Ex-situ and in-situ X-ray diffractions of corrosion products freshly formed on the surface of an iron-silicon alloy

Ex-situ X-ray diffraction measurements of a small amount of samples extracted from wet corrosion products freshly formed on a pure iron and iron-2 mass% silicon surfaces have been conducted using synchrotron radiation for clarifying the formation process of corrosion products. The results showed that γ-FeOOH was formed on the outer side of wet corrosion products formed on the surface of the pure iron by sodium chloride solution, while γ-FeOOH, α-FeOOH, Fe₃O₄, and green rusts were formed on the inner side. On the other hand, in comparison to the case of the pure iron, a significant formation of β-FeOOH was observed in the iron-silicon alloy. Influences of silicon alloying on corrosion products formed by aqueous solution containing sulfate ions were also observed. Furthermore, in-situ diffraction measurements by a conventional X-ray source were conducted for analyzing corrosion products formed on the pure iron and iron-silicon alloy surfaces by cyclic exposure to wet and dry atmospheres. The results obtained by the in-situ diffraction and ex-situ diffraction measurements on the corrosion products were consistent.     

低碳钢在模拟酸雨大气条件下的锈蚀演化

利用干湿循环加速腐蚀实验和电化学极化曲线法研究了低碳钢在模拟工业酸雨大气条件下的锈蚀演化过程.结果表明:实验初期锈蚀速度随干湿循环次数的增加而增大,随后转为随干湿循环次数的增加而降低;带锈低碳钢的干湿循环下的腐蚀产物促进阴极过程抑制阳极过程;在干湿循环加速腐蚀进程中低碳钢表面铁锈的化学组成、结构变化表现为在锈蚀初期α-FeOOH含量较低,锈层疏松,锈蚀速度随干湿循环次数增加呈上升趋势;后期随α-FeOOH含量的增加和锈层变得更加致密,腐蚀速度转变为随干湿循环次数增加而下降.     

The effect of ion implantation on the corrosion behaviour of pure iron-III. Tantalum ion implantation

Pure iron specimens were implanted with doses of 5 × 10¹⁴ and 2 × 10¹⁵ tantalum ions/mm². Using a three sweep potentiokinetic polarization technique the corrosion behaviour of these surface alloy layers was compared with that of pure iron and of a conventional Fe-4.9 wt% Cr alloy. It was found that tantalum implantation improved the corrosion resistance of pure iron and in particular reduced both the critical current density for passivation and the passive current density. The corrosion resistance of tantalum implanted iron was similar to that of the conventional Fe-4.9 wt% Cr alloy. The beneficial effects of tantalum implantation were unexpectedly persistent.     

Versuche zum Verständnis des Ablaufs der Atmosphärischen Korrosion des Eisens

Investigations towards understanding of the atmospheric corrosion processes of pure iron In an investigation of the atmospheric corrosion of pure iron the metal loss of a foil was determined by magnetic measurements and the oxygen consumption gasvolumetrically. The results showed that in the period immediately following short wetting iron dissolution takes place with reduction of the rust layer. The corrosion current density thereby reaches a value of 500 m?A/cm² temporarily falling rapidly with time. Apart from this the corrosion under conditions of critical humidity of the metal surface during the drying-out of the corrosion system plays a dominant role. The corrosion current density reaches here values up to 1 mA/cm².     

工业纯Sn和Sn-3Ag-0.5Cu合金在沈阳大气环境下自然暴露的初期腐蚀行为

通过沈阳大气环境下的自然暴露实验研究工业纯Sn和Sn-3Ag-0.5Cu合金的早期大气腐蚀行为。采用扫描电子显微镜(SEM)和X射线光电子能谱仪(XPS)分析了表面腐蚀产物的形貌与成分。结果表明:工业纯Sn与Sn-3Ag-0.5Cu合金在沈阳工业大气环境下自然暴露后都很快发生了表面腐蚀并失泽,早期腐蚀产物疏松、龟裂并易于剥落。XPS深度分析表明:自然条件下暴露18 d后,表面腐蚀产物层厚度约为400 nm。工业大气中的悬浮物颗粒对腐蚀的形核和扩展起重要作用,Sn-3Ag-0.5Cu合金中的第二相Ag3Sn和Cu6Sn5作为阴极存在,但对大气腐蚀的加速影响不大。     

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.     

