Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.     

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。     

High-temperature corrosion behaviors of typical nickel alloy coatings in a simulated boiler coal ash/gas environment in the Zhundong region

The high-temperature corrosion behaviors of five nickel alloy coatings used in coal-fired boilers in the Zhundong region (Xinjiang province) were investigated in simulated coal ash and coal-combusted flue gas environment at 650°C for 250 and 500 hr. The samples were analyzed by weight gain experiment, X-ray diffraction test, and scanning electron microscopy technique with energy-dispersive spectroscopy. The results indicated that the corrosion level is in the order of NiCrMo13 ≈ Hastelloy C-276 (276) > NiCrBSi > Inconel 718 (718) > 45CT. The compositions of the corrosion scale in five nickel alloy coatings mainly consist of NiO, Ni₃S₂, and Cr₂O₃. The enrichment of Cr in the corrosion scale in 45CT, 718, and NiCrBSi coatings inhibits the formation of oxide and sulfide on the coating surface. The presence of W and Mo in nickel alloy coatings accelerates the formation of corrosion products, thus weakening the corrosion resistance of NiCrMo13 and 276 in simulated coal ash and coal-combusted flue gas environment.     

Das Korrosionsverhalten von sauerstoffionenimplantiertem Magnesium in schwach saurem Elektrolyten

The corrosion behaviour of oxygen implanted magnesium in weekly acid solution Under atmospheric conditions magnesium forms oxide layers which consist mainly of magnesium hydroxide. The protective action of Mg(OH)₂ is minor in sulphate‐, carbonate‐ or chloride containing solutions [2, 18]. Additionally, the crystallographic misfit between magnesium and magnesium hydroxide leads to crack formations inside the hydroxide layer and the metallic surface is then exposed. By using the ion implantation of oxygen (1*10¹⁸ O⁺/cm²) a MgO‐layer can be formed and buried within the bulk metal. Pre‐tests of MgO‐layers produced by ion implantation suggest that it is successful as corrosion protection [17]. Crystallographic stresses between metal‐ and oxide phase will be reduced, depending on the gradient of oxygen concentration. The investigation in weekly acid sodium solution shows, that the oxygen ion implantation leads to a significant improvement in the corrosion behaviour of magnesium.     

The corrosion of two NiNb alloys under 1 atm O2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O₂ at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb₂O₆. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both alloy phases Ni₈Nb and Ni₃Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.     

Effect of a magnetron sputtered (Al2O3-Y2O3)/(Pt-Au) laminated coating on hot corrosion resistance of 8Nb-TiAl alloy

Novel (Al₂O₃-Y₂O₃)/(Pt-Au) laminated coatings were prepared on 8Nb-TiAl alloy by magnetron sputtering methods. (Na₂SO₄-NaCl)-induced hot corrosion and cyclic oxidation tests were adopted to investigate the high-temperature corrosion behaviours and the mechanical properties of the as-prepared laminated coatings. The results revealed that the multi-sealed (Al₂O₃-Y₂O₃) and (Pt-Au) layers can effectively suppress the inward diffusion of oxygen and corrosive fused salt to an extremely low level at 1000 °C and 900 °C respectively. Consequently, the high-temperature corrosion resistance of the 8Nb-TiAl alloy can be significantly improved. In addition, cyclic tests revealed the (Al₂O₃-Y₂O₃)/(Pt-Au) laminated coating can exhibit enhanced spallation and fracture resistance, which are attributable to the brittle/ductile laminated composite structure by means of energy release mechanisms and the increased thermal expansion coefficient of the coating by the addition of Pt-Au alloy.     

Models describing the degradation of FeAl and NiAl alloys induced by ZnCl2-KCl melt at 400-450 °C

In this study, a mechanism to account for the corrosion of Fe-Al and NiAl alloys beneath molten ZnCl₂-KCl at 400-450 °C in air is described. All the examined materials experienced enhanced corrosion, with the formation of porous surface scales or, in some cases, in combination with local attack of the matrix. Comparatively, the corrosion resistance of Fe-Al and NiAl alloys is markedly improved with increased Al contents, whereas different microstructural evolutions are observed for the two systems. For Fe-Al alloys, the outermost layer of corrosion product is a mixed region composed of a large amount of Fe₂O₃, KCl and metallic zinc particles, overlying a mixture of aluminium oxide and KCl that acts as the intermediate layer. Moreover, a multi-layered Fe₂O₃ scale is produced on the surface of the matrix. For Ni-Al alloys, however, the outermost layer is mainly a mixture of KCl, metallic Zn, and some alumina particles in local regions, but no nickel oxide is detected. Beneath this outer layer is an Al-rich oxide layer combined with KCl impurity, and then a metallic nickel bulk containing aluminium oxide precipitation, in contact with the matrix.     

