Depassivation-repassivation behavior of a pure iron surface investigated by micro-indentation

Depassivation-repassivation behavior on a pure iron surface in borate buffer solution was examined under potentiostatic control by a micro-indentation test. Current peaks emerge during both downward and upward drives of the indenter due to depassivation which is caused by plastic deformation of the substrate but not elastic deformation and repassivation. The total electric charge of the current peaks is proportional to the maximum load. The total electric charge also increases with increase in intermission time of the indentation, indicating that the passive film is ruptured even during stress relaxation. It is estimated from the electric charge balance that 82% and 18% of the film rupture occurs during the downward drive and intermission, respectively, and that no rupture occurs during the upward drive. Furthermore, the film-ruptured area is estimated to be 80% of the plastic deformed surface area. The partial retainment of the passive film on iron suggests that the ductility of the passive film is higher than that of the substrate.     

A numerical model for current transients during micro-indentation of passive iron surface

During in situ micro-indentation of passive iron in pH 8.4 borate solution, a couple of anodic current peaks emerged; the first peak during loading and the second peak during unloading. The current transients, associated with rupture and repair of the passive film, were influenced by the indentation conditions. For instance, the current peak height, the current peak area and the time required for complete repassivation are strongly dependent on indentation rate. A numerical model was proposed to correlate the current transient during loading with mechanical deformation of the passive surface. The comparison between the current transient measured experimentally and that estimated from the load transient suggested that the ruptured area of passive film was about 10% of the surface area deformed by the contact with the indenter.     

Electrochemical investigation of locally depassivated iron. A comparison of various techniques

Electrochemical aspects of the damage caused in situ to the passive film on iron have been investigated by means of ac impedance and spectral analysis of the current fluctuations. These techniques can provide useful information on the individual depassivation-repassivation transient and its interaction with the surrounding passive surface. It has been shown that it is rather difficult to define an intrinsic rate of repassivation which could be independent of the depassivation technique. Firstly the repassivation has been studied by potential jump experiments. Secondly depassivation by abrasion due to the projection of a suspension of particles or focused laser pulse have been developed. The results are discussed in terms of the classical model involving a sequence of depassivation-repassivation events induced by local breakdown of the film.     

Current transients of passive iron observed during micro-indentation in pH 8.4 borate buffer solution

Micro-indentation test of passive iron electrode in deaerated pH 8.4 borate buffer solution was carried out to investigate the rupture and repair of passive film. During driving a conical diamond micro-indenter with a load of 0.1 N order downward to the electrode and driving upward from the electrode, a couple of anodic current peaks were observed. The first current peak spiked during loading was responsible for partial exposure of iron substrate to the solution due to rupture of the passive film when the indenter tip contacted. The second peak emerged during unloading when elastic deformation recovered, which was ascribed to repair at the ruptured sites. The second peak current was larger than the first one. Both peaks were sensitively influenced by electrode potential or concentration of sulfate ions containing in solution. The model for a series of rupture and repair processes of the passive film by micro-indentation was proposed to discuss the current transients.     

Potassium sorbate solutions as copper chemical mechanical planarization (CMP) based slurries

Copper depassivation and repassivation characteristics in potassium sorbate solutions, subsequent to mechanical abrading are reported. The identification of copper repassivation kinetics obtained subsequent to mechanical damage of copper protective films formed in sorbate based solutions is discussed. The repassivation rate of copper in sorbate based solutions was measured by means of a slurryjet system capable of measuring single particle impingments on microelectrodes. Copper repassivation rates measured by this slurryjet system in sulfate solutions containing 10 g L⁻¹ potassium sorbate were found to be in the range of 0.5-1.5 ms. An increase in the potassium sorbate concentration leads to a decrease in copper repassivation time at potentials ranging from 200 to 600 mV_Ag/AgCl. The impingement angle between the copper surface and a single abrasive particle has no impact on copper repassivation time nor peak current (I_max) values. XPS studies revealed that copper passivation in potassium based solution was due to the formation of a thin film which is constituted of: Cu₂O, Cu(OH)₂ and Cu(II)-sorbate, while copper(II)-sorbate is mainly present at the top levels of the passive film. It is therefore recommended that the use of potassium sorbate as a passivating component in conjunction with the addition of strong oxidizing agents in chemical mechanical planarization (CMP) slurry design should be considered.     

