Photoelectrochemical study of the growth of the passive film formed on Fe-20Cr-15Ni in a pH 8.5 buffer solution

The structural change of the passive film on Fe-20Cr-15Ni at its growth stage was examined in-situ using a photoelectrochemical technique. The photocurrent spectra showed that the passive film on Fe-20Cr-15Ni for 0.5 h ∼ 25 h in a pH 8.5 buffer solution was composed of (Cr, Ni)-substituted γ-Fe₂O₃ mixed with NiO. Photocurrent spectral analysis suggested that a crystalline film of (Cr, Ni)-substituted γ-Fe₂O₃ formed before 30 min, with its thickness exceeded that of the space charge layer after 1 h of passivation. NiO particles appeared to have gradually precipitated from (Cr, Ni)-substituted γ-Fe₂O₃ film in the early stage of passivation of 1 h.     

Effects of nitrogen on the passivity of Fe-20Cr alloy

Influences of nitrogen on the passivity of Fe-20Cr-(0, 1.1)N alloys were examined by in situ electrochemical techniques. Nitrogen was incorporated in the form of (Fe, Cr)-nitrides in the passive film, and Cr was enriched in the film of the alloy with nitrogen. Photocurrent analysis demonstrated that the structure of passive film formed on Fe-20Cr-1.1N alloy is Cr-substituted γ-Fe₂O₃ with (Fe, Cr)-nitrides. Mott-Schottky analysis revealed that the film formed on Fe-20Cr-1.1N contained higher Cr⁶⁺ and lower Cr³⁺ vacancy concentrations compared with that on Fe-20Cr alloy. All of these results were associated with the enhanced protectiveness of the film on Fe-20Cr-1.1N.     

光电化学响应分析Ni201在中性溶液中形成表面钝化膜的半导体性质

应用光电化学响应法和:Mott-Schottky曲线法,研究了Ni201在500℃空气中生成的氧化膜和在pH值为8.4的中性缓冲溶液中阳极氧化生成钝化膜的半导体性质,分析了Ni201表面钝化膜的结构和组成.Mott-Schottky曲线表明,Ni20l在该中性溶液中生成钝化膜的平带电位约为0.40 V,其在500℃空气中生成的氧化膜的平带电位约为0.15 V,前者的载流子浓度约是后者的34倍.在中性缓冲溶液中生成钝化膜的光电流谱表明,Ni201的结构由内层NiO和外层Ni（OH）₂构成,其带隙宽度分别为2.8和1.6 eV.其中,具有晶体结构的内层NiO的带隙宽度与Ni201在500℃空气中生成的氧化膜的带隙宽度2.4 eV相似.通过光电化学法和Mott-Schottky曲线建立Ni201表面钝化膜的电子能带结构模型,解释了其内层NiO和外层Ni（OH）₂同是p型半导体组成的钝化膜的半导体性质.     

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.     

The passivation of nickel in aqueous solutions—II. An in situ investigation of the passivation of nickel using optical and electrochemical techniques

The film responsible for the passive behavior of nickel and the mechanisms of film formation was studied in situ. Electrochemical polarization techniques were used in conjunction with Differential Reflectometry. It was found that the film which causes nickel to passivate in 0.15 N Na₂SO₄ (pH = 4.0–12.0) is Ni(OH)₂. It is suggested that the film forms via a precipitation mechanism in the acid solutions subsequent to active metal dissolution. At low values of pH, this precipitated film, once formed, generally does not change in thickness as the potential is made increasingly anodic. Conversely, the Ni(OH)₂ film formed in higher pH solutions (pH = 8.0 open, pH = 12.0 open and de-aerated) grows by virtue of a solid state mechanism. Both NiO and NiOOH are observed to form subsequent to Ni(OH)₂ but only at higher values of pH. Although the films formed at alkaline pH values are thinner than those in the lower pH environments, they were found to be more protective.     

