Semiconducting behavior of passive film formed on stainless steel in borate buffer solution containing sulfide

The semiconducting behavior of passive film formed on 316L stainless steel in borate buffer solution containing sulfide was studied by capacitance measurements (Mott–Schottky approach), electrochemical impedance spectroscopy and potentio dynamic polarization curves. The results reveal that the measured capacitance values of the stainless steel electrode have frequency dependence and hysteresis, which shows amorphous or highly doped semiconductor property of the passive film. The Mott–Schottky plots indicate p-type semiconducting behavior related to chromium oxide and n-type semiconducting behavior to iron oxide at different potential range of stainless steel electrodes. The existence of sulfide in the solution increases the acceptor densities obviously which increase more than five times with the sulfide concentration of 9 mg L⁻¹ and enables a more conductive behavior. The presence of sulfide also decreases the impedance values and enlarges the passive current of the electrode.     

Effect of alternating voltage passivation on the corrosion resistance of duplex stainless steel

Potentiodynamic polarization and E _corr versus t curves were obtained, together with electrochemical impedance spectroscopy (EIS) measurements, in order to understand the effects of alternating voltage (AV) passivation on the corrosion resistance of duplex stainless steel (DSS). SEM, EDS and XPS were employed to further investigate the influence of AV passivation on the properties of the passive film. The results of the electrochemical measurements showed that AV passivation significantly improved the corrosion resistance of DSS. SEM images indicated that the surface exhibited a unique morphology after AV passivation treatment, and XPS results suggested that AV passivation greatly increased the thickness of the passive film. Furthermore, significant chromium enrichment and a higher ratio of Fe³⁺/Fe²⁺ were observed in the passive film after AV passivation. Mott–Schottky results confirmed that AV passivation had a strong influence on the semiconducting properties of the passive film.     

Effects of solution temperature on the kinetic nature of passive film on Ni

Effects of solution temperature on the kinetic nature of passive film on Ni were investigated using polarization test, Mott–Schottky analysis and electrochemical impedance spectroscopy to reveal why the corrosion resistance of Ni is degraded with an increase in solution temperature. The increase in the corrosion rate of Ni with solution temperature was confirmed by the increase in the passive current density and also in the steady-state current density. Mott–Schottky analysis revealed that the passive film formed on Ni exhibits a p-type semiconducting characteristics irrespective of the solution temperature, and the concentration of cation vacancy in the passive film increases with temperature. By optimizing the reduced PDM (point defect model) on the experimental impedance data, base rate constants and transfer coefficients for the charge transfer reactions occurring at the metal/film and film/solution interfaces were extracted, and the Warburg coefficient for the cation vacancy transport was also determined. According to the calculated kinetic parameters (rate constants for the interfacial reactions, diffusivity of cation vacancy, etc.), the mechanism for the degradation of corrosion resistance of Ni with solution temperature was explained.     

The corrosion behavior of sputter-deposited aluminum-tungsten alloys

Thin films on aluminum-tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al₈₀W₂₀-Al₆₇W₃₃ composition range. Passivity and corrosion behavior of amorphous Al-W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al-W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al₆₇W₃₃) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al-W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W⁺⁶) and mobile cation vacancies, which restrict the transport of Cl⁻ through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al-W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.     

Effect of Al content on the electrochemical properties of Mg2−xAlxNi thin film hydride electrodes

Thin films of Mg_2−xAl_xNi alloys have been prepared by magnetron sputtering, and the effects of partial substitution of Al for Mg on the electrochemical properties of the films were studied. EIS results indicate the rate-limiting process for the thin film hydride electrode is the charge transfer reaction during the process of total discharge. A theoretical model has been derived for the impedance of a thin film hydride electrode based upon the assumption that hydrogen diffusion is neglected in the electrode. The charge-transfer reaction rate at the electrode surface and hydrogen diffusivity in the Mg_2−xAl_xNi thin film hydride electrodes were observed to initially decrease then increase with increasing Al content. Results from capacitance measurements indicate n-type semiconductor properties for the corrosion layer during the charge–discharge process. Hydrogen atom and OH⁻ transfer became more difficult with increasing Al content until x = 0.3, after which a significant drop in the barrier resistance was observed.     

