Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

B2O3 modified SiC–MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation

To protect carbon/carbon (C/C) composites against oxidation, a B₂O₃ modified SiC–MoSi₂ coating was prepared by a two-step pack cementation. The microstructure and the oxidation resistant property of the coating were studied. The results show that, the as-received coating is a dense structure, and is composed of α-SiC, β-SiC and MoSi₂. The B₂O₃ modified SiC–MoSi₂ coating has excellent oxidation resistant property, and can protect C/C composites from oxidation at 1773 K in air for more than 242 h. The failure of the coating was considered to arise from the existence of the penetration cracks in the coating during the slow cooling from 1873 to 673 K.     

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.     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.     

Coke formation and metal dusting of electroplated Ni3Al-CeO2-based coatings in CO-H2-H2O

Coke formation and metal dusting of electrodeposited pure, 5 μm CeO₂-dispersed, and 9-15 nm CeCO₂-dispersed Ni₃Al coatings were investigated in CO-H₂-H₂O at 650 °C for a period of 500 h. All Ni₃Al coatings showed the inferior long-term resistance to coke formation and metal dusting to the Fe-Ni-Cr alloy due to failure to form a continuous Al₂O₃ scale. CeO₂-dispersed Ni₃Al coatings, especially 9-15 nm CeCO₂-dispersed coatings, exhibited more severe coke formation and metal dusting than the pure Ni₃Al coating. The detrimental effect of CeO₂ is believed to be caused by the enhanced formation of NiO/Ni crystals on the coating surfaces or at the grain boundaries, which catalysed the carbon deposition and promoted the carbon attack on Ni₃Al coatings.     

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.     

Al2O3-ZrO2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance

Corrosion resistance and wear resistance are the two important parameters for high performance of zinc galvanic coating. In the present work, the improvement of these two characteristics was achieved by the incorporation of Al₂O₃-ZrO₂ mixed oxide composite in the coating. Al₂O₃-ZrO₂ mixed oxide composite was synthesized from ZrOCl₂·8H₂O. Aluminium rich zinc coatings with high sliding wear resistance was developed from a galvanic bath containing the mixed oxide. Based on the performance of the coating during physicochemical and electrochemical characterization, the concentration of mixed oxide composite in the bath was optimized as 0.50 wt% Al₂O₃-0.50 wt% ZrO₂. While rich in Al-metal content in the coating caused high corrosion resistance, the incorporation of the mixed oxide improved structural characteristics of the coating resulting in high wear resistance also. The coating was nonporous in nature and even the interior layers had high stability. The coatings have potential scope for high industrial utility.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.     

Influence of the iron content in Cu–Ni based inert anodes on their corrosion resistance for aluminium electrolysis

Mechanically alloyed (Cu_3.25Ni)_100−xFe_x materials (x = 0, 15 and 30 wt.%) were evaluated as inert anodes for aluminium electrolysis in KF–AlF₃ (700 °C) electrolyte. For x = 0, the cell voltage was unstable and high (5–6 V) due to the formation of an insulting NiF_x layer at the metal–oxide interface. For x = 15 and 30, the formation of a Cu₂O-rich external scale with a protective NiFe₂O₄-rich intermediate layer was favoured, resulting in a lower (∼4 V) and more stable cell voltage. The purity of the produced Al was 98.96, 99.31 and 99.20 wt.% for x = 0, 15 and 30, respectively.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Effect of sintering temperature on corrosion properties of sol–gel based Al2O3 coatings on pre-treated AZ91D magnesium alloy

A novel anti-corrosion sol–gel based Al₂O₃ coating was developed on the AZ91D magnesium alloy. The morphology, microstructure and composition of the coatings were investigated by scanning electron microscope coupled with energy dispersive spectroscopy, Fourier transform infrared spectrum analysis, X-ray diffraction, thermo-gravimetric and differential thermal analysis. The corrosion resistance of the coatings in 3.5 NaCl wt.% solution was studied using electrochemical measurements. The results demonstrated that a homogeneous Al₂O₃ coating could be obtained and the sol–gel coated samples sintered at 380 °C had the best corrosion resistance properties as compared to the specimens sintered at 120 and 280 °C.     

Corrosion behaviour of cermet-based coatings with a bond coat in 0.5 M H2SO4

The corrosion behaviour of an HVOF Ni–5Al/WC–17Co coating on Al-7075 is investigated in 0.5 M H₂SO₄. In the temperature range of 25–45 °C, the coating exhibits pseudopassivity that effectively protects from localized corrosion. At 25 °C, pseudopassivity proceeds via three stages: during the first stage, oxidation of W in the binder phase occurs. The second stage is characterized by oxidation of W in both the binder and the carbide particles. The third stage is characterized by intensive hydration of WO₃ and formation of Co₃O₄. During the second and third pseudopassive stages, the formation of a bi-layer surface film is postulated. The inner layer, consisting of anhydrous oxides, has a barrier character. The outer layer, composed of WO₃ · xH₂O, is unstable. In case of surface film disruption, the bond coat successfully hinders corrosion propagation into the Al-alloy. Higher electrolyte temperatures lead to faster corrosion kinetics and higher tendency for pitting.     

