The transpassive dissolution mechanism of 316L stainless steel

The transpassive dissolution mechanism of AISI 316L stainless steel was studied using electrochemical impedance spectroscopy (EIS). A generalized model of the transpassivity is proposed. The transpassive film is modeled as a highly doped n-type semiconductor-insulator-p-type semiconductor (n-i-p) structure. Injection of negative defects at the transpassive film/solution interface results in their accumulation as a negative surface charge. It alters the non-stationary transpassive film growth rate controlled by the transport of positive defects (oxygen vacancies). The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the transpassive film via parallel reaction paths.     

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.     

Passive and transpassive behaviour of CoCrMo in simulated biological solutions

In this work, the behaviour of a CoCrMo alloy under simulated body conditions was investigated. More specifically, the electrochemical properties of the alloy and the relevant mechanisms in the passive and transpassive states were studied in detail. Electrochemical techniques such as potentiodynamic and potentiostatic polarisation, cyclic voltammetry, rotating disc electrode and electrochemical impedance spectroscopy were employed. Further, ex situ X-ray photoelectron spectroscopy analysis of the passive films was carried out. A good correlation between the results obtained from all the experimental techniques was achieved. Overall, it was found that the passive film on CoCrMo changed in composition and thickness with both potential and time. The passive behaviour of the CrCrMo alloy is due to a formation an oxide film highly enriched with Cr (≈90% Cr oxides) on the alloy surface. The passive and transpassive behaviour of the alloy is hence dominated by the alloying element Cr. In the transpassive region, strong thickening of the oxide film takes place, combined with a change in the composition of the film, and strongly increased dissolution rate. In the transpassive region, all alloying elements dissolve according to the composition of the alloy. The metal ion release is also very strongly enhanced by cyclic variation of the potential between reducing and oxidizing conditions. In this case, during activation/repassivation cycles, cobalt dissolution is greater than expected from the composition of the alloy. Therefore, active dissolution behaviour is mainly dominated by the alloying element Co.     

Anodic dissolution of amorphous Ni-P alloys

Anodic behaviour of amorphous Ni-P alloys containing 23 and 27 at.% P was compared to that of pure Ni. Measurements were performed in 0.1 M sulphate solutions of different pH by means of potentiodynamic polarization and impedance spectroscopy at selected anodic potentials. Significant discrepancies in the course of anodic polarisation curves and impedance spectra between amorphous Ni-P and pure Ni were seen. Amorphous Ni-P alloys exhibit a suppression of dissolution in the active range of Ni, whereas they show a faster dissolution in the passive domain of Ni. From the other side, there are minor modifications in impedance spectra taken for Ni-P over a wide range of anodic potentials, whereas Ni exhibit essential changes of impedance spectra within the same potential region, related to the active-passive transition of this metal. The charge transfer resistance of the anodic dissolution for amorphous Ni-P decreases gradually with increasing anodic potential in contrast to a sharp increase of this parameter for pure Ni in the region of its passivation. In contrast to Ni, the anodic dissolution of amorphous Ni-P exhibits a slight dependence on pH. These findings prove that the anodic dissolution suppression of amorphous Ni-P cannot be associated with the oxide passivity typical of pure Ni.     

Photo-electrochemical and impedance investigation of passive layers grown anodically on titanium alloys

The anodic behaviour of two titanium cast alloys, obtained by fusion in a voltaic arc under argon atmosphere, was analyzed in aerated aqueous solutions having different pH values. In all solutions the alloys, having nominal compositions Ti-50Zr at.% and Ti-13Zr-13Nb wt.%, displayed a valve-metal behaviour, owing to the formation of barrier-type oxide films. Passive films, grown potentiodynamically up to about 9 V, were investigated by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). These passive layers show photoactivity under anodic polarizations, with optical gaps close to 3.55 and 3.25 eV for the binary and the ternary alloy, respectively, independent of the anodizing electrolyte. Films grown on the binary alloy present insulating behaviour and anodic impedance spectra with one time constant; this was interpreted in terms of a single-layer mixed Ti-Zr oxide enriched in Ti with respect to the alloy composition. Also for the ternary alloy the results are consistent with the formation, upon anodization, of Ti-Nb-Zr mixed oxide films, but they display n-type semiconducting behaviour, owing to their poor content of ZrO₂ groups.     

Thermal and electrochemical properties of cobalt containing Finemet type alloys

This paper presents results concerning the effect of cobalt substitution on bulk (thermal) and surface (electrochemical) properties of Finemet type alloys. Thermal properties of the alloys were determined by differential scanning calorimetry (DSC). The partial substitution of iron for cobalt up to 10 at.% improves the stability of the alloys. The results obtained using various electrochemical techniques show that the new component does not improve notably the electrochemical characteristics of the basic composition alloy sample in KOH solutions. On the other hand, the high corrosion rate registered in HCl solutions does not allow us to obtain quantitative data.     