Potential dependence of normalized friction coefficient of passive iron surface evaluated by nano-scratching in solution

Nano-scratching in solution was performed to the single-crystal iron (1 0 0) surface passivated at 0.0-1.0 V (SHE) in pH 8.4 borate buffer solution to evaluate the friction coefficient of the iron (1 0 0) surface kept in the passive state and its potential dependence. The friction coefficient obtained with nano-scratching for the passive iron surface depended on normal force, i.e., normal displacement, which resulted mainly from the geometry of the diamond tip. In order to avoid the effect of the tip geometry on friction coefficient, the normalized friction coefficient was newly defined with dividing friction coefficient by geometrical factor. The normalized friction coefficient obtained with nano-scratching in solution for the iron (1 0 0) surface kept in the passive sate was significantly larger than those obtained with nano-scratching in air after passivation. The normalized friction coefficient obtained with nano-scratching in air after passivation was almost independent of potential in the passive region. On the other hand, the normalized friction coefficient obtained with nano-scratching in solution increased with increasing potential in the passive region.The difference between normalized friction coefficients obtained with nano-scratching in solution and in air was discussed by taking into account a series of mechano-electrochemical reaction (film rupture, active dissolution and repassivation) which would take place at the moving front of the diamond tip during nano-scratching in solution. The large potential dependence of the normalized friction coefficient obtained with nano-scratching in solution was explained in terms of the increase in repassivation rate at the film rupture sites with increasing potential in the passive region.     

Passivity and pitting of carbon steel in chromate solutions

The passivity and pitting behavior of A516-70 carbon steel in chromate solutions were studied using electrochemical measurements. The anodic Tafel slopes in the active region show that carbon steel dissolution involves two mechanisms in this range: formation and further oxidation of a pre-passive film of Fe(OH)₂. The first current peak at −0.228 V (Ag | AgCl) in cyclic voltammograms is caused by the oxidation of the pre-passive film and the formation of a stable passive film of Cr³⁺+Fe³⁺. The second peak at 0.612 V is ascribed to the oxidation of Cr³⁺ in passive film to Cr⁶⁺. The charge-transfer step at the electrode/solution interface controls the film formation and dissolution; the role of diffusion is negligible. Chromate ions play a prominent role in the formation of passive film, but hardly affect the stability of the passive state. More chromate ions in solution enhance the dissolution of Cr³⁺ at the second peak potential. Upon addition of chloride ions metastable pits are initiated, as indicated by a typical current transient: a quick current rise followed by a slow recovery. A maximum exists in the potential dependence of the pit initiation rate. Metastable pit growth is controlled by the ohmic potential drop mainly across the cover over the pits. Increasing potential is beneficial to the repassivation of metastable pits, as indicated by the decreasing average repassivation time. A pit stabilization criterion, the ratio of peak pit current to pit radius, must exceed 6×10⁻² A cm⁻¹ during pit growth to avoid repassivation in the present system.     

Effect of stress on the passivation of Si-DLC coating as stent materials in simulated body environment

DLC coating can be used for vascular stents to prevent the stainless steel substrate from eluting Ni and Cr by plastic deformation and corrosion environment. The stress corrosion cracking (SCC) of Si-diamond-like carbon (Si-DLC) coated on 316L stainless steel was studied in a simulated body environment of a deaerated 0.89 wt.% NaCl electrolyte at 37 °C. This paper investigated the effect of Si-DLC coating on the SCC of 316L SS by slow-strain-rate test (SSRT), constant load test (CLT), and electrochemical impedance spectroscopy (EIS). The EIS data were monitored for the elastic and plastic regions under CLT to determine the electrochemical behavior of the passive film during SCC phenomena. The Si-DLC coated steel exhibited more ductility than uncoated steel and less susceptibility to SCC in this environment. According to X-ray photoelectron spectroscopy (XPS) analysis, the film repassivation occurs due to the presence of the silicon oxide layer on the Si-DLC film surface.     

An SECM observation of dissolution distribution of ferrous or ferric ion from a polycrystalline iron electrode

The dissolution of iron as ferrous or ferric ion from a polycrystalline iron electrode during anodic polarization in pH 2.3 sulfate solution was evaluated by using scanning electrochemical microscopy (SECM). A graphite reinforcement carbon (GRC) microelectrode was employed as a probe electrode of SECM to detect ferrous or ferric ions dissolved from the iron electrode in the active-dissolution, passive or trans-passive region. The probe current above the iron electrode surface subjected to active-dissolution showed the dissolution distribution of ferrous ion, depending on the substrate grains. It was found that the active-dissolution rate of iron as ferrous ions from the grain on which the thicker film was formed in the passive region, was lower than that from the grain on which the thinner film was formed in the passive region.     