Characterization of passivity and pitting corrosion of 3003 aluminum alloy in ethylene glycol–water solutions

In this work, the passivity and pitting corrosion behavior of 3003 aluminum (Al) alloy in ethylene glycol–water solutions was investigated using various electrochemical measurements, Mott–Schottky analysis and surface analysis techniques. Results demonstrate that the passive film formed on Al alloy contains both Al oxide and Al alcohol, showing an n-type semiconductor in nature. There is an enhanced corrosion resistance of the Al-alcohol film, which is resistant to adsorption of chloride ions. The pitting corrosion of 3003 Al alloy occurs in the solutions containing a low concentration of ethylene glycol only, where the formed film is dominated by Al oxide. Chloride ions attack and replace the oxygen vacancies in the film, resulting in a local detachment of the film from the Al alloy. A galvanic effect exists between Al alloy substrate and the adjacent second phase particles. Pits form when Al alloy substrate is dissolved away and the second phase particles drop off from the substrate.     

The role of inhibitors on the repassivation of pitting corrosion of carbon steel in synthetic carbonated concrete pore solution

The semiconductive properties and pitting susceptibility of passive films formed on carbon steel in synthetic carbonated concrete pore solution containing Cl⁻ ions were studied by Mott–Schottky (M–S) plots and electrochemical noise (EN). The results show that both nitrite and tetraethylenepentamine (TEPA) can reduce the donor density in passive film, leading to a more positive film breakdown potential. Moreover, nitrite can rehabilitate metastable pits and passivate the broken passive film more quickly than TEPA. Polarization curves also show that nitrite can increase the pitting potential more than TEPA at same concentration. Atomic force microscopy (AFM) imaging shows that Cl⁻ ions could induce large metastable pits on passive film, but the size of pits decreases and the density of pits increases with addition of nitrite. However, AFM imaging fails to detect the metastable pits under TEPA adsorbed film in spite of a smooth-like image on the steel surface is observed. The force curves of AFM indicate a stiffness (elasticity) order of films: nitrite oxidation film > original passive film > enamine adsorbed film.     

Evaluation of the characteristics using slow strain rate tests of 5456 Al-Mg alloy for ship construction

Recently, there has been increased interest in using aluminum alloys in ship construction instead of fiber-reinforced plastic (FRP). Aluminum alloy ships are faster, have a greater load capacity, and are easier to recycle than FRP ships. We investigated the mechanical and electrochemical properties of aluminum alloys using slow strain rate and potentiostatic tests under various potential conditions. Aluminum and aluminum alloys do not corrode due to the formation of an anti-corrosive passive film, such as Al₂O₃ or Al₂O₃·3H₂O, which resists corrosion in neutral solutions. In seawater, however, Cl⁻ ions destroy this passive film. The optimum protection potential range with regards to hydrogen embrittlement and stress corrosion cracking was determined to lie between −1.5 and −0.7 V (SSCE). These results can be used as reference data for ship design.     

Corrosion, passivation and breakdown of passivity of Al and Al-Cu alloys in gluconic acid solutions

The pitting corrosion of Al, and two Al-Cu alloys, namely (Al-2.5% Cu) and (Al-7.0% Cu) was investigated in gluconic acid (HG) solution through linear polarization and cyclic voltammetry techniques complemented with ex situ EDX and SEM examinations of the electrode surface. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method of chemical analysis was used to study the effect of alloyed Cu on the rate of the uniform corrosion of Al in these solutions. Results obtained from ICP were compared with those obtained from polarization measurements. For the three Al samples, the anodic responses did not involve active/passive transition due to spontaneous passivation. Addition of HG induced pitting (confirmed from SEM) within the passive oxide film due to the aggressive attack of gluconate (G⁻) anions. Relationships between pitting potential (E_pit), HG concentration, temperature, pH and potential scan rate were established. Alloyed Cu was found to enhance uniform corrosion, while it suppressed pitting attack. Local acidification model is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of G⁻ anions.     

Corrosion behavior of composition modulated multilayer Zn–Co electrodeposits produced using a single-bath technique

Composition modulated alloy (CMA) electrodeposits of Zn–Co were produced from acid chloride baths by the single-bath technique. Their corrosion behavior was evaluated as a function of the switched cathode current densities and the number of layers. The process was optimized with respect to the highest corrosion resistance. Enhanced corrosion resistance was obtained when the outer layer was slightly richer with cobalt. At the optimum switched current densities 40/55 mA cm⁻², a coating with 600 layers showed ~6 times higher corrosion resistance than monolithic Zn–Co electrodeposit having the same thickness. The CMA coating exhibited red rust only after 1,130 h in a salt-spray test. The increased corrosion resistance of the multilayer alloys was related to their inherent barrier properties, as revealed by Electrochemical Impedance Spectroscopy. The corrosion resistance was explained in terms of n-type semiconductor films at the interface as supported by Mott–Schottky plots.     