In situ ESTM and ESTS studies of the nanostructure and energetic properties of Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloy surfaces

Fe-20% Cr-40% Ni and Fe-20% Cr-70% Ni alloys are studied in air and in 0.01 N HCl and 0.1 N H₂SO₄ solutions at the controlled potentials of the specimen and needle using in situ techniques of electrochemical scanning tunneling microscopy (ESTM) and electrochemical scanning tunneling spectroscopy (ESTS). The changes in the potential of Fe-20% Cr-40% Ni alloy specimen fall in ranges of −0.3 to 1.1 V and 0–0.3 V (N.H.E.) in 0.01 N HCl and 0.1 N H₂SO₄, respectively, and the changes in the potential of Fe-20% Cr-70% Ni specimen, in ranges of 0.09–0.94 V and −0.1 to 0.5 V in 0.01 N HCl and 0.1 N H₂SO₄, respectively. Local spectral dependences of the tunnel current on the tunnel voltage are obtained and processed and the coefficients that characterize the local electrophysical properties of the surfaces are calculated on their basis. It is found that, in the studied acid solutions at the controlled potential, the surfaces are more energetically homogeneous than in air, where a substantially larger dispersion of the properties is observed. It is concluded that the energetic characteristics of separate surface sites of these alloys are determined on the atomic scale by the mutual effects of the neighboring atoms involved in its composition, i.e., by the matrix effect, which was discovered previously in the case of Kh18N10T stainless steel [1].     

Photoelectrochemical investigation of passive layers formed on Fe in different electrolytic solutions

The passive films formed on Fe in different electrolytic solutions, spanning almost the overall pH range, and with different growth procedures were investigated systematically by photocurrent spectroscopy. The potentiodynamic growth curves are compared both in aerated and de-aerated electrolytes and a potentiostatic growth procedure is also employed. For high positive formation potentials, similar anodic spectra are recorded in all solutions giving an optical gap very close to that expected for crystalline Fe₂O₃. The origin of photocurrent spikes is also investigated and the effect of the formation potential upon the measured absorption threshold is discussed taking into account the ordered or disordered structure of the films and its Fe(II) species content. Films formed at quite negative potentials display different photocurrent spectra, characterized by cathodic stationary photocurrent, absence of spikes and lower absorption in the visible range, indicating the formation of Fe(II) passive layers. The same behaviour is observed with film formed at positive potentials, after a long negative biasing.     

X-ray photoelectron spectroscopy and electrochemical characterization of a FeCrMoNiRu alloy in 0.5 M HCl solution

The spontaneous passivation behaviour of a Fe-22Cr-3Mo-5Ni alloy system containing small amounts of ruthenium in 0.5 M HCl solution has been investigated by means of immersion tests, electrochemical measurements and the XPS technique. The weight loss and electrochemical measurements show that increasing amounts of ruthenium (up to 0.3 wt%) in Fe-22Cr-3Mo-5Ni substantially improve resistance to attack by hydrochloric acid. The XPS results show that the spontaneously formed passive film on this alloy with ruthenium mainly consists of Cr and Mo oxyhydrox-ides. Molybdenum is incorporated as Mo⁴⁺ and Mo⁶⁺, while neither ruthenium nor nickel are found in the passive film. The effects of ruthenium additions on the spontaneous passivation of FeCrMoNi alloys are discussed.     

Passivation and local activation of α-Fe + Fe3C-based nanocomposites in neutral media

Regularities of the anodic dissolution and processes of local activation of α-Fe + Fe₃C nanocomposites containing 9 to 92 mol % of cementite are studied. It is shown that the sequence of anodic processes in borate solutions is the same for pH 6.3 and 7.4, namely, the dissolution and passivation of the ferrite component, dissolution and passivation of the cementite component, and anodic oxygen evolution at the nanocomposites’ passive surface. With an increase in the pH the passivation potentials decrease, which agrees with a thermodynamic model of the formation of two-layer passive film Fe₃O₄/γ-Fe₂O₃. It is found that the chloride concentration range, in which the local activation of α-Fe + Fe₃C is possible, narrows with an increase in the cementite content. Original Russian Text ? A.V. Syugaev, S.F. Lomaeva, S.M. Reshetnikov, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 1, pp. 58–64.     