Simulation study on function mechanism of some precipitates in localized corrosion of Al alloys

The function mechanism of different types of aging precipitates in localized corrosion of Al alloys was studied. The function mechanism of the precipitates of θ (Al₂Cu) and η (MgZn₂) is validated. The precipitate of θ containing noble element Cu is cathodic to the alloy base, resulting in the anodic dissolution and corrosion of the alloy base at its adjacent periphery. The precipitate of η containing active element Mg is anodic to the alloy base, anodic dissolution and corrosion occur on its surface. Meanwhile, a localized corrosion mechanism conversion associated with the precipitate of T1 (Al₂CuLi) is advanced, which contains noble element Cu and active element Li simultaneously. The precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface at the beginning. However, during its corrosion process, the preferential dissolution of Li and the enrichment of noble element Cu make its potential move to a positive direction. As a result, the corroded T1 precipitate becomes cathodic to the alloy base at a later stage, leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery.     

The Influence of KCl and HCl on the High-Temperature Oxidation of a Fe-2.25Cr-1Mo Steel at 400 °C

The influence of alkali- and chlorine-containing compounds on the corrosion of superheater alloys has been studied extensively. The current paper instead investigates the corrosive effects of KCl and HCl under conditions relevant to waterwall conditions. A low-alloy (Fe-2.25Cr-1Mo) steel was exposed to KCl(s), 500 vppm HCl(g) and (KCl?+?HCl) in the presence of 5%O₂ and 20% H₂O at 400 °C. The results indicate that alloy chlorination by KCl occurs by an electrochemical process, involving cathodic formation of chemisorbed KOH on the scale surface and anodic formation of solid FeCl₂ at the bottom of the scale. The process is accompanied by extensive cracking and delamination of the iron oxide scale, resulting in a complex, convoluted scale morphology. Adding 500 vppm HCl to the experimental environment (KCl?+?HCl) initially greatly accelerated the formation of FeCl₂ at the scale/alloy interface. The accelerated alloy chlorination is attributed to HCl reacting with KOH at the scale surface, causing the cathodic process to be depolarized. A rapid slowing down of the rate of chlorination and corrosion in KCl?+?HCl environment was observed which was attributed to the electronically insulating nature of the FeCl₂ layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions.

     

Electrochemical and surface analytical studies on hydrogen permeation with Fe-Cu alloys in sulfuric acid with and without H2S

Cathodic and anodic polarization curves and hydrogen permeation have been determined for Fe, Fe-0.5Cu and Fe-3.4Cu in sulfuric acid. During cathodic polarization at potentials more noble than -400 mV (NHE) a black layer consisting of metallic Cu and Fe-oxides or -hydroxides is formed on the surface of the alloys by preferential dissolution of Fe. The black layer acts as a barrier for hydrogen diffusion and a catalyst for H₂ evolution, the layer reduces the permeation rate by a factor of two compared to pure Fe, in the presence of H₂S the reduction is by a factor of six. Addition of H₂S and/or an increase in pH can shift the cathodic polarization curve to more negative potentials where the scale is not stable. Under such conditions the permeation rate is faster on the alloys than on Fe.     

A peculiar cathodic process during iron and steel corrosion in sulfate reducing bacteria (SRB) media

Features related to the cathodic reduction of iron sulfides precipitation on iron surface during its exposure to SRB culture were studied. Electrochemical measurements were performed with pure iron and platinum electrodes plated with a thin iron film in de-aerated SRB culture. The study reveals that iron sulfide precipitation is being cathodically reduced just below a potential of −0.1 V_SCE, and if iron corrosion process occurs at potentials below that threshold potential, then the reduction of iron sulfide may provide an alternative cathodic depolarization mechanism in SRB. This cathodic process can maintain iron and corrosion at potentials above RHE potential.     

Effect of phosphine on acid corrosion of iron

The effect of the phosphine-concentration (10^?8–10^?3 M) in strong acid solutions (pH = 0) on the corrosion rate of pure iron and Fe- 0.12% P alloy has been investigated. It was shown that at low concentrations (10^?7–10^?6M), phosphine accelerates the corrosion of iron, whereas at adequately high concentrations (10^?5–10^?3 M) it has inhibitory properties. In the presence of PH₃ the cathodic reduction of hydrogen ions is strongly accelerated. At the same time, the increase in PH₃ concentration causes an increase of surface coverage by PH₃ molecules, which results in the inhibition of both partial electrode processes.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

The electrochemical reduction of oxygen on copper in dilute sulphuric acid solutions

