Effects of temperature on the oxide film properties of 304 stainless steel in high temperature lithium borate buffer solution

The paper mainly investigated the effects of temperature on the oxide film properties of 304SS in lithium borate buffer solution by electrochemical measurements and XPS analysis. As temperature increased, the protective property of the film degraded and structure varied from a single layer to a double-layer. Whatever the temperature, the oxide film exhibited an n-type and p-type semiconductor in the potential range above and below the flat band potential, respectively. The electronic properties were assigned to a Fe–Cr spinel inner layer and a defective Fe–Cr oxide outer layer. The related growth mechanisms of the oxide film were also discussed.     

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.     

Corrosion behaviour of sputter-deposited Mg–Zr alloys in a borate buffer solution

Corrosion behaviour of sputter-deposited Mg–Zr alloys was examined in a borate buffer solution of pH 8.7. XRD measurements showed that the alloys were supersaturated with Zr. The addition of 29 at.% Zr or more was very effective in increasing the corrosion resistance of Mg by more than four orders of magnitude. XPS analysis of spontaneously formed passive films revealed that the passive film consisted of double oxyhydroxide composed of enriched tetravalent Zr and divalent Mg cations. The enrichment of Zr cations in the passive film is responsible for the enhanced corrosion resistance of Mg–Zr alloys.     

Electrochemical behaviour of the Mg alloy AZ91D in borate solutions

Corrosion and passivation behaviour of Mg-based alloy AZ91D was investigated in aqueous sodium borate solutions (pH 9.2) in relation to some test parameters, using electrochemical techniques. Increasing borate concentration (0.01–0.10 M) or temperature up to 298 K leads to increase the corrosion rate of the alloy. However, at temperatures higher than 298 K borate anions have stronger propensity to passivate the alloy, thereby decreases its corrosion rate. For a fixed borate concentration increasing Cl⁻ addition is correlated with a more negative corrosion potential and a higher corrosion rate, as well as increase the vulnerability of the anodic passive film for breakdown. The influence of oxidizing potentials over the range −1.5 V to 2.75 V (SCE) on the performance of the alloy in the most aggressive borate solution (0.10 M) reveals that higher potentials, induces better passivation due to formation of a rather thick and more protective n-type semiconducting film. A modified Randles circuit including Warburg impedance to account for the diffusion of reactants or products through the surface film was adopted to analyse the EIS data, that gave impedance parameters in good agreement with the results of open circuit potential and dc polarization measurements.     

Electrochemical and XPS studies of the passive film formed on stainless steels in borate buffer and chloride solutions

The passivity of AISI 304L and AISI 316L stainless steels in a borate buffer solution, with and without the addition of chloride ions, was studied using cyclic voltammetry and potentiodynamic measurements. The passive layers formed by electrochemical oxidation at different passivation potentials on both the stainless steels were studied by X-ray photoelectron spectroscopy, their compositions were analysed as a function of depth, and the cationic fraction of the passive film was determined. The passive films established on the two stainless steels in the borate buffer solution at pH = 9.3 contained the oxides of two main elements, i.e., Fe and Cr. The oxides of the alloying elements Ni and, optionally, Mo, also contribute to the passive layer. In the presence of chloride ions a strong chromium enrichment was observed in the passive layers.     

The aging of the anodic oxide of titanium during potentiostatic condition by ellipsometry

The aging effect of the anodic oxide film on titanium under constant potential oxidation was investigated by 3-parameter (3-P) ellipsometry in 0.1 mol dm⁻³ sulfuric acid solution. The oxide film was initially formed by a potential sweep oxidation, where the film with a low refractive index grew at relatively high rate. From the low value of the refractive index, the oxide film is assumed to be a hydrated structure. After the sweep oxidation to 4.26 V vs. reversible hydrogen electrode, the constant potential oxidation was performed at a potential of 4.26 V to examine the aging effect of the hydrated oxide film by 3-P ellipsometry. It was found that the refractive index of the oxide film increased from 2.39 to 2.59 during the aging of the constant potential oxidation, and the thickness decreased. The increase of the refractive index and the decrease of thickness may be explained by conversion from the hydrated oxide to a dehydrated oxide during the aging.     