Passive film formed on nickel in a neutral solution

A bright nickel was passivated in the pH 8·39 boric acid-borate buffer solution. The oxide formation reactions were mainly investigated by means of polarization experiments, alternating anodic and cathodic polarization, potential decay experiments and colorimetric analysis. From the results, the chemical composition and structure of passive films formed on nickel were discussed. The experimental results are summarized as follows.1. A considerable amount of nickel ions dissolved during the anodic formation of passive films. The dissolution ceased because of the formation of NiO₂. 2. The passive films on bright nickel has a duplex structure consisting of NiO and Ni₃O₄. At higher potentials, NiO₂ was produced on them. NiO and Ni₃O₄ were formed directly from Ni, and NiO₂ was transformed from Ni₃O₄. 3. NiO and NiO₂ were reversibly produced and reduced, but Ni₃O₄ was very difficult to be reduced. 4. It was observed that there was a reversible charging and discharging layer at the potential where Ni₃O₄ formed. The electric capacity was calculated to be about 100 μF/cm², assuming that roughness factor was 2. It would be reasonable to think that a space charge was established on nickel surface.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

Use of Electrochemical Noise (EN) Technique to Study the Effect of sulfate and Chloride Ions on Passivation and Pitting Corrosion Behavior of 316 Stainless Steel

In the present paper, studies were conducted on AISI Type 316 stainless steel (SS) in deaerated solutions of sodium sulfate as well as sodium chloride to establish the effect of sulfate and chloride ions on the electrochemical corrosion behavior of the material. The experiments were conducted in deaerated solutions of 0.5 M sodium sulfate as well as 0.5 M sodium chloride using electrochemical noise (EN) technique at open circuit potential (OCP) to collect the correlated current and potential signals. Visual records of the current and potential, analysis of data to arrive at the statistical parameters, spectral density estimation using the maximum entropy method (MEM) showed that sulfate ions were incorporated in the passive film to strengthen the same. However, the adsorption of chloride ions resulted in pitting corrosion thereby adversely affecting noise resistance (R _N). Distinct current and potential signals were observed for metastable pitting, stable pitting and passive film build-up. Distinct changes in the values of the statistical parameters like R _N and the spectral noise resistance at zero frequency (R°_SN) revealed adsorption and incorporation of sulfate and chloride ions on the passive film/solution interface.     

A study of the passive films on chromium by capacitance measurement

Using capacitance measurement and Mott-Schottky analysis, the semiconducting properties of passive films formed on chromium within the passive potential range under different conditions were investigated. The study reveals a p-type behavior of the passive layers. Two semiconductive parameters, i.e., the acceptor density (N_A) and the flatband potential (E_FB), which are mainly related to composition and surface charges of the passive films, have been measured. The effect of film formation potential, passivation time, pH and composition of solutions on the parameters are discussed. N_A increases either with lowering film formation potentials or with prolonging passivation times. This is attributed to the transformation of a less hydrated oxide film into a more hydrated form. The changes of E_FB are discussed as a function of adsorptive anions and pH values of electrolyte solution.     

An XPS study of passive films on Nickel and alloy 600 in acids

Compositions of surface films formed on nickel and Alloy 600 in I M HCI, 0.5 M H₂SO₄ and I/3 M H₃PO₁ solutions were investigated as a function of polarization potential. The main constituent of surface films formed on Ni in 0.5 M H₂SO₄ or 1/3 M H₃PO₄ solution was hydrated nickel oxyhydroxide, in which the ratio of O₂ to OH⁻ increased when passivation occurred. The surface films formed on Ni and Alloy 600 at lower potentials in 0.5 M H₂SO₄ solution contain S² ions other than SOP_4²⁻ ions, whereas S²⁻ ions were not incorporated in the passive film. Passivation of Alloy 600 took place by the formation of hydrated chromium oxyhydroxide. Pitting led to no substantial change in the average composition of the film.     