Gold dissolution in acidic thiourea and thiocyanate solutions

Gold dissolution in acidic solutions containing thiourea (Tu) and thiocyanate was studied using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and surface enhanced Raman spectroscopy (SERS). A synergistic response found in that mixed ligand system promoted a higher dissolution rate than either lixiviant alone. The passivation of gold occurs in a Tu only solution, but when thiocyanate is mixed with Tu, passivation is significantly alleviated. The dissolution rate of gold in the mixed lixiviant system increases with increasing Tu and thiocyanate concentration. Results from LSV and EIS indicate that gold dissolution is controlled by a combination of charge transfer and diffusion in the mixed lixiviant system. The optimum concentration for Tu and thiocyanate is about 5 mM and 0.05 M, respectively for the rate of gold dissolution. SERS results suggest a possible formation of a mixed ligand complex involving the interaction of Au(Tu)₂⁺ and SCN⁻. Further study is necessary to identify the mixed ligand complex.     

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 improvement of the corrosion resistance of 309 stainless steel in the transpassive region by nano-crystallization

The effect of nano-crystallization on the corrosion behavior of 309 stainless steel in the transpassive region was investigated in 0.5 M Na₂SO₄ (pH 2) solution. Three parts defined as transpassive dissolution, secondary passivity and oxygen evolution can be observed in the transpassive potential region of the anodic polarization curves. In the whole transpassive region the nano-crystalline coating has a smaller corrosion current density than the bulk steel, which indicates the transpassive dissolution rate is decreased by nano-crystallization. In addition, there is an obvious difference in the surface micrographs of the two materials at transpassive potentials, as scanning electron microscopy (SEM) shows. The electron probe microanalysis (EPMA) reveals that nano-crystallization improves the homogeneity of Cr on the surface. Mott-Schottky plots displays that the carrier density of the oxide film in the transpassive region is decreased by nano-crystallization. Thus, the interfacial reactions are decelerated and the corrosion resistance of the stainless steel in the transpassive region has been greatly improved by nano-crystallization.     

Polymeric sol–gel coatings as protective layers of aluminium alloys

Sol–gel route is an emerging technology to synthesize coatings of a wide variety of properties taylored. In this work three low temperature cured coatings has been studied to evaluate their protective properties in order to be used as protective barrier coatings for aluminium alloys with potential architectural and automotive applications. These three coatings are novel modified silane nanocomposites coatings obtained mixing two sols separately prepared: a pre-hydrolysed 3-glycidyloxypropyl trimethoxysilane (GPTS) with acidic catalyst and another obtained from tetraethylorthosilicate/methyltriethoxysilane (TEOS/MTES). Particulated coatings were obtained by addition of 25 wt.% particles of Aerosil 300 and Aerosil R972, respectively. The protective properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) which showed notably differences among them not only from the protective viewpoint but the hydrophobic nature of the coatings and the controlling corrosion mechanism in each case.     

Aspects of the chemistry of donor solvent coal dissolution. The role of phenol in the reaction

The relative reactivity of phenol in exchange reaction with tetralin-d₁₂ and naphthalene-d₈ indicates that hydrogen exchange occurs by a free radical process rather than by an electrophilic substitution reaction at 427 °C. Substances such as benzyl phenyl ether that initiate free radical reactions increase the rate of the exchange reaction between phenol and tetralin-d₁₂ whereas weak acids have only a modest influence on the exchange. Phenol and 1 -naphthol accelerate the rates of decomposition of some ethers and amines in tetralin. These results suggest that phenolic compounds enhance thermal coal dissolution reactions in donor solvents via their influences on the rates of the homolytic cleavage of carbon-oxygen and carbon-nitrogen bonds.     

Complexities of corrosion behaviour of copper-nickel alloys under liquid impingement conditions in saline water

Flow-assisted electrochemical corrosion of Cu-Ni disc electrodes, subjected to a submerged impinging jet in saline water, was evaluated and quantified using the anodic current densities (j_a), corrosion potentials (E_corr), interfacial capacitances (C) and polarization resistances (R_p). The Reynolds numbers in the range of applied impinging velocities suggested that the flow between the solution and the electrode is turbulent in the systems investigated.The role of Ni content in the resistance of spontaneously formed barrier oxide film and diethyldithiocarbamate inhibitor film on Cu-Ni alloys (10, 20, 30 and 40 at.% Ni) was examined and evaluated using electrochemical impedance spectroscopy in dependence of the impinging velocity and the immersion time.The results are discussed in terms of the corrosion mechanisms and their relevance to the use of Cu-Ni alloys in desalination plants and in the marine engineering field because fluid hydrodynamics plays an important role in Cu-Ni alloys application. Based on the analysis presented in the paper, it is anticipated that the Cu-10Ni alloy is the best choice for application in the marine environment.     