Electronic structure and pitting behavior of 3003 aluminum alloy passivated under various conditions

Passivity of aluminum (Al) alloy 3003 in air and in aqueous solutions without and with chloride ions was characterized by electrochemical measurements, including cyclic polarization, electrochemical impedance spectroscopy (EIS), localized EIS and potential of zero charge, Mott-Schottky analysis and secondary ion mass spectroscopy (SIMS) technique. Stability, pitting susceptibility and repassivation ability of Al alloy 3003 under various film-forming conditions were determined. Results demonstrated that passive films formed on 3003 Al alloy in air and in Na₂SO₄ solution without and with NaCl addition show an n-type semiconductor in nature. The passive film formed in chloride-free solution is most stable, and that formed in chloride-containing solution is most unstable, with the film formed in air in between. Pitting of Al alloy 3003 passivated both in air and in aqueous solutions is inevitable in the presence of chloride ions. There is the strongest capability for the air-passivated Al alloy 3003 to repassivate, and the weakest repassivating capability for Al alloy 3003 passivated in chloride-containing solution. The resistance of the passivated Al alloy 3003 to pitting corrosion is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). According to the differences between corrosion potential and potential of zero charge, passive film formed in air has the strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Chloride ions causing pitting of passivated Al alloy 3003 in air and in chloride-free solution come from the test solution, while those resulting in pitting of passivated Al alloy 3003 in chloride-containing solution mainly exist in the film during film-forming stage.     

304 Stainless steel oxidation in sulfate and sulfate+ bicarbonate solutions

The electrochemical characterization of 304 stainless steel in 0.1–0.5m Na2SO4 and Na2SO4 + NaHCO3 aqueous solutions at pH8 was done in combination with SEM surface analysis. Passivation of the surface film without any pitting events is observed for the current–potential and current–time experiments, and no anodic current spikes, which are associated with depassivation events, are distinguishable above the background current level. However, SEM pictures of an electrode surface polarized at potentials above 0.4V show microscopic pit nucleation. Even when the metal may be regarded as passive from a current–time or current–potential characteristics, passivity is not stable and localized film breakdown can still occur. The events are ascribed to pit nucleation at the active inclusion sites. The pits do not grow even into the metastable state but die through repassivation by metal salt precipitation immediately after birth. The effect is ascribed to the solubility of metal salts with the electrolyte produced by dissolution in the nucleation sites. The results show that pit nucleation and pit growth are two distinct processes. The importance of solution composition and the protective effect of bicarbonate ions is also discussed.     

Hydrodynamic effects on erosion-enhanced corrosion of stainless steel in aqueous slurries

This paper studies the hydrodynamic effects of erosive slurry on corrosion of passive target (type 304 stainless steel). The transient current response to disruption of passive film is investigated using the single particle impingement technique. The transient current density over the damaged surface is approximately independent of the hydrodynamics of fluid when the flowing velocity is in range of 5-10 m/s. It is characterized by a sharp rise caused by disruption of passive film and a slow decay due to repassivation. Therefore, the difference in transient current response is a result of different damaged surface area produced by solid particle impingement. After the kinetic parameters of repassivation are determined, the hydrodynamic effects on average corrosion rates of the passive target are quantitatively predictable with aid of the physical model developed by Lu and Luo.     

High Rate Anodic Dissolution of 100Cr6 Steel in Aqueous NaNO 3 Solution

The high rate anodic dissolution of 100Cr6 steel in NaNO₃ electrolytes of various concentrations and at different temperatures was investigated. Galvanostatic flow channel experiments were used to examine the current efficiency of the steel substrate. Below 6 A cm^–2(zone A), oxygen evolution dominates, while at higher current densities iron dissolution prevails (zone C). Potentiodynamic polarization studies indicated a complete substrate surface passivation up to +1.8V (vs NHE), and periodic fluctuations of the current density at higher anode potentials (> +1.8 V) due to severe oxygen evolution. Rotating cylinder measurements served for polarization studies at lower current densities in the region of dominating oxygen evolution. Scanning electron micrographs revealed a correlation between the current efficiency and the coverage of the substrate surface with an electronically conductive film at current densities of 2, 9 and 20 A cm^–2. The microstructure of the black, solid surface film developing during the high rate anodic dissolution of the steel was found to be heterogeneous and very porous. The main film components, as determined by X-ray diffraction and Auger electron spectroscopical measurements, were amorphous iron oxides, Fe _x O _y , and inert carbides, M₃C, originating from the steel matrix. An activation–repassivation process is proposed, which is responsible for the development of the complex multilayer (multiphase) structure observed at the steel substrate surface.     

