variations on the intensity of the generated photocurrent indicated the presence of adsorbed ions on the surface. Different behaviour between the three metals investigated was also observed.     

Electrochemical behaviour of heat-treated AlZnMg alloys in chloride solutions containing sulphate

The electrochemical corrosion and passivation of Al5Zn1.7Mg0.23Cu0.053Nb alloys, submitted to different heat treatments (cold-rolled, annealed, quenched and aged, and quenched in two steps and aged), in sulphate-containing chloride solutions, has been studied by means of cyclic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The cyclic polarization curves showed that sulphate addition to the chloride solution produced a poor reproducible shift of the breakdown potential to more positive potentials. The repassivation potentials, much more reproducible, and practically separating the passive from the pitting potential region, were slightly displaced in the negative direction with that addition. When the alloys were potentiodynamically polarized in the passive potential region, sulphate was incorporated in the oxide film, thus precluding chloride ingress. In addition, Zn depletion was favoured, whereas Mg losses were avoided. Different equivalent circuits corresponding to different alloys and potentials in the passive and pitting regions were employed to account for the electrochemical processes taking place in each condition. This work shows that sulphate makes these alloys more sensitive to corrosion, increasing the fracture properties of the surface layer and favouring the pitting attack over greater areas than chloride alone.     

Influence of rare earths addition on the corrosion behaviour of Zn-5%Al (Galfan) alloy in neutral aerated sodium sulphate solution

The effect of rare earth metal (Ce, Er, Y) additions on the corrosion behaviour of Zn-5Al Galfan alloy has been investigated. The corrosion resistance of Zn-5Al-1Ce, Zn-5Al-1Er and Zn-5Al-1Y alloys has been assessed by various electrochemical tests, such as corrosion potential measurements, polarization curves and electrochemical impedance spectroscopy. They have been performed in a 0.1 M Na₂SO₄ solution, at approximately neutral pH, without stirring and in contact with the air. For comparison, the electrochemical tests have also been carried out on the Zn-5Al alloy. Moreover, the surface morphology and nature of the corrosion products have been investigated. The results indicated that rare earths’ addition improves the corrosion behaviour of Galfan, the effect being more pronounced for the Er- and Y-containing alloys. The mechanism by which the corrosion resistance of Zn-5Al alloy is enhanced in presence of the rare earths is discussed.     

Oxidative dissolution and anion-assisted solubilisation in the transpassive state of nickel-chromium alloys

The transpassive state of pure Ni, Cr and two Ni-Cr alloys (10 and 20 wt.% Cr) in 14.8 M H₃PO₄ is studied by voltammetric, rotating ring-disc electrode and impedance spectroscopic measurements. The results indicate that the rate of transpassive dissolution of the alloys in a major part of the transpassive range is higher than the rate of this process for both pure metals. A kinetic model retaining the essential features of the corresponding models for the pure metals proposed in the literature has been found to reproduce both the steady-state current versus potential curves and the ac impedance spectra in the transpassive range. The parameters of the model are determined by a fitting procedure and the relevance of their values for the relative importance of two possible pathways in the transpassive state—oxidative dissolution and anion-assisted solubilisation—are discussed.     

The growth kinetics and properties of potentiodynamically formed thin oxide films on aluminium in citric acid solutions

The growth kinetics and properties of potentiodynamically formed thin oxide films on Al were investigated in 0.05 M citric acid solutions of different pH (5, 6 and 7) by means of potentiodynamic polarization and a.c. electrochemical impedance spectroscopy (EIS) measurements. Al showed passive behaviour within the pH range that was examined. The potentiodynamic growth of the oxide film on Al takes place due to ionic conductivity under the influence of the high electric field. Characteristic kinetic oxide film growth parameters such as the high-field growth exponential law constants (A and B), ionic conductivity through the oxide film, field strength and half barrier width have been calculated. Impedance measurements were used to determine the parameters related to the characteristic sizes and properties of oxide film. The capacitive response of the impedance spectrum was related to the thickness and dielectric properties of the barrier oxide film. The oxide film resistance values were very high, indicating that the oxide films formed under potentiodynamic conditions are highly uniform in thickness and very resistant. The anodic behaviour of Al in the citric solutions under potentiodynamic conditions were characterized by the rapid growth of the oxide film which diminished the influence of relaxation processes on the growth kinetics and structural characteristics of the aluminium/anodic oxide film/electrolyte system.     

Transpassive dissolution of Ni-Cr alloys in sulphate solutions—comparison between a model alloy and two industrial alloys

The application of a general model for the transpassive dissolution mechanism of binary Ni-based alloys to industrial alloys, Alloy 600 and Alloy C276, containing Ni, Cr, Fe and Mo, in 1 M sulphate solutions at pH 0 and 5 is described. A comparison of the electrochemical behaviour of these two alloys to a binary Ni-15%Cr alloy is also included. The techniques used were ring-disc voltammetry, impedance spectroscopy and resistance measurements. Soluble high-valency products were found to be released in a considerable amount from all the materials. The presence of Mo in Alloy C276 was found to increase the transpassive oxidation rate in comparison to alloys 600 and Ni-15%Cr at pH 0, but the same effect of Mo is not so well pronounced at pH 5. The mechanism of transpassive dissolution was found to be similar on every material at pH 0. At pH 5 the mechanism of the transpassive dissolution on Alloy C276 at high overpotentials is different from that at low overpotentials or from that at pH 0. This change is concluded to be due to the increased effect of adsorbed intermediates at the film/solution interface. The model was found to reproduce the steady state current and the impedance spectra satisfactorily.     

Electrochemical corrosion characterization of Al-Ni alloys in a dilute sodium chloride solution

The aim of this study is to characterize the electrochemical corrosion resistance of Al-Ni alloy samples which were directionally solidified under upward unsteady state heat flow conditions. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques were used to analyze the corrosion resistance in a dilute 0.05 M NaCl solution at 25 °C. Equivalent circuit analyses were also conducted. It was found that microstructural features such as the dendritic arrangement and the distribution of Al₃Ni intermetallic particles have important roles on both the resulting pitting potential and the general corrosion resistance of Al-Ni alloys.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Effect of N+ ion implantation on the corrosion behaviour of Ti—6Al—7Nb and Ti—5Al—2Nb—1Ta orthopaedic alloys in Hanks solution

Ti—6Al—7Nb and Ti—5Al—2Nb—1Ta alloys were implanted with N⁺ ions with an ion energy of 75 keV at a dose rate of 1 × 10¹⁷ and 1 × 10¹⁸ ions cm^–2. Open circuit potential (OCP) and potentiodynamic cyclic polarization measurements were carried out on the titanium alloys in Hanks solution to assess their corrosion resistance. The tendency for repassivation is higher in the case of implanted alloys than in untreated alloys owing to the formation of stable passive films. The impedance data showed a decrease in the double layer capacitance and an increase in the charge transfer resistance of the treated alloys. Nitrogen ion implanted Ti—6Al—7Nb was found to be more corrosion resistant than implanted Ti—5Al—2Nb—1Ta alloy.     

Pitting corrosion of Al and Al–Cu alloys by ClO4 ions in neutral sulphate solutions

The influence of various concentrations of NaClO₄, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na₂SO₄ solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al₂O₃ passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density (j_pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO₄⁻ ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO₄⁻ ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO₄⁻ ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO₄²⁻ ions was also discussed.