Prevention of passive film breakdown on iron in a borate buffer solution containing chloride ion by coverage with a self-assembled monolayer of hexadecanoate ion

Breakdown of a passive film on iron in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻ was suppressed by coverage of the passive film surface with a self-assembled monolayer (SAM) of hexadecanoate ion C₁₅H₃₁CO₂⁻ (C₁₆A⁻). The pitting potential of an iron electrode previously passivated in the borate buffer at 0.50 V/SCE increased by treatment in an aqueous solution of sodium hexadecanoate for many hours, indicating protection of the passive film from breakdown caused by an attack on defects of the film with Cl⁻. No breakdown occurred over the potential range of the passive region by coverage with the SAM of C₁₆A⁻ in some cases. Structures of the passive film and the monolayer were characterized by X-ray photoelectron and Fourier transform infrared reflection spectroscopies and contact angle measurement with a drop of water.     

Effect of oxide film on oxygen reduction current for the platinum-cobalt alloy electrodes in PEFC

Effect of surface oxide on Pt-Co alloy electrodes on the oxygen reduction reaction (ORR) was investigated in 0.5 M sulfuric acid solution by electrochemistry, ellipsometry, laser Raman scattering spectroscopy, and XPS. The oxide as thick as 1-2 nm increases the overpotential of ORR and falls down efficiency of PEFC. The thickness of the oxide films is precisely determined by ellipsometry. The oxide film 1.9 nm thick was formed on Pt-50 mol% Co electrode by constant potential oxidation at 1.20 V and the film 1.5 nm thick remains on the electrode at 0.6 V at which ORR already starts. The remaining oxide decreases the current density of ORR and increases the overpotential. On pure Pt electrode, the similar influence of the oxide film was observed.     

氧含量对烧结钕铁硼磁体Dy晶界扩散的影响研究

本文主要研究了烧结钕铁硼磁体中氧含量变化对Dy晶界扩散后的Dy含量及矫顽力增加量的影响。选取多种高、低氧磁体进行Dy扩散处理后比较发现,低氧磁体的Dy扩散量和矫顽力提高量均明显高于高氧磁体。对9个0wt.%Dy的不同氧含量样品进行扩散再次确认,氧含量减少有利于Dy扩散量、矫顽力的提高。各样品成分梯度结果显示,低氧磁体的Dy扩散量由表及里全面高于高氧磁体,内外浓度梯度也小于后者。电子探针表征结果表明,低氧磁体Dy扩散后晶界处Dy富集条纹更明显、连续,完整包裹各个主相晶粒。这种结构优化也使低氧磁体各向异性场提高幅度大于高氧磁体。磁体中氧含量降低使富钕相在主相周边均匀连续分布,为后续进入磁体内部的Dy元素提供连续的扩散通道,从而使磁体的Dy扩散量和矫顽力提高量进一步提高。     

Passivity breakdown on AISI Type 403 stainless steel in chloride-containing borate buffer solution

Passivity breakdown on AISI Type 403 stainless steel (SS), a commonly employed blade alloy in low pressure steam turbines, has been studied and the data are interpreted in terms of the point defect model (PDM). The near normal distribution in breakdown potential measured in deaerated borate buffer solution (pH = 8.1 ± 0.1) with different chloride concentrations is in satisfactory agreement with the quantitative characterization of the breakdown potential distribution using the PDM. The linear dependence of breakdown potential on the square root of potential scan rate (υ^1/2), as predicted by the PDM, yields an estimate of the critical areal concentration of condensed vacancies at the metal/film interface (ξ < 7.0 × 10¹⁴ cm⁻²) that leads to passivity breakdown. This is in excellent agreement with that calculated from the unit cell dimensions of the substrate Fe-Cr alloy (ξ ≈ 10¹⁵ cm⁻²) and the barrier layer oxide (Cr₂O₃) (ξ ≈ 10¹⁴ cm⁻²) for vacancy condensation on the alloy lattice or on the cation sublattice, respectively, of the film. These provide convincing evidences for the validity of the PDM for modeling passivity breakdown on Type 403 SS.     