Electrochemical dissolution of surface alloys in acids: Thermodynamic trends from first-principles calculations

A simple procedure is introduced to use periodic Density Functional Theory calculations to estimate trends in the thermodynamics of surface alloy dissolution in acidic media. With this approach, the dissolution potentials for solute metal atoms embedded in the surface layer of various host metals (referenced to the dissolution potential of the solute in its pure, metallic form) are calculated. Periodic trends in the calculated potentials are found to be related to trends in surface segregation energies of the various solute/host pairs. The effects of water splitting and concomitant hydroxyl adsorption on the dissolution potentials are also considered; these effects do not change the potentials for highly oxophilic solutes embedded in less active hosts, but they do decrease the dissolution potential for more inert solutes on oxophilic hosts. Finally, the dissolution of Pt “skin” layers from Pt₃X (X = Fe, Co, and Ni) bulk alloys is analyzed; the Pt skins are found to be stabilized compared to pure Pt.     

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.     

Passive behavior of magnesium alloys (Mg-Zr) containing rare-earth elements in alkaline media

The passive behavior of magnesium alloys ZK31, EZ33 and WE54 was studied in alkaline media (NaOH - pH 13) in the presence and absence of chloride ions. The electrochemical properties were investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements.X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed for the study of the chemical composition and surface morphology of the surface films, respectively.The electrochemical impedance results revealed that the film formed on the surface of the three alloys is characterized by an increasing resistance, which stabilized with time. In the absence of chloride the film resistance was identical for all the three alloys. However, in the presence of chloride, the resistance of the film formed on the EZ33 alloy dropped nearly one order of magnitude comparatively to the other alloys. Generally, in the presence of chloride there was a decrease of the conductive character of the film.The films are homogeneous and, according to the XPS results, the outer layer seemed mainly composed of Mg(OH)₂ and the internal layer composed of MgO, independently of the presence of chloride. The AFM study revealed that the presence of chloride affected film morphology, namely nano-crystallites dimensions and aggregates size that increased.     

Estimation of kinetic parameters of the passive state of carbon steel in mildly alkaline solutions from electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data

The unambiguous interpretation of electrochemical impedance spectra of complex systems such as passive metals and alloys in terms of an unique kinetic model is often hampered by the large number of adjustable modeling parameters. In this paper, a combination of in situ electrochemical data and ex situ surface analytical information is employed to validate the estimates of kinetic and transport parameters of the passive state of carbon steel. For the purpose, electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data for the oxidation of carbon steel in mildly alkaline solutions are quantitatively compared with the predictions of the Mixed-Conduction Model for oxide films that represent the passive oxide as an intermediate phase between magnetite and maghemite. Estimates of the kinetic rate constants at the film interfaces, as well as the diffusion coefficients and field strength in the film are obtained and their relevance for the corrosion mechanism of carbon steel is discussed.     

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.     

Quantitation of the dissolution of the graphite-coated copper foil in lithium-ion battery electrolytes by flame atomic absorption spectroscopy

Atomic absorption spectroscopy (AAS) was used to quantitatively evaluate the stability of copper foils coated with graphite and binder in lithium (Li)-ion battery electrolytes at open-circuit. The results showed that copper dissolution was extremely small in fresh electrolyte. However, atmospheric exposure markedly influenced the stability of copper and caused large amounts of dissolution. In addition, copper dissolution was found to increase (up to several hundred ppm) in electrolyte that had been aged for 6 months. The study showed that the condition of the electrolytes was critical to the stability of the graphite-coated copper foils.     

Quantitation of the dissolution of battery-grade copper foils in lithium-ion battery electrolytes by flame atomic absorption spectroscopy

Atomic absorption spectroscopy (AAS) was shown to be an effective method to quantitatively evaluate the stability of battery-grade copper foils in lithium-ion battery electrolytes at open-circuit. The results showed that copper stability was different between “fresh” electrolyte and electrolytes that had been “aged” for 6 months. In the “fresh” electrolyte, the copper foils showed a small amount of dissolution (up to ∼50 ppm) during their storage for up to 20 weeks. In the “aged” electrolyte, a large amount of copper dissolution (up to several hundred ppm) was found. The study showed that the condition of the electrolytes was critical to the stability of the copper foils.     

A theoretical approach of impedance spectroscopy during the passivation of steel in alkaline media

A theoretical impedance function is deduced for a proposed mechanism of passive film formation of steel in contact with alkaline aqueous media involving two reaction intermediates: mixed oxide with similar stoichiometry to magnetite and Fe(III)-oxides. The reduction reaction of dissolved oxygen is considered as the only cathodic reaction compensating the anodic current induced by the formation of iron oxides at open circuit potential. The iron dissolution takes place through a chemical dissolution of ferric oxide. A two-layered passive film with 3D structure is considered. A satisfactory agreement between the digital simulations on the basis of the theoretical impedance function and experimental spectra validates the proposed model.