Electrochemical investigation of the effects of hydrogen on the stability of the passive film on iron

The effects of hydrogen on the stability of passive films on iron were investigated by electrochemical methods: open circuit potential decay, cathodic galvanostatic reduction, electrochemical impedance spectroscopy, and breakdown potential measurements. The results show that hydrogen decreases the final static open circuit potential, the cathodic charge for reduction and the charge transfer resistance of the passive film, indicating that hydrogen decreases the stability of the passive film. The charge transfer resistance of the passive film formed on the charged specimen does not change with increasing the film formation potentials, suggesting that increasing film formation potentials under hydrogen charging conditions cannot improve the stability of the passive film. Hydrogen decreases the breakdown potential of the passive film, especially at lower chloride ion concentrations, confirming that hydrogen promotes the susceptibility of the passive film on iron to pitting corrosion. The reasons why hydrogen decreases the stability of the passive film were discussed.     

Electrodeposited amorphous iron(III) oxides as anodes for photoelectrolysis of water

Deposition of amorphous iron(III)-oxide films on a conducting glass substrate was achieved via a cathodic bias in a 0.1 M hydrated ammonium iron(II) sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O) solution at −1.6 V versus Ag/AgCl. Analysis by X-ray absorption near edge structure confirmed the iron(III) feature of the amorphous films. The deposited films exhibited n-type semiconducting characteristics by showing photoresponses under an anodic bias. The Mott–Schottky method and cyclic voltammetry were employed to characterize the semiconducting properties of the deposited films, which included the band gap (2.2 eV), the potentials of the conduction and valence band edges and flat band (−0.6, +1.6 and −0.58 V versus Ag/AgCl at pH 7, respectively), and the donor density (1 × 10²²/cm³). The deposited iron(III)-oxide films were suitable to serve as an anode for water splitting under illumination.     

Electrochemical properties of metallurgical-grade silicon in hydrochloric acid

The electrochemical properties of metallurgical-grade silicon (MGS) and its native oxide (passive layer) were investigated using standard potentiostatic, potentiodynamic and impedance methods. The corrosion rate was found to reach up to 37 μm year⁻¹ in undiluted hydrochloric acid. The passive oxide layer was found to thicken with time as well as with potential. Increasing the potential from −0.5 to 1.0 V vs. SHE increases the thickness of the passive layer from approximately 2 to 6 nm. The interfacial impedance data for MGS in contact with the electrolyte was modeled. The model takes into account the capacitive behaviour of the space charge region and passive film. Due to the non-ideal behaviour of the passive film, a constant phase element (CPE) was incorporated in the model. Extraction of Mott–Schottky plot data from the EIS tests showed that the passive layer on the surface of the MGS is a p-type semiconductor. Scanning electron microscopy was used to determine that the impurities in the silicon (mostly iron, aluminum and calcium) were located almost exclusively at grain boundaries.     

Passivity and passivity breakdown of 304 stainless steel in alkaline sodium sulphate solutions

The passivity and passivity breakdown of 304 stainless steel were investigated in 0.25 M Na₂SO₄solutions of pH 10. The effect of applied potential and the presence of Cl^– ions in the electrolyte were also studied. Different electrochemical methods such as open circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results showed that the steel electrode passivates under open circuit conditions and also under potentiostatic control. The rate of passive film thickening under open circuit conditions follows a simple logarithmic law. Addition of Cl^– ion shifts the polarization curves in the active direction and above a critical chloride concentration, [Cl^– ] 0.15 M, pitting corrosion occurs and the pitting potential, E _pit, decreases linearly with the logarithm of [Cl^–]. The addition of sulphate ions to the chloride-containing solutions was found to inhibit the pitting process, and at [SO^2- ₄] 0.25 M, a complete immunity to pitting corrosion was recorded. The impedance measurements provided support for film thickening and film breakdown reactions. An equivalent circuit model which consists of a pure resistor, R , in series with a parallel combination of a pure resistor, R _p, and a constant phase element, Q, was proposed to describe the electrode/electrolyte interface. The passive film thickness was found to increase with applied potential up to a critical value of 0.3 V. At higher voltages, breakdown of the passive film occured.     