Correlation between repassivation kinetics and corrosion rate over a passive surface in flowing slurry

The effect of hydrodynamics of flowing slurry on anodic dissolution rate of passive metals was quantitatively evaluated using a theoretical model recently developed by the authors. The enhanced anodic dissolution over a passive metal in flowing slurry is dominated by the passive film breakdown caused by the impingement of solid particles and the decay of local current density over the impacted area due to repassivation. In the present study, the anodic current densities of 304 stainless steel and carbon steels were measured in flowing slurries under potentiostatic control condition. The difference in the repassivation modes indicates different repassivation mechanisms that depend on electrode material and corrosive medium. The parameters of repassivation kinetics experimentally determined enable estimation of the average anodic current density on the electrode surface in flowing slurry using the theoretical model. The theoretical predictions are in good agreement with the experimental data.     

Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Nano-scale study of passive films and chloride-induced depassivation of carbon steel rebar in simulated concrete pore solutions using FIB/TEM

Analytical TEM techniques were used to characterize passive film on carbon steel and how it breaks down with chloride addition. Before chloride addition, the thicknesses of the oxide films on rebars were found to be uniform between 5 and 13 nm. The films consisted mainly of iron oxides that included nano-crystalites of FeOOH with an epitaxial relationship to the metal surface. Elements from the passivating solutions were located in regions of the oxide closest to the free surface. With the addition of chloride at concentrations below depassivation thresholds, there was evidence for change in the oxide closest to the free surface, but generally the oxides were similar to those grown before chloride addition. After exposure to chloride concentrations above depassivation thresholds, the oxide films were no longer uniform, with parts of the surface bare, and the average oxide thickness reduced. Pitting and enhanced corrosion were seen to accompany cementite lamella.     

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.     

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3 GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8 GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10 GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.     

Transpassive dissolution of iron in sulphuric acid solution

Anodic oxidation characteristics of iron in 3, 10 and 12 mol dm^–3 sulphuric acid solutions have been studied in the transpassive region. Dissolution current efficiency measurements have been carried out using potentiostatic current-voltage curves and solution analysis techniques. The current-voltage curves were split into metal dissolution and oxygen evolution curves assuming that the iron goes into solution as Fe³⁺. The current density value in the passive region increased whereas the current density in the transpassive region decreased with the increase of sulphuric acid solution concentration. In order to obtain information about the nature of the films present on the surface, potential decay curves from different anodic potentials in the transpassive region have been recorded. It seems that there is no passive film present on the specimen surface in 12 mol dm^–3 sulphuric acid solution and a better surface finish is obtained after the dissolution. Depth profile analysis of oxide films by the AES technique in 3 mol dm^–3 sulphuric solution reveals that the sulphur concentration is maximum at the metal/oxide interface rather than at the oxide/electrolyte solution interface as required by an ion exchange mechanism for film dissolution.     

Effect of small addition of Mn on the passivation of Zn in 0.1 M NaOH solution

The passivation of pure Zn (99.995 wt%) and Zn–0.4Mn (0.4 wt% Mn) alloy in a deaerated 0.1 M NaOH solution (pH 12.9) was investigated by electrochemical measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The potentiodynamic polarization and electrochemical impedance measurements show that addition of 0.4 wt% Mn can decrease the passive current density of Zn in the passive region. XPS surface analysis indicates that there is approximately 1.0–2.0 at% Mn²⁺ being incorporated into the passive film on Zn–0.4Mn alloy with Mn content being higher in the outer layers. Mott–Schottky analysis shows that the incorporated Mn can decrease concentration of defects in the film. AFM observations disclose that Mn can decrease the grain size of the film. The mechanism by which Mn additions improve the passivity of Zn is that the incorporated Mn can inhibit ions transportation in the film and inhibit its growth. Meanwhile, Mn can also promote the nucleation of Zn oxides and decrease film porosity.