Sintering of Fe2NiO4 with an internal binder: a way to obtain a very dense material

The coupled synthesis and sintering of Fe₂NiO₄ can be carried out from the calcination under air at high temperatures (>1200 °C) of precompacted (under 12 MPa) pellets of different mixtures: NiO/α-Fe₂O₃; NiO/α-Fe₂O₃/Fe; NiO/α-Fe₂O₃/Ni. The densest material is obtained at 1200 °C only from the following mixture: NiO (40 mol%), α-Fe₂O₃ (50 mol%) and Ni (10 mol%). Because the metallic nickel is very ductile, it is used as an internal binder in order to enhance the precompacting of the samples. Moreover, the role of nickel is to enhance the sintering reaction. This route leads to a final material of relative density close to 98±2%.     

Inhibition mechanism of S-containing additives towards intergranular corrosion of a sensitized stainless steel

The comparison of the polarization curves recorded on both pure Fe, Cr, Ni, and Fe-18Cr-8Ni, Fe-8Cr-8Ni alloys, and Fe-10Ni, Fe-17Cr alloys in 1 N H₂SO₄ at 70°C clarifies the mechanism of inhibition of IG corrosion on sensitized AISI 304 SS by S-containing additives. These additives stimulate the anodic dissolution process of Fe-18Cr-8Ni alloy and inhibit this process on Fe-8Cr-8Ni alloy. The anodic behaviour of Fe-18Cr-8Ni is similar to that of pure chromium, while the behaviour of Fe-8Cr-8Ni is similar to that of pure iron.     

Diffusion of cations in chromia layers grown on iron-base alloys

Diffusion of the cations Cr, Fe, Mn, and Ni in Cr₂O₃ has been investigated at 1173 K. The diffusion measurements were performed on chromia layers grown on the model alloys Fe-20Cr and Fe-20Cr-12Ni in order to consider effects of small amounts of dissolved alien cations in Cr₂O₃. The samples were diffusion annealed in H₂-H₂O at an oxygen partial pressure close to the Cr₂O₃/Cr equilibrium. For all tracers the lattice-diffusion coefficients are 3–5 orders of magnitude smaller than the grain-boundary diffusion coefficients. The lattice diffusivity of Mn is about two orders of magnitude greater than the other lattice-diffusion coefficients, especially in Cr₂O₃ grown on Fe-20Cr-12Ni. The values of the diffusion coefficients for Cr, Fe, and Ni are in the same range. Diffusion of the tracers in Cr₂O₃ grown on different alloys did not show significant differences with the exception of Mn.     

Oxidation of Ni films electroplated on steel

The high-temperature oxidation of Ni films electroplated on a steel substrate was studied at 700 and 800°C in air. During oxidation, outward diffusion of Ni and Fe ions and inward diffusion of oxygen ions occurred. NiO formed initially, and as the oxidation progressed, Fe₂O₃ and NiFe₂O₄ also formed. The oxidation resistance of Ni films was inferior to that of bulk Ni, which was attributed to the involvement of Fe during oxidation.     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

Electrochemical behaviour of high nitrogen stainless steel in acidic solutions

The electrochemical behaviour of high nitrogen stainless steel in acidic solutions was studied by potentiodynamic polarization, EIS, Mott-Schottky and XPS. The passive film formed in neutral NaCl solution was very stable, but the stability of the film decreased with the addition of H₂SO₄ into the solution. The passive film formed in acidic Na₂SO₄ has a superior protective ability than that in acidic NaCl solution. The stability of the film formed in tested solution decreased with increase of applied potentials. The film formed on steel surface was of n-type semiconductor. Chloride penetration mechanism was proposed for the observed passive film breakdown.