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.     

Potential dependence of surface crystal structure of iron passive films in borate buffer solution

The effect of passivation potential on surface crystal structure, apparent thickness and passivity of oxide films formed on pure iron prepared by plasma sputter deposition was investigated. The crystallinity was improved with passivation potential and the width of atomically flat terraces was expanded to 6 nm when passivating at 750 mV for 15 min, as observed by ex situ scanning tunneling microscopy (STM) after aging in air (<30% RH). Apparent thickness and passivity are linearly dependent on passivation potential. The former weakly depends on passivation duration, the latter strongly depends on passivation duration. This is well explained by the correlation between crystal structure and passivity.     

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.     

Effects of solution temperature on electronic properties of passive film formed on Fe in pH 8.5 borate buffer solution

Effects of solution temperature on the electronic properties of passive film formed on Fe in pH 8.5 borate buffer solution were investigated to clarify why the passive current density of Fe increases with solution temperature. The Mott-Schottky analysis and photocurrent measurement were employed to determine the electronic properties of the film. The Mott-Schottky analysis and photocurrent measurement revealed that as solution temperature increased, the concentration of oxygen vacancy in the passive film increased accompanying with an increase in the concentration of Fe²⁺ ion in the γ-Fe₂O₃ passive film due to a charge neutrality reaction in the film. Further, the increase in passive current density of Fe with solution temperature was found due primarily to the increase in the concentration of oxygen vacancy in the passive film, which is well explained by the point defect model (PDM).     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Photoelectrochemical analysis on the passive film formed on Fe-20Cr in pH 8.5 buffer solution

The electronic properties of passive film formed on Fe-20Cr stainless steel in pH 8.5 buffer solution were examined by the photocurrent measurements of the film. The passive film on Fe-20Cr exhibited an n-type semiconductor, which was confirmed by anodic photocurrent. The photocurrent spectrum for the passive films formed on Fe-20Cr was almost same in shape to that for the passive film on Fe except for the large difference in photocurrent intensity, which demonstrated that the passive film on Fe-20Cr is composed of Cr-substituted γ-Fe₂O₃ involving the d-d and p-d electron transitions. However, the large difference in photocurrent intensity for passive film between Fe and Fe-20Cr was due presumably to the fact that Cr³⁺ ions in passive film act as effective recombination sites of electron-hole pairs. The abrupt increase in the photocurrent intensity for the passive film formed on Fe-20Cr at a transpassive potential is due presumably to the decrease in the recombination site resulting from the reduction of Cr³⁺ content accompanying the oxidation of Cr³⁺ to Cr⁶⁺ in transpassive region.     

Oxide film growth on Al-In alloys in a borate buffer solution in conditions of galvanostatic anodising

Anodic oxide films have been formed galvanostatically on Al-In alloys (containing up to 0.074% In) in a borate buffer solution (pH 7.8) at different current densities (20-100 μA cm⁻²). The mechanism, kinetics of growth and properties of formed oxide films have been investigated. The study of charge curves suggests that the growth of oxide films on Al-In alloys occurs by an activation-controlled ionic conduction under the influence of the high electric field through the oxide film according to an exponential law, like on valve metals. The following parameters have been calculated: the constants of the exponential law, ionic conductivity through the film, the effective activation distance for ion movement and the corresponding field strength. The values for the field strength, of the order of magnitude of 10⁶ V cm⁻¹, justify the application of the high field migration mechanism. Properties of anodic oxide films have been determined by means of electrochemical impedance spectroscopy; the resistance and thickness of the oxide film have been found to increase with the increase in the indium content in the alloy and with increased anodic current density. It has been established that the current efficiency in oxide films formation on Al-In alloys is lower than 100%: the increase of the indium content in the alloy, as well as the increase in anodic current density, increases the value of current efficiency.     

Changes on iron electrode surface during hydrogen permeation in borate buffer solution

Hydrogen interaction with oxide films grown on iron electrodes at open circuit potential (E_oc) and in the passive region (+0.30 V_ECS) was studied by chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy techniques. The results were obtained in deaerated 0.3 mol L⁻¹ H₃BO₃ + 0.075 mol L⁻¹ Na₂B₄O₇ (BB, pH 8.4) solution before, during and after hydrogen permeation. The iron oxide film modification was also investigated by means of in situ X-ray absorption near-edge spectroscopy (XANES) and scanning electrochemical microscopy (SECM) before and during hydrogen permeation. The main conclusion was that the passive film is reduced during the hydrogen diffusion. The hydrogen permeation stabilizes the iron surface at a potential close to the thermodynamic water stability line where hydrogen evolution can occur. The stationary condition required for the determination of the permeation parameters cannot be easily attained on iron surface during hydrogen permeation. Moreover, additional attention must be paid when obtaining the transport parameters using the classical permeation cell.     