High-temperature SO2-gas corrosion of TiN–Ti5Si3 composites prepared by polymer pyrolysis

By pyrolyzing a mixture of Si-containing pre-ceramic polymers and TiH₂ powders in a N₂ atmosphere, a TiN–Ti₅Si₃ composite was synthesized. The composite was then corroded between 700 °C and 1000 °C for 20 h in an Ar–0.2% SO₂ atmosphere. TiN was mainly oxidized to rutile TiO₂. Ti₅Si₃ was oxidized to TiO₂ supersaturated with Si ions, and sulfidized to Ti₂S supersaturated with Si ions. At initial stage of corrosion, oxidation dominated sulfidation. As corrosion proceeded, sulfidation progressively occurred underneath the oxide scale based on the decreased oxygen potential and increased sulfur potential near the scale/matrix interface.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

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.     

Microstructure and oxidation of multi-layer MoSi2-CrSi2-Si coatings for SiC coated carbon/carbon composites

Multi-layer MoSi₂-CrSi₂-Si anti-oxidation coatings with different compositional ratios were prepared on the surface of SiC coated carbon/carbon (C/C) composites by a two-step pack cementation method. The microstructure and anti-oxidation performance of the coating were studied. The results show that the multi-layered coatings could protect the C/C composites from oxidation in air at 1773 K for 1000 h or 1873 K for 750 h, respectively. The anti-oxidation performance of the multi-layer MoSi₂-CrSi₂-Si coating is mainly attributed to their dense and microcrack-free structure, appropriate thermal expansion coefficient and the well dispersed MoSi₂ and CrSi₂ in the coating.     

NiFe1 . 98Nd0 . 02O4-Fe 双层吸波涂层设计及制备

目的制备高性能NiFe1.98Nd0.02O4-Fe双层吸波涂层。方法采用溶胶凝胶自燃烧法制备钕掺杂NiFe1.98Nd0.02O4。借助X射线衍射仪、扫描电镜和矢量网络分析仪对NiFe1.98Nd0.02O4和羰基铁的结构、形貌、电磁参数进行测试分析。采用遗传算法对NiFe1.98Nd0.02O4-羰基铁双层涂层的厚度进行优化设计。结果以环氧树脂为基体,以羰基铁和NiFe1.98Nd0.02O4为吸波剂的双层吸波涂层具备较好的吸波性能,厚度约1 mm,反射率在9.25~11.35 GHz范围内均小于-10 dB。结论 NiFe1.98Nd0.02O4层和羰基铁层在吸波性能上有很好的互补性,理论优化结果和实验结果相同。     

Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments

Passive films formed on Alloy 690 in different pH solutions at high temperatures were studied by potentiodynamic polarization, Auger electron spectroscopy, thermodynamic diagrams and the Mott–Schottky relation. The chemical compositions and electronic structures of the passive films were found to be strongly pH-dependent. In alkaline solutions, a secondary passivation was clearly observed on potentiodynamic polarization curves. The passive films were a mixture of Cr₂O₃ and FeCr₂O₄ below the flat band potential of nickel oxide and were NiFe₂O₄ above this potential. Electronic structure models, describing the electrochemical properties of the passive films, are proposed and discussed.     

Hot corrosion behaviour of Ni3Al in sulphate-chloride mixtures in the atmosphere

Hot corrosion of Ni₃Al intermetallic compound in the presence of sulphate-chloride mixtures was studied. A comminuted Ni₃Al mixed with NaCl-Na₂SO₄, NaCl-Li₂SO₄, LiCl-Na₂SO₄, LiCl-Li₂SO₄ additions was oxidized in the air up to 1000 °C with linearly increasing temperature and isothermally within the temperature range of 500-700 °C. The corrosion process was observed by thermogravimetric method using Mettler thermoanalyzer.The experiments indicated that LiCl (∼10 wt.%)-Li₂SO₄ mixture was the most corrosive agent. It was also found that by addition of MgO the corrosion of Ni₃Al was reduced. Phase composition of the corrosion products was established by X-ray diffraction analysis; there were detected Al₂O₃, Al₂S₃, NaAlO₂ (or LiAlO₂) as intermediate products, nickel sulphides, NiO and NiAl₂O₄. NiAl₂O₄ spinel was formed only at the highest temperatures, while at lower temperatures alumina was present instead of spinel.Hot corrosion behaviour of Ni₃Al in sulphate-chloride mixtures in air atmosphere.