Corrosion and sulphate ion reduction studies on Ni and Pt surfaces in with and without V2O5 in (Li,Na,K)2SO4 melt

In the present investigation corrosion and sulphate ion reduction on the nickel and platinum surfaces, respectively have been studied in (Li,Na,K)₂SO₄ melt in the presence and absence of V₂O₅ at 550 °C. The corrosion or oxidation rate of nickel was derived by Tafel extrapolation method while cyclic voltammetry measurement was employed for the determination of electrochemical behaviour of sulphate ion. According to result obtained, corrosion of nickel increases very fast after the addition of V₂O₅ and becomes more than two orders of magnitude higher in the presence of 3% V₂O₅ in the melt. Three well defined cathodic peaks related to vanadate ion reduction, were clearly observed in the cyclovoltammogram obtained on the Pt surfaces in presence of V₂O₅ in the melt. Observation of three reduction peaks could be due to subsequent reduction of vanadate ion to its lower oxidation state. From the results, a conclusion has been drawn that a strong ability of vanadium to reduce in its lower oxidation state by consuming electrons released by sulphate ion oxidation, is mainly responsible for enhancement of corrosion or oxidation of the nickel in the sulphates melt.     

The corrosion behavior of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 alloy

The potentiodynamic polarization curves in 0.5 M NaCl solution before and after crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy have been studied in relation to the microstructure and alloy composition. It was shown that the corrosion resistance of the alloy strongly depending on these two factors. The observed decrease in corrosion resistance of the alloy after the heat treatment up to 480 °C in comparison to the corrosion resistance of the alloy in the as prepared state is attributed to the increased inhomogeneity of the alloy that coincides with the first appearance of Fe₃Si phase. Further heating (up to 600 °C) resulted in an increase in the number of Fe₃Si nanocrystallites and the appearance of a FeCu₄ phase. After annealing at 600 °C the lowest corrosion rate, 0.004 mm a⁻¹, was observed. Annealing of the samples at higher temperatures (>600 °C) induced formation of six crystalline phases which proved detrimental to the corrosion resistance of the Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy. Solid corrosion products were identified on the surface of the samples after anodic polarization.     

铝基表面等离子喷涂Al2O3-TiO2-Ta涂层的制备及防腐性能

采用等离子喷涂技术在铝基表面构建Al₂O₃-TiO₂涂层和Al₂O₃-TiO₂-Ta涂层。由于钽元素的引入,Al₂O₃-TiO₂-Ta涂层表面形貌更均匀、致密。同时钽金属具有极强的耐酸碱特性,因此,Al₂O₃-TiO₂-Ta涂层相对于Al₂O₃-TiO₂涂层具有更强的耐腐蚀性。Tafel曲线结果显示,Al₂O₃-TiO₂涂层使得基体的腐蚀电位仅正移了99.6 mV,Al₂O₃-TiO₂-Ta涂层使得铝基体腐蚀电位正移了208.9 mV。因此,由于耐蚀性极强的Ta金属的掺入,Al₂O₃-TiO₂涂层的防腐性得到了极大的增强,Al₂O₃-TiO₂-Ta涂层有效地防止铝合金腐蚀。     

Influence of oxygen ion irradiation on the corrosion aspects of Ti-5%Ta-2%Nb alloy and oxide coated titanium