Electrochemical investigation of the passive behaviour of biomaterials based on Ag–Sn and Cu–Zn–Al in carbonate buffer in the absence and presence of chloride

The electrochemical behaviour of biomaterials based on Cu–Zn–Al (cubic Cu₃Zn phase) and Ag–Sn (orthorhombic Ag₃Sn and hexagonal Ag₄Sn phases) alloys was investigated in carbonate buffer solutions (pH 9.66) in the absence and presence of chloride, using different electrochemical techniques. Analyses of the open circuit potential and the potentiodynamic polarisation curves showed that the passivation domain and the corrosion parameters depend on alloy composition and chloride concentration. Chronoamperometric studies showed that passivation kinetics and corrosion of the passive film are both well described by a linear ln(i) versus ln(t) relation. The passive film formed on the Ag–Sn alloy is less susceptible to corrosion when compared to the Cu–Zn–Al system. The impedance data obtained in the passive region for the Cu–Zn–Al alloy showed that the passive layer is compact. In contrast, the impedance data obtained for the Ag–Sn alloy showed that the passive layer is formed by a compact oxide layer covered by a porous oxide gel layer. Mott–Schottky analysis showed that the passive film formed on the Cu–Zn–Al alloy behaves as a p-type semiconductor.     

Initiation and propagation of pitting and crevice corrosion of hydrogen-containing passive films on X70 micro-alloyed steel

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E_p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO₃ solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E_p and open circuit potential E_corr of the steel in 0.5 M NaHCO₃ solution. However, a synergistic effect of hydrogen and Cl⁻ on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO₃ solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.     

Electrochemical investigation of oxide films formed on nickel alloys 182, 600 and 52 in high temperature water

Nickel-based alloys 182, 600 and 52 were exposed to simulated Pressurized Water Reactor (PWR) primary water (1000 ppm B, 2 ppm Li, O₂ <10 ppb, 325 °C) under different dissolved hydrogen (DH) conditions [0, 2, 25 and 50 cm³ H₂(STP) kg⁻¹] for times up to 1 month in a recirculating autoclave. The influence of exposure time and DH on oxide films formed on the alloys was evaluated by means of electrochemical tests; electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S). The in situ EIS was performed every day, allowing the monitoring of the oxide layer formation and change. M–S was performed at room temperature after the full exposure time. The results showed that the maximum in the defect concentrations obtained via M–S analysis and the maximum crack growth rate are at the same DH content, thus relating electrochemical testing to stress corrosion cracking observations. A conceptual separation between the electrochemical behavior of (1) the oxide layer (visible in the higher frequencies of EIS) and (2) the oxide layer – solution interface (visible in the lower frequencies of EIS) was able to explain the effect of hydrogen on the low-frequency EIS impedance results.     

Uniform and pitting corrosion events induced by SCN anions on Al alloys surfaces and the effect of UV light

The influence of the alloying elements on the uniform and pitting corrosion processes of Al-6061, Al–4.5%Cu, Al–7.5%Cu, Al–6%Si and Al–12%Si alloys was studied in 0.50 M KSCN solution at 25 °C. Open-circuit potential, Tafel polarization, linear polarization resistance (LPR) and ICP-AES measurements were used to study the uniform corrosion process on the surfaces of the tested alloys. Cyclic polarization, potentiostatic current-time transients and impedance techniques were employed for pitting corrosion studies. Obtained results were compared with pure Al. Passivation kinetics of the tested Al samples were also studied as a function of applied potential, [SCN⁻] and sample composition by means of potentiostatic current transients. The induction time, after which the growth of stable pits occurs, decreased with increasing applied potential and [SCN⁻]. Regarding to uniform corrosion, alloyed Cu was found to enhance the corrosion rate, while alloyed Si suppressed it. Alloying elements of the tested samples diminished pitting attack to an extent depending on the percentage of the alloying element in the sample. Among the investigated materials, Al–Si alloys exhibited the highest corrosion resistance towards uniform and pitting corrosion processes in KSCN solutions. The passive and dissolution behaviour of Al was also studied under the conditions of continuous illumination (300–450 nm) based on cyclic polarization and potentiostatic techniques. The incident photons had a little influence on pit initiation and a marked effect on pit growth. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to pitting. The effects of UV photons energy and period of illumination on the morphology of the pitted surfaces were also studied.     

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. 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 ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Electrochemical impedance spectroscopy study of the passive films of alloy 22 in low pH nitrate and chloride environments

Utilizing electrochemical impedance spectroscopy (EIS), we characterize the passive film properties of alloy 22 during immersion in low pH nitrate and chloride solutions. In pure HCl, the passive film grows thinner with increasing acid concentration. In contrast, in HNO₃, the passive film corrosion protection properties are enhanced, which leads to low corrosion rates, even at pH < −0.5. The combined influence of both HCl and HNO₃ in contact simultaneously with the alloy 22 surface shows multiple phases in the passive film properties depending on the pH. EIS results show that the passive film changes either thickness and/or composition as the system is driven chemically through different corrosion states, including: active, passive, active/passive and transpassive.