Grain boundary microstructure in DyF3-diffusion processed Nd-Fe-B sintered magnets

The effect of post-sinter tempering on the DyF₃-diffusion processed Nd-Fe-B was investigated using two kinds of starting magnets. The increase of coercivity after diffusion process using as-sintered magnet was higher than that using two-stage tempered magnet. The grain boundary phase of the tempered magnet became discontinuous upon further annealing at the temperature of diffusion process. This clearly indicates that a continuous grain boundary phase is helpful to the DyF₃-diffusion process. When sufficiently diffused, there is no enrichment of Dy in the grain boundary phase. The excess Nd as a result of Dy substitution in the Nd₂Fe₁₄B matrix phase forms Nd-O phase at grain boundary and on the surface of the magnet. The increase of coercivity can be related to the (Nd,Dy)₂Fe₁₄B grains as well as to the improved decoupling by the grain boundary phase.     

Effect of magnetization state on the corrosion behaviour of NdFeB permanent magnets

The corrosion behaviour of uncoated NdFeB permanent magnets in the non-magnetized and magnetized state was comparatively investigated in H₂SO₄ solutions by potentiodynamic and potentiostatic polarization experiments. Depth profiles of the corroded surfaces were recorded and an effect of magnetization on the localization of the corrosion attack was identified. The anodic behaviour is discussed on the basis of previously reported results on the corrosion of neodymium and iron and on basis of the magnetic forces acting on the electrochemical system.     

The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of (PMo₁₂) and anions for stabilization of the passive oxide film at the metal-polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo₁₂. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy-PMo₁₂/PPy-DHNDS < PPy-PMo₁₂/PPy-NDS < PPy-PMo₁₂/AnqDS < PPy-PMo₁₂/PPy-DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.     

Direct modification of a passivated iron electrode with alkyltriethoxysilanes for preventing passive film breakdown in a borate buffer containing 0.1 M of chloride ion

A passive film on an iron electrode was modified with alkyltriethoxysilanes directly. In order to examine the protective ability of the modified passive film against breakdown, the pitting potential, E_pit was measured by anodic polarization of the modified electrode in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻. The value of E_pit for the modified electrode shifted in the positive direction from that of the unmodified electrode, indicating prevention of passive film breakdown. The modified passive film was not broken down in the passive and transpassive regions of polarization curve in some cases. However, many current spikes appeared in the all curves of the modified electrodes. The modified surface of passivated electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement. There were defects and clusters of associated water within the modified film and hence, Cl⁻ could permeate through the defects, leading to appearance of current spikes and occurrence of breakdown.     

Passivity of 316L stainless steel in borate buffer solution studied by Mott-Schottky analysis, atomic absorption spectrometry and X-ray photoelectron spectroscopy

The passivity of 316L stainless steel in borate buffer solution has been investigated by Mott-Schottky, atomic absorption spectrometry (AAS) and X-ray photoelectron spectroscopy (XPS). The results indicate that the polarization curve in the passive region possesses several turning potentials (0 V_SCE, 0.2 V_SCE, 0.4 V_SCE, 0.6 V_SCE and 0.85 V_SCE). The passive films formed at turning potentials perform different electrochemical and semiconductor properties. Further, the compositions of the passive films formed at turning potentials are investigated. The results reasonably explain why these potentials appear in the passive region and why specimens perform different properties at turning potentials.     