The corrosion resistance of Ti-5%Ta-2%Nb alloy and DOCTOR (double oxide coating on titanium for reconditioning) coated titanium by O⁵⁺ ion irradiation were compared and investigated for their corrosion behaviour. O⁵⁺ ion irradiations were carried out at a dose rate of 1 × 10¹⁷, 1 × 10¹⁸ and 1 × 10¹⁹ ions/m² at 116 MeV. The surface properties and corrosion resistance were evaluated by using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray (EDX), glancing-angle X-ray diffraction (GXRD) and electrochemical testing methods. The results of electrochemical investigations in 11.5 N HNO₃ indicated that the open circuit potential (OCP) of DOCTOR coated titanium is nobler than Ti-5%Ta-2%Nb alloy. The potentiodynamic polarization study of Ti-5%Ta-2%Nb alloy and DOCTOR coated specimen indicated decrease in passive current density with increase in ion doses (1 × 10¹⁷ to 1 × 10¹⁹ ions/m²) indicating decrease in anodic dissolution. Nyquist arc behaviour in the electrochemical impedance study substantiated the enhancement in oxide layer stability by O⁵⁺ ion irradiation. AFM results revealed that the DOCTOR coated Ti surface was dense without gross voids, and the surface roughness decreased by O⁵⁺ ion irradiation, but increased after corrosion test. EDX and GXRD patterns of DOCTOR coated Ti sample indicated that the coating was mainly composed of rutile TiO₂. Based on the above results, the O⁵⁺ ion irradiation effect on corrosion behavior of Ti-5%Ta-2%Nb alloy and DOCTOR coated titanium are discussed in this paper.     

H2S and O2 influence on the corrosion of carbon steel immersed in a solution containing 3 M diethanolamine

Aqueous solutions with 3 mol L⁻¹ (M) diethanolamine (DEA) concentration are extensively used in the gas processing industry to remove acid gases. However, the degradation of the DEA and the formation of heat-stable salts (HSS) lead to severe corrosion problems. Even worse, equipment corrosion can be magnified by the unavoidable presence of sulphide acid and dissolved oxygen as a result of hydrocarbon (natural gases and crude oil) processing. The aim of this work is to study the combined corrosion effects of DEA, sulphide acid and oxygen on carbon steel. Electrochemical methods revealed that in the 3 M DEA medium without oxygen, corrosion processes are modulated by adsorbed DEA film formation. Furthermore, it was shown that the addition of oxygen and 15 × 10⁻³ mol L⁻¹ (15 mM) H₂S produced the formation of an adherent film on the carbon steel surface. Chemical analyses by EDAX revealed a homogeneous film of corrosion products composed of iron oxide and sulphide formed in DEA solution containing O₂ and H₂S, respectively. Equivalent circuits were used to estimate the parameters associated with ion diffusion through the formed corrosion films. The results showed that the presence of H₂S induced the formation of thin iron sulphide films that provide protective properties to the metal. It is concluded that the presence of oxygen in a sweetening plant should be avoided as DEA degradation can be produced with the subsequent decrease in chelating process efficiency and the increase in corrosion problems.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

The corrosion of pure niobium in oxidizing,sulfidizing, and oxidizing-sulfidizing gas mixtures at 600–800°C

The corrosion of pure niobium has been studied at 600–800°C in various environments as part of a study of the corrosion resistance of its alloys with iron, cobalt, and nickel to atmospheres of low-oxygen and/or high-sulfur activities. The results have shown that not only the sulfidation but also the corrosion in mixed atmospheres and particularly the oxidation under low oxygen pressures of pure niobium are quite slow, with kinetics rather similar in the three types of gas mixtures used. The good corrosion resistance of niobium to attack by oxygen under low pressures is quite interesting because this element is corroded very rapidly by oxygen under high oxygen pressures, due to the formation of the nonprotective highest oxide Nb₂O₅ as a main corrosion product.     

Inert anode composed of Ni–Cr alloy substrate, intermediate oxide film and α-Al2O3/Au (Au–Pt, Au–Pd, Au–Rh) surface composite coating for aluminium electrolysis

An inert anode composed of alloy substrate, intermediate oxide film and surface composite coating for aluminium electrolysis has been fabricated. The intermediate oxide film (ZrO₂/Y₂O₃) provides good adhesion and mutual diffusion resistance between the substrate and the surface coating which consists of α-Al₂O₃ particles embedded in Au (Au–Pt, Au–Pd, Au–Rh) matrix. The results of electrolysis test revealed that aluminium with high purity (>99.999%) can be produced. It is demonstrated that the inert anode exhibits superior erosion and corrosion resistance during aluminium electrolysis, especially in low-temperature (800 °C) electrolytes.     

Corrosion processes of Mg–Y–Nd–Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg–Y–Nd–Zr (WE43 commercial alloy) was investigated in Na₂SO₄ electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)₂) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y₂O₃) and hydroxide (Y(OH)₃), mixed with a small amount of MgH₂, zirconium oxide (ZrO₂) and neodymium oxide (Nd₂O₃). The Y₂O₃ and Y(OH)₃ signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO₂ and Nd₂O₃ compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.     