Corrosion and passivation of iron and its nitrided layer in borate buffer

The aim of this work was to gain better understanding of the reasons of enhanced resistance to pitting corrosion of nitrogen-containing iron. Gas nitrided (570 °C, 4 h) and untreated Armco iron were examined in a borate solution of pH 8.4 without and with chlorides or ammonia. Enhanced pitting resistance and enhanced anodic currents were exhibited by nitrided Fe and also by untreated Fe in the solution with added ammonia. XPS showed that anodic films on nitrided Fe contained much larger amounts of iron oxides, in particular of magnetite, than those on untreated Fe. It is suggested that the anodic behaviour of nitrided Fe is determined mainly by the effect of evolving ammonia on corrosion products. Increased anodic dissolution can be explained by the formation of soluble complexes with ammonia, whereas increased amounts of magnetite can be due to the ammonia-promoted conversion of FeOOH + Fe(II) to Fe₃O₄. It is proposed that the enhanced pitting resistance of nitrided Fe results mainly from the formation of large amounts of iron oxides and from binding of chloride anions into a Fe-NH₃-Cl complex.     

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.     

Surface enhanced Raman spectroelectrochemical studies of the corrosion films on iron and chromium in aqueous solution environments

“In situ” laser Raman spectra of the corrosion films on iron have been observed in aerated 5 M KOH and 0.15 M NaCl solutions via surface enhancement by the electrodeposition of a silver overlayer. Essentially the same spectra are observed in the two solutions as a function of applied potential in spite of a breakdown of passivity on iron in the chloride solution. Fe(OH)₂ and Fe₃O₄ are found in the prepassive potential region while FeOOH is present in the passive region. A film which is very difficult to reduce appears to be always present on the electrode surface even at hydrogen evolution potentials; this film is believed to be -FeOOH. Surface enhanced Raman spectra of the corrosion films on chromium have also been obtained in NaCl solution for the first time. The passive film has a composition that corresponds most closely to an amorphous form of Cr₂O₃, with some Cr(OH)₃ also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO²⁻₄. Reversible reduction of this species to Cr₂O₃ is indicated.     

Ozonation kinetics of iron and manganese in aqueous solution

Ozonation kinetics of iron and manganese in aqueous solution is experimentally investigated. The effects of pH, initial ion concentration and the ozone dosage on the kinetic rate parameters are examined. It has been found that ozonation of those ions in general follows zero order kinetics.     

Analysis of electrochemical parameters in time domain during the passive layer cracking occurring on the 304L stainless steel in chlorides solution under tensile stresses

Dynamic electrochemical impedance spectroscopy (DEIS) has been applied for detailed analysis of the passive–active transient region during the passive layer cracking process. The effect of applied potential and tensile stresses on the passive layer rupture of type 304L stainless steel (SS) immersed in 0.5 M NaCl solution at room temperature was examined. This paper presents instantaneous impedance spectra obtained for 304L stainless steel samples at the different potential values that refer to the transition from passive into active state while crack of the passive layer took place. Besides, differential dependencies of electrochemical parameters versus relative elongation have been presented to illustrate the system's dynamics changes.     

防氧化脱碳涂料对钢铁材料热处理的保护研究

分别研究了硅酸盐防氧化脱碳涂料对45#钢和40Cr钢热处理时的保护效果。1 100℃下保温时间对钢样氧化失重率的影响实验表明,随着保温时间的延长,无涂料保护的钢样的氧化失重率迅速增大,但有涂料保护的钢样其氧化失重率变化不大。钢样经1 100℃×2 h热处理后油淬的金相照片显示,45#钢的氧化脱碳程度比40Cr钢更严重,而有涂料保护的钢样其氧化脱碳作用明显减少。经过860℃×4 h与1 100℃×4 h热处理后淬火脱落的涂层其截面SEM形貌显示,涂层的内侧出现较厚的致密层,并呈明显的烧结状态,由能谱分析可知,它应为基体上的氧化膜或氧化膜与涂层成分的反应产物。初步探讨了涂料的防护原理,指出涂层不能避免钢样氧化,而轻微氧化形成的氧化膜能与涂层成分反应形成复杂的氧化物,从而达到较好的防护效果。     

铁盐水处理剂生产装置设计及运行过程若干问题

针对铁盐水处理剂生产装置及运行过程存在的问题,即反应器型式、换热降温系统、气体管路静电消除系统、气室压力测控系统,以及反应结束后氮氧化物的脱除与吸收系统等进行了工程化探索。结果表明,各批次的反应时间大约为2.5 h;质量分数为38%的三氯化铁生产过程温度未超过80 ℃;未发生过管路燃爆事故;产品在储运及使用过程中未出现“黄烟”逸出现象。研究使铁盐水处理剂制造行业生产过程更加安全高效。