The electrolytic reduction of O₂ on Cu was studied in dilute H₂SO₄ at 25°C by steady-state and transient electrochemical techniques. A distinctive hump (i.e. a current maximum) is observed in the steady-state potentiostatic polarization curve approximately 50 mV negative to the open-circuit corrosion potential. Results of transient studies, of varying the solution agitation, and varying the concentrations of oxygen, sulphuric acid, and sulphate ions indicate that the hump is due to cathodic film formation which inhibits the rate of oxygen reduction. The inhibiting film appears to be composed of intermediates from the O₂ reduction process and also may contain adsorbed sulphate ions. The film is removed under corrosion conditions by rapid dissolution of the substrate copper. Extrapolation of the cathodic Tafel curve to the corrosion potential yields corrosion rates lower than those measured because of the cathodic passivation.     

Hydrogen entry into steel during atmospheric corrosion process

Hydrogen entry and permeation into iron were measured by an electrochemical method during atmospheric corrosion reaction. The hydrogen permeation was enhanced on passive films because the hydrogen adsorption increased by the hydrogen evolution mechanism which is different from that on a bear iron surface. The permeation rate during a wet and dry corrosion cycle showed a maximum in the drying process depending upon the surface pH and the corrosion potential. The pollutant such as Na₂SO₃ which decreases the pH and the corrosion potential causes an increase in the permeation rate. The mechanism of the change in the permeation rate during the wet and dry cycles is explained by the polarization diagram of the electrode covered by thin water layer.     

The electrochemical behaviour of leaded brass in neutral CL and SO4 media

The electrochemical behaviour of lead brass with different leaded content in neutral chloride and sulphate solutions was investigated using the EIS technique. For comparison, the behaviour of the pure components of the alloy was investigated under the same conditions. The corrosion process was found to proceed via oxygen reduction following a diffusion controlled mechanism. The Cu electrode showed a higher polarization resistance due to film formation during oxygen reduction. Zn and Pb showed markedly lower impedance values due to continuous dissolution. The two investigated brass alloys (1.8% and 3.5% Pb, respectively) showed higher impedance values indicating the passivation of the surface in the Cl⁻ or SO₄⁻ media. Brass II was found to be more stable against corrosion indicating the beneficial effect of the lead content in the alloy.At cathodic potentials, the only process is the oxygen reduction. Anodic polarization leads to selective dissolution of Zn. At more positive potentials simultaneous dissolution of the alloy components with the deposition of Cu(I) salt takes place leading to the passivation of the alloy surface. At higher potentials, film breakdown occurs producing Cu(II) compounds whose diffusion control the corrosion process. At potentials higher than − 0.1 V, pitting corrosion was observed and a transmission line type in the impedance spectra was recorded.     

In situ studies of the corrosion during drying of confined zinc surfaces

The corrosion process during the drying out of zinc surfaces confined in crevices was studied using real time photograpy and in situ FTIR microspectroscopy. A pH‐indicator was used to visualise differences in the pH during the drying process. The distribution and the composition of the corrosion products after several wetting and drying cycles were studied with FTIR microspectroscopy and SEM‐EDS. An area with high pH formed during the drying process at the border of the electrolyte, with a zone of white corrosion products that contained zinc hydroxycarbonate in the electrolyte inside this area. A differential aeration cell is present at the border of the electrolyte, and the cathodic oxygen reduction reaction takes place close to the border of the electrolyte during the drying process. The corrosion attack and the distribution and composition of the corrosion products on the surface depend strongly on the drying process of the surface. The corrosion attack of confined surfaces was localised, with a significantly higher corrosion attack in some areas. Outside the drying front a thin layer of electrolyte formed as a result of surface tension driven flow of electrolyte from the electrolyte border. This effect was attributed to the alkaline pH of the electrolyte due to the oxygen reduction reaction at the border. A galvanic element was formed between the local cathodes in the area outside the drying front and the anode in the area with bulk electrolyte. The main corrosion products detected after several wet dry cycles were ZnO, Zn₅(OH)₆(CO₃)₂ and Zn₅(OH)₈Cl₂ · H₂O, but Na₂CO₃ · 10H₂O was also detected. The corrosion products were non‐homogeneously distributed on the surface and the distribution was related to the anodic and cathodic processes that took place in different regions on the surface during the corrosion process.     