Surface characterization of NiTi modified by plasma source ion implantation

This paper reports the novel application of an oxygen ion plasma for surface modification of a shape memory alloy. The surface characterization of oxygen ion implanted Ti–50.7 at% Ni alloy samples was performed with the assistance of Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy techniques (TEM). TEM identified amorphous TiO₂ and crystalline phases of Ti₁₁Ni₁₄ and Ti₄Ni₂O_x. XPS results reveal that the surfaces of control samples are covered with predominantly TiO₂ and traces of TiO and NiTi, as well as NiO and Ni₂O₃. At the surface of oxygen-implanted samples, however, only TiO₂ and trace amounts of Ni₂O₃, NiO and NiTi were observed. TiO and NiO exist in much larger range than other oxides, and TiO extends to the oxide–metal interface. These results are especially noteworthy because of their implications for interpreting corrosion, wear, and biocompatibility behavior.     

Characterization of plasma electrolytic oxide formed on AZ91 Mg alloy in KMnO4 electrolyte

The aim of this work is to investigate microstructure, corrosion resistance characteristics and nanohardness of the oxide layer on AZ91 Mg alloy by applying different voltage with KMnO₄ contained solution. There are lots of closed pores that are filled with another oxide compound compared with the typical surface morphology with pore coated until 350 V of coating voltage. The thickness of oxide layer increases with increasing coating voltage. The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium permanganate consists of MgO and Mn₂O₃. Corrosion potential of the oxide layer on AZ91 Mg alloy obtained at different plasma electrolytic oxidation(PEO) reaction stages increases with increasing coating voltage. The corrosion resistance of AZ91 Mg alloy depends on the existence of the manganese oxide in the oxide layer. The inner barrier layer composed of the MgO and Mn₂O₃ may serve as diffusion barrier to enhance the corrosion resistance and may partially explain the excellent anti-corrosion performance in corrosion test. Nanohardness values increase with increasing coating voltage. The increase in the nanohardness may be due to the effect of manganese oxide in the oxide layer on AZ91 Mg alloy coated from electrolyte containing KMnO₄.     

Evaluation of microstructural evolution of thermal barrier coatings exposed to Na2SO4 using impedance spectroscopy

Impedance spectroscopy as a non-destructive evaluation technique was employed to study the microstructural evolution of thermal barrier coatings exposed to Na₂SO₄ at 950 °C. The results showed that the resistance and capacitance of yttria-stabilized zirconia top coat increased with corrosion time. The resistance of thermally growth oxide (TGO) from 20 to 60 h decreased slowly, which indicates there is little change in the composition of TGO. The fast decrease in the resistance of TGO from 60 to 100 h may correspond to the compositional change of TGO from a-Al₂O₃ to more conductive NiO and Ni(Cr, Al)₂O₄ with corrosion time.     

Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy

The effect of equal-channel angular pressing(ECAP) on the pitting corrosion resistance of anodized Al-Cu alloy was investigated by electrochemical techniques in a solution containing 0.2 mol/L AlCl₃ and also by surface analysis. Anodizing was conducted for 20 min at 200 and 400 A/m² in a solution containing 1.53 mol/L H₂SO₄ and 0.018 5 mol/L Al₂(SO₄)₃·16H₂O at 20 °C. Anodized Al-Cu alloy was immediately dipped in boiling water for 20 min to seal the micro pores present in anodic oxide films. The time required before initiating pitting corrosion of anodized Al-Cu alloy is longer with ECAP than without, indicating that ECAP process improves the pitting corrosion resistance of anodized Al-Cu alloy. Second phase precipitates such as Si, Al-Cu-Mg and Al-Cu-Si-Fe-Mn intermetallic compounds are present in Al-Cu alloy and the size of these precipitates is greatly decreased by application of ECAP. Al-Cu-Mg intermetallic compounds are dissolved during anodization, whereas the precipitates composed of Si and Al-Cu-Si-Fe-Mn remain in anodic oxide films due to their more noble corrosion potential than Al. FE-SEM and EPMA observation reveal that the pitting corrosion of anodized Al-Cu alloy occurs preferentially around Al-Cu-Si-Fe-Mn intermetallic compounds, since the anodic oxide films are absent at the boundary between the normal oxide films and these impurity precipitates. The improvement of pitting corrosion resistance of anodized Al-Cu alloy processed by ECAP appears to be attributed to a decrease in the size of precipitates, which act as origins of pitting corrosion.