Ab initio study of the interaction of chlorides with defect-free hydroxylated NiO surfaces

DFT periodic calculations have been performed to model the adsorption and sub-surface insertion of Cl atoms at increasing coverage on undefective hydroxylated NiO(1 1 1) terraces characteristic of the surface of the passive film on nickel. The results evidence that structural relaxation, splitting the mixed topmost OH/Cl anionic plane, releases the repulsive interactions induced by Cl adsorption, but the effect decreases with increasing Cl coverage with a loss of ∼1.1 eV in stability at surface saturation. Cl sub-surface insertion in the first inner anionic plane also releases the repulsive interactions in the topmost plane but the effect becomes favourable only at high Cl coverage because of inter-layer mixing after surface reconstruction. At low Cl coverage, an unrelaxed diatomic Ni-OH layer weakly bonded to the oxide and likely to dissolve is formed after sub-surface insertion, suggesting a hybrid mechanism of local oxide thinning. Adsorption of Cl by place exchange is energetically favourable but does not seem to promote dissolution. Insertion into the lattice may promote dissolution but it is energetically favourable only at high Cl surface coverage, which is not a likely situation due to Cl repulsive interaction on the surface. For undefective terraces of the passive film, the reported calculations do not confirm the existing hypotheses of adsorption-induced oxide thinning or chloride sub-surface penetration.     

Insight of DFT and atomistic thermodynamics on the adsorption and insertion of halides onto the hydroxylated NiO(1 1 1) surface

Spin polarized, DFT + U periodic calculations have been used to study the interaction of halides (X) with a (1 × 1)-hydroxylated NiO(1 1 1) surface, a model of passivated nickel. The exchange of surface OH groups by the X ions and the insertion of the halides in the anionic sub-surface layer have been investigated. The substitution of OH by halides is favored by a smaller size of the halide ions and by a lower substitution proportion. An atomistic thermodynamic approach including solvent effects allows us to construct phase diagrams of the surface terminations as a function of the Cl and F concentrations in the aqueous solution. The higher proportion of OH substitution by F, and the lower insertion energy, as compared to Cl, may be related to stronger corrosion caused by F as compared to Cl.     

Effects of temperature on the protective property, structure and composition of the oxide film on Alloy 625

The paper mainly investigated the protective property, structure and composition of the oxide film on Alloy 625 in a lithium borate buffer solution (pH_300°C = 6.93) in the temperature range of 25–300 °C. The methods used were electrochemical measurements and XPS analysis. As temperature increased, the protective property of the oxide film degraded, and the structure varied from a singe-layer to double-layer. The oxide film consisted of Cr₂O₃ and Cr(OH)₃ at 25 and 150 °C, while it contained Ni(OH)₂, in addition to Cr₂O₃ and Cr(OH)₃ at 250 and 300 °C. This was mainly attributed to the temperature-induced variation of composition and protective property of the barrier layer.     

Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Early stage oxidation of Ni–Cr binary alloy (1 1 1), (1 1 0) and (1 0 0) surfaces: A combined density functional and quantum chemical molecular dynamics study

First-principles and tight-binding quantum chemical molecular dynamics were used in this study. The chemisorption energies of O and OH on the Ni–Cr (1 1 0) surface are lower than those of other surfaces. The oxygen 2p orbitals hybridise with Ni 3d, 4s and small amounts of p orbitals for the (1 0 0) surface while Ni p orbitals have no contribution for the (1 1 0) surface, which might reduce the adsorption energy. Additionally, oxygen acquires the maximum depth into the Ni–Cr (1 1 0) surface. Applied strain increases the oxygen diffusivity. This study reveals that the Ni–Cr (1 1 0) surface is easier for oxygen diffusion accordingly oxidation accelerates.     

Environmental factors affecting the corrosion behaviour of reinforcing steel: I. The early stage of passive film formation in Ca(OH)2 solutions

Oxide film thickening on reinforcement steel at early stage of formation is followed in naturally aerated Ca(OH)₂ solutions, recalling the natural behaviour in concrete, by measuring the open-circuit potential, E, with time up to 4 h. The final potentials, E_fin, are reached from negative values indicating oxide film growth. E varies with the Ca(OH)₂ concentration according to a straight line relationship. Oxide film thickening, at early stage of immersion, follows a direct logarithmic growth law as evident from the linear relationship between E and log t. The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature. The free activation energy of oxide film thickening is determined and found to be 29.28 kJ/mole, indicating that the process of oxide film growth is under diffusion control.     

Electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al3Mg2

A corrosion mechanism is proposed for Al₃Mg₂, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (∼2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ∼10 nm. After exposure to ultra-pure water, the composition was AlMg_1.3O_0.2(OH)_5.1 corresponding to Al(OH)₃·1.1 Mg(OH)₂·0.2MgO. After exposure to 0.01 M Na₂SO₄, the composition was AlMg_0.2O_0.4(OH)_2.5 corresponding to Al(OH)₃·0.1Al₂O₃·0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH₂. Three possibilities are identified for MgH₂ formation. Al(OH)₃ formation is attributed to a precipitation reaction. Bulk nanoporous Al₃Mg₂ formation is predicted to be possible by Mg dealloying of Mg₁₇Al₁₂.     

The passivation of nickel in aqueous solutions—I. The identification of insoluble corrosion products on nickel electrodes using optical and ESCA techniques

In situ differential reflectometry in conjunction with ESCA techniques are used to identify various surface films which form electrochemically on nickel. The film observed on pure Ni in a 0.15 N Na₂SO₄ electrolyte has been identified to be primarily Ni(OH)₂. At different values of pH and potential, NiO has been found to form simultaneously with Ni(OH)₂. At solutions with pH > 8, the Ni(OH)₂ film partially transforms into a third oxide, presumably NiOOH, which can only be observed in situ.     

Surface characterization and corrosion behavior of 70/30 Cu-Ni alloy in pristine and sulfide-containing simulated seawater

The corrosion behavior of the 70/30 Cu-Ni alloy in stagnant, aerated pristine and sulfide-containing simulated seawater as a function of exposure time was investigated with polarization curve measurement and electrochemical impedance spectroscopy (EIS). It was demonstrated that the compact protective oxide film formed on the 70/30 Cu-Ni alloy resulted in the decrease of corrosion rate in aerated pristine seawater; while the corrosion rate of 70/30 Cu-Ni alloy in aerated sulfide-containing seawater increased dramatically due to the catalysis of the sulfide ions or sulfide scale for both the cathodic and anodic reactions. The impedance spectra and the corresponding equivalent circuits confirmed that a duplex layer of a surface film was formed on the 70/30 Cu-Ni alloy in aerated pristine seawater after a period of time and that the inner layer was responsible for the good resistance of the alloy; while only a porous and non-protective corrosion product layer formed on the 70/30 Cu-Ni alloy in aerated sulfide-containing seawater, which made small values of charge transfer resistance (R_ct) to last for a abnormally long time by interfering with the growth of the protective oxide film. The composition of the surface film on the alloy in pristine and sulfide-containing seawater for different exposure times were investigated thoroughly by XPS. It was found that the duplex corrosion product layer formed on the alloy in pristine seawater was composed of an inner Cu₂O and an outer CuO layer. The porous and non-protective corrosion product layer formed on the alloy in aerated sulfide-containing seawater was a mixture of CuCl, Cu₂S, NiS, Cu₂O and NiO with trace amounts of CuO and Ni(OH)₂ and that the most significant component was Cu₂S. In addition, SEM was used to analyze the topography of the 70/30 Cu-Ni alloy in both solutions after different exposure times.     

A first quantitative XPS study of the surface films formed, by exposure to water, on Mg and on the Mg-Al intermetallics: Al3Mg2 and Mg17Al12

An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al₃Mg₂ and Mg₁₇Al₁₂. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH)₂ layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al₃Mg₂ was AlMg_1.4O_0.2(OH)_5.4 whilst on Mg₁₇Al₁₂ the composition was AlMg_2.5(OH)₈.     

Ellipsometry of passive oxide films on nickel in acidic sulfate solution

Nickel passive film has been studied in acidic sulfate solutions at pH 2.3 and 3.3 by ellipsometry. During anodic passivation followed by cathodic reduction, the roughness increases with dissolution of nickel, being indicated by gradual decrease of reflectance. However, the ellipsometric parameters, Ψ (arctan of relative amplitude ratio) and Δ (relative retardation of phase), are relatively insensitive to the roughness increase. From the change of Ψ and Δ, δΨ and δΔ, during the anodic passivation and reduction, thickness of the passive oxide film was estimated with assumption of refractive index of n_f = 2.3 of the film. The thickness estimated is a range between 1.4 and 1.7 nm in the passive potential region from 0.8 to 1.4 V vs. RHE, having a tendency of thickening with increase of potential. Cathodic reduction at constant potential induces a change of the oxide film to an oxide film with lower refractive index of n_f = 1.7, accompanied by thickening of the film about 30% more in the initial stage of reduction for 30 s. The gradual decrease of thickness takes place for the oxide with the lower refractive index in the latter stage. The potential change from the passive region to cathodic hydrogen evolution region may initially cause hydration of the passive oxide of NiO, i.e., NiO + H₂O = Ni(OH)₂, and during the latter stage of reduction, the hydrated nickel oxide gradually dissolves.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