The influence of ion implantation on the corrosion behaviour of iron in acid solution

The influence of ion implantation on the aqueous corrosion of pure iron in IN H₂SO₄ was studied. The iron was bombarded with 5 × 10¹⁵–10¹⁷ ions.cm^?2 of Ne, Ar, Cu, Pb and Au. The current density-potential curves of the implanted samples were measured and compared with that of untreated pure iron. Ne⁺ and Cu⁺ bombardments lead to a slightly higher corrosion rate in comparison with untreated iron. Pb⁺ depressed the corrosion rate by orders of magnitude, Au⁺ enhanced it by a factor of more than ten. The effect is attributed to a reduction or an increase of the activity of the electrode surface with respect to the cathodic hydrogen evolution reaction, i.e. the ion implantation influences strongly the exchange current density of the hydrogen evolution reaction. A marked influence of the implantation on the anodic behaviour of the corroding metal could also be observed.     

低氧压预氧化对FeCrAlSiY合金耐LBE熔液腐蚀性能的影响

为进一步提高核反应堆系统的结构材料 Fe-Cr 基合金抗 LBE 腐蚀的性能,对其进行成分优化和表面处理十分重要。 对 Fe-12Cr-xAl-2Si-0.6Y 合金进行低氧压预氧化试验,分析并讨论氧分压和 Al 含量对合金预氧化产物的影响,并研究预氧化前后的合金在 LBE 合金熔液中的耐腐蚀性能。结果表明,Fe-12Cr-2Al-2Si-0.6Y 合金在不同氧分压下进行预氧化后生成的表面氧化物均以 Al2O3 为主,氧分压越大,合金表面发生选择性氧化速度越快。不同 Al 含量的 Fe-12Cr-xAl-2Si-0.6Y 合金在 10^?15 atm(1 atm=0.101 3 MPa)氧分压下进行预氧化,随着 Al 含量的增加,合金表面氧化物由以 Cr₂O₃为主逐渐变为以 Al₂O₃ 为主,还混合有少量复合氧化物。进行低氧压预氧化处理能有效提高合金在 450 o C 的 LBE 合金熔液中的耐腐蚀性能。研究成果可为设计和开发耐 LBE 熔液腐蚀的 FeCrAlSiY 合金提供参考,促进该体系合金在核电工业领域的运用。     

Comparative investigation on copper atmospheric corrosion by electrochemical impedance and electrical resistance sensors

Electrochemical impedance (EIS) and thin electrical resistance (ER) sensors were invented for atmospheric corrosion measurement of copper (Cu) during cyclic wetting–drying/high–low temperature tests and field exposure tests. Three-month field exposure results showed that average corrosion rate of Cu measured by ER sensor was well in accordance with that by weight loss method. During cyclic wetting–drying test, EIS was proven to reflect sensitively time of wetting and drying on the surface of sensor. Although corrosion rate obtained from EIS had a similar tendency to that obtained from ER sensors, the former was more dependent on environmental humidity than the latter. When relative humidity was low than 60%, corrosion rate of Cu measured by EIS was much lower than that by weight loss method, mainly attributing to the fact that impedance sensor failed to detect corrosion current of interlaced Cu electrodes due to the breakdown of conductive passage composed of absorbed thin liquid film under low humidity condition. Promisingly, ER sensor was proven to be more suitable for atmospheric corrosion monitoring than electrochemical techniques because it could sensitively monitor thickness loss of Cu foil according to the Ohmic law, no matter how dry or wet the sensor surface is.     

Influence of a prefilming treatment on the corrosion resistance of aluminium brass in aqueous solutions

The influence of a short-time prefilming treatment with benzimidazole-2-thiol on the corrosion resistance of Cu Zn 22Al 2 has been examined. The efficiency of the prefilming treatment has been evaluated by recording the anodic and the cathodic polarization curves in 0.1 N NaCl. Ellipsometric measurements have been carried out in order to obtain some information on the properties of the layer. Comparative data on “prefilmed” and “bare” Cu Zn 22Al2 specimens have been obtained in aqueous chloride solutions circulating in a corrosion loop and by tests in moist atmosphere containing SO₂. Galvanic corrosion testing have been carried out by using small not prefilmed areas and large prefilmed ones, simulating in this way the partial removal of the film from the metallic surface. The results suggest that a metal-inhibitor film forms on Al-brass, which mainly affects the cathodic reaction rate. Furthermore, the tests clearly indicate that the effect of benzimidazole-2-thiol is strongly influenced by a pre-existing oxide layer on the alloy surface. A good protection against atmospheric corrosion is reached by prefilming treatment. Corrosion loop testing indicate that prefilming does not hinder the formation of protective corrosion products on the surface of the alloy in flowing aqueous solutions.