放电等离子烧结-热变形各向异性Nd-Fe-B磁体的微观组织及磁性能

采用放电等离子烧结及后续热变形技术制备各向异性Nd-Fe-B磁体,研究烧结温度对放电等离子烧结Nd-Fe-B磁体微观组织和磁性能的影响。随着烧结温度在650~900°C范围内的升高,烧结态Nd-Fe-B磁体的剩磁、内禀矫顽力及最大磁能积呈现先升后降的趋势。在800°C下烧结所获得磁体的磁性能最佳。随后,对800°C烧结后具有最佳磁性能的磁体采用放电等离子烧结技术进行后续热变形处理。与初始吸氢-歧化-脱氢-再复合粉末和烧结态磁体相比,热变形磁体拥有更显著的各向异性和更好的磁性能。当热变形温度为800°C且压缩比为50%时,热变形磁体中的Nd2Fe14B晶粒呈扁平片状且不发生异常长大;磁体沿热压方向具有最佳的磁性能：Br、Hcj和（BH）max分别为1.16 T、449 k A/m和178 k J/m3。     

Oxide film formation on zinc in borate solutions under open circuit

The open circuit potentials of Zn electrode were followed as a function of time in different concentration of Na₂B₄O₇ solution until steady-state, E_st., values were attained. The potential shifts immediately towards positive values, indicating film thickening and repair. The rate of oxide film thickening was determined from the linear relationship between the open circuit potential, E, of the Zn electrode and the logarithm of immersion time t as E = a₁ + b₁ log t. The liner plots consist of two segments indicating the duplex nature of the formed oxide film on the Zn surface. The final steady- state potential, E_st., varied with the logarithm of molar concentration of Na₂B₄O₇ solution according to: E_st. = a₂ − b₂ log C_Na2B4O7. The effect of rising pH and temperature was also studied. It was found that the rising of pH and temperature of the solution affect on the rate of oxide film thickening and the final steady- state potential.     

The remarkable passivity of austenitic stainless steel in sulphuric acid solution and the effect of repetitive temperature cycling

The passivation of 316L austenitic stainless steel in sulphuric acid solution has been measured at a single temperature, and then further, after a series of temperature sweeps. After 15 h at 20 °C and constant electrode potential, the passive current density has fallen to 16 nA cm⁻² and still continues to decline, with no evidence of an approach to the steady state. After subjecting the metal to a series of temperature sweeps the passive current density has decayed much further to ca. 0.5 nA cm⁻² and probably still decaying slowly. The metal is thereby rendered extremely passive. It is argued that the passive current density as measured by conventional potential sweep voltammetry is not in fact the steady state current density, because the true approach to a steady state is far too slow.     

Influence of pH and flow on the polarisation behaviour of pure magnesium in borate buffer solutions

The influence of pH and flow on the polarisation and impedance behaviour of pure Mg was investigated in 0.6 wt% NaCl borate buffer solutions at pH 6.7, 7.6, and 9.3 using a rotating electrode. The existence of flow prevented the accumulation of the corrosion product and promoted uniform corrosion, leading to an increase of the anodic current density and a decrease of the impedance regardless of the pH of the bulk solution. The influence of the pH did not obviously appear under the static condition, whereas, under the dynamic condition, the anodic current density at pH 6.7 and 7.6 was higher than that at pH 9.3. This result suggests that pH near the surface can be buffered at the pH of bulk solution by borate buffer with the acceleration of ion diffusion.     

The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water

Oxide films formed on Alloy 690 exposed to 290 °C water containing 3 ppm O₂ were investigated. It was found that Cr rich oxides form initially through solid-state reactions. Ni–Fe spinels gradually develop on surface layer by precipitation with increasing immersion time. Initially formed Cr rich oxides react with outwards diffusing Ni and Fe to form small spinel particles which then vanish gradually. An inner layer develops from oxide/matrix interface through inward diffusion of oxidant. Cr is preferentially oxidized and tends to dissolve into solution. The resultant inner layer consists of predominant NiO which cannot serve as a protective barrier layer.