An ESEM in situ investigation of initial stages of the KCl induced high temperature corrosion of a Fe–2.25Cr–1Mo steel at 400 °C

The initial oxidation of a low-alloyed steel (Fe–2.25Cr–1Mo) in the presence of small amounts of KCl(s) have been investigated through ESEM in situ exposure and analysis at 400 °C. The samples were also characterized by XRD, SEM/EDX and FIB. The present study shows the corrosive nature of KCl towards the low alloyed steel. It is concluded that the initial KCl distribution is important and that a KCl/FeCl₂ liquid phase film forms on large parts of the oxide surface in the presence of KCl. It is proposed that Cl increases the oxidation rate (by decorating oxide grain boundaries) and decreases the oxide scale adhesion.     

Improved corrosion resistance in simulated concrete pore solution of surface nanocrystallized rebar fabricated by wire-brushing

Continuous surface nanocrystallization (SNC) of rebar was achieved through wire-brushing process. A uniform NC layer with thickness of 25 μm and average grain size of 50 nm was formed on the rebar surface. Due to the enhanced passivation performance of the NC layer, corrosion resistance of the SNC rebar was significantly improved in Cl⁻-containing saturated Ca(OH)₂ solution. High-energetic crystal defects of the nano-grains leads to the faster passivation and enhanced stability of the passive film of the SNC rebar.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments

The mechanism of corrosion product flaking on bare copper sheet and three copper-based alloys in chloride rich environments has been explored through field and laboratory exposures. The tendency for flaking is much more pronounced on Cu and Cu–4 wt%Sn than on Cu–15 wt%Zn and Cu–5 wt%Al–5 wt%Zn. This difference is explained by the initial formation of zinc and zinc–aluminum hydroxycarbonates on Cu15Zn and Cu5Al5Zn, which delays the formation of CuCl, a precursor of Cu₂(OH)₃Cl. As a result, the observed volume expansion during transformation of CuCl to Cu₂(OH)₃Cl, and concomitant corrosion product flaking, is less severe on Cu15Zn and Cu5Al5Zn than on Cu and Cu4Sn.     

Kinetics and mechanism of the nickel electrode—II. Acid solutions containing a high concentration of sulphate and nickel ions

The kinetics of the Ni electrode in acid solutions with a high sulphate and nickel ion concentration has been investigated in the range of 25–75°C. The active dissolution and the passive regions and the active-passive transition phenomena have been studied employing different potential perturbation techniques using both still and stirred solutions.Two electrochemical processes are competing within the prepassive film potential region, namely, the active Ni dissolution to Ni(II) ions and the Ni(OH)₂ film formation. Nickel passivation is explained by reaction pathways involving successive electrochemical and chemical steps implying the occurrence of hydroxo- and oxo-species as reactions intermediates. The advanced mechanism is based upon a positively charged Ni surface structure and attributes the onset of the complete passivity to a particular surface oxide species.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Structural changes during the reaction of Ni thin films with (1 0 0) silicon substrates

Ultrathin films of nickel deposited onto (1 0 0) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO₂ on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5 nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15 nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi₂ with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.     

Effect of nickel ion from autoclave material on oxidation behaviour of 304 stainless steel in oxygenated high temperature water

Characteristics of the oxide films formed on 304 stainless steel exposed to 290 °C oxygenated water in a nickel-lined autoclave were examined. The oxides evolve from dominating irregularly shaped hematite to faceted spinels with increasing immersion time. The surface layer of oxide film is first Cr-enriched and then Ni-enriched as immersion time increases. The oxides nucleate by solid-state reactions with selective dissolution of Fe and Ni, and then grow up through precipitation of cations from solution. Nickel ions dissolved from the nickel lining could promote the stability of NiFe₂O₄ spinel and influence the oxidation behaviour of 304 stainless steel significantly.