不同(Mo+B)/(Ni+Cr)质量比的原位合成MoB/NiCr涂层的组织结构与性能

采用超音速火焰喷涂技术沉积含3种不同(Mo+B)/(Ni+Cr)质量比（1:1,2:1和3:1）的Mo-B-Ni-Cr球磨复合粉末以原位反应制备获得MoB/NiCr涂层。采用扫描电子显微镜（SEM）和X射线衍射仪（XRD）分析了MoB/NiCr涂层的组织结构和物相。同时讨论了不同(Mo+B)/(Ni+Cr)质量比对涂层的组织结构、硬度、结合强度和耐腐蚀性能的影响。研究结果表明,(Mo+B)/(Ni+Cr)质量比为1：1的MoB/NiCr涂层孔隙率最低及涂层厚度最大。在3种涂层中均原位反应生成了Mo₂NiB₂三元硼化物,且随着(Mo+B)/(Ni+Cr)质量比的增加,涂层中三元硼化物含量随之增加,涂层的硬度值增加,结合强度反而随之降低;由于涂层中三元硼化物的原位生成,MoB/NiCr涂层的硬度值均高于316L不锈钢基体。通过能谱和XRD分析发现,经过360 h熔融锌腐蚀试验后,涂层表层中没有发现锌元素及其金属间化合物,然而随着(Mo+B)/(Ni+Cr)质量比的增加,涂层的孔隙率增加及厚度降低。最后,综合分析可得,相比其他涂层,(Mo+B)/(Ni+Cr)质量比为1:1的MoB/NiCr涂层具有更好的耐熔融锌腐蚀能力。     

Microstructure evolution and corrosion behavior of Fe-Al-based intermetallic aluminide coatings under acidic condition

Two Fe-Al-based intermetallic aluminide coatings were fabricated on 430-SS (Fe-Cr) and 304-SS (Fe-Cr-Ni) substrates by pressure-assisted solid diffusion bonding with coating on pure Fe as control. The microstructure and intermetallic phases of the coatings were characterized by SEM, EDS and EBSD. A network of Cr₂Al₁₃ with matrix of Fe₄Al₁₃ was formed by inter-diffusing of Al with the substrates. The corrosion behavior of intermetallic coatings was investigated in 0.5 mol/L HCl solution by mass-loss, OCP, Tafel plot and EIS. It was found that corrosion resistance was greatly enhanced by dozens of times after the addition of Cr and Ni compared with that on pure Fe. The presence of cracks in the coating on 430-SS provided a pathway for corrosion media to penetrate to the substrate and accelerated the corrosion rate. Moreover, the corrosion product was analyzed by XRD, demonstrating that the addition of Cr and Ni facilitated the formation of more corrosion resistant phases, and therefore improved corrosion resistance.     

Electrodeposition of Zn-Ni, Zn-Fe and Zn-Ni-Fe alloys

Zn-Fe, Zn-Ni and Zn-Ni-Fe coatings were electrodeposited galvanostatically on mild steel from acidic baths (pH 3.5) consisted of ZnCl₂, NiCl₂, FeCl₂, gelatin, sulfanilic (p-aminobenzenesulfonic) acid and ascorbic acid. Cyclic voltammetry showed that the effect of gelatin was more pronounced than that of sulfanilic acid, and that the deposition of the ternary alloy behaved differently from the deposition of the binary alloys. In all three systems, the Faradaic efficiency was higher than 88%, the rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, and the deposition was of anomalous type. For each applied current density, the concentrations of Ni and Fe in the ternary alloy were higher than the corresponding concentrations in the binary alloys. The hardness of Zn-Ni coatings was the highest, while that of Zn-Fe coatings was the lowest. The Zn-Ni-Fe coatings were the smoothest, had distinguished surface morphology, and contained ZnO in the bulk, not just on the surface. The lowest corrosion rate in each alloy system (214, 325 and 26 μm year⁻¹ for Zn-Ni, Zn-Fe and Zn-Ni-Fe, respectively) was characteristic of coatings deposited at 30, 30 and 40 mA cm^− 2, respectively. The higher corrosion resistance of the ternary alloy was also reflected by a higher corrosion potential, a higher impedance and a higher slope of the Mott-Schottky line. The enhanced corrosion behavior of the ternary alloy was thus attributed to its chemical composition, phase content, roughness and the synergistic effect of Ni and Fe on the n-type semiconductor surface film.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Effect of Chemical Compositions and Surface Morphologies of MCrAlY Coating on Its Isothermal Oxidation Behavior

Chemical composition and surface morphology of MCrAlY coatings are factors which influence the oxidation behavior and the thermal durability of thermal barrier coatings. In this study, Cold-sprayed Ni20Cr10AlY and Ni23Co20Cr8.5Al4.0Ta0.6Y coatings with polished surfaces were employed to study the effect of composition on the oxidation behavior. The cold-sprayed MCrAlY coatings at the as-sprayed and shot-peened surface conditions, along with the low pressure plasma-sprayed MCrAlY coating with sputters adhered weakly on the surface, were employed to investigate the effects of surface morphologies of MCrAlY coatings on their oxidation behavior. Cold-sprayed Ni20Cr10AlY coating exhibited a two-stage oxidation behavior and a higher TGO growth rate than that of the cold-sprayed Ni23Co20Cr8.5Al4.0Ta0.6Y coating at the rapid growth stage. After 10-h oxidation, the TGO on the as-cold-sprayed coating surface was mainly constituted by Al₂O₃, while the TGO on the coating surface attached with sputters was composed of Al₂O₃ and Cr/Ni-oxides. After 500-h oxidation, Cr₂O₃ and porous spinel appeared in the TGO on the surface of the as-cold-sprayed coatings with different compositions. The growth of Cr/Ni-oxides was attributed to the Al depletion. The content of spinel decreased on the cold-sprayed NiCrAlY with a shot-peened surface compared with the as-sprayed coating.     

真空高频感应熔覆Ni60-WC复合涂层的耐蚀性

在不同的WC质量分数和不同的熔融状态下,用真空高频感应熔覆法制得Ni60-WC复合涂层,分析复合涂层的化学成分和组织形貌,研究WC质量分数和熔融状态对高频感应熔覆Ni60-WC复合涂层耐蚀性的影响。结果表明:复合涂层具有优良的耐蚀性能,其中在盐酸(质量分数10%)中最大耐蚀性是Q235-A的100倍左右,在硫酸(质量分数10%)中最大耐蚀性是Q235-A的20倍左右。在同种腐蚀溶液中的复合涂层材料耐蚀性一般随着WC质量分数的增加而降低。刚熔状态比过熔状态耐蚀性能要好,这是由于合金粉末中含有耐腐蚀元素Ni、Cr等,提高了涂层的耐蚀性。但随着熔烧时间的延长,涂层与基体之间产生扩散,涂层中Fe元素质量分数增加,腐蚀元素Ni、Cr的质量分数相对减少,导致涂层的耐蚀性降低。     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Investigations on role of HVOF sprayed Co and Ni based coatings to combat hot corrosion

Abstract

This paper aims to investigate the hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed cobalt based (Stellite-6) and nickel based (Ni–20Cr) coatings deposited on the superalloy Superni-718 (Ni–19Cr–18˙5Fe–5˙13Ta–3˙05Mo–0˙9Ti–0˙5AI–0˙18Mn–0˙18Si–0˙15Cu–0˙04C) in the Na₂SO₄–60%V₂O₅ salt environment at 900°C under cyclic conditions. The X-ray diffractometry, scanning electron microscopy/energy dispersive analysis and electron probe microanalyser techniques were used to study the corrosion products with respect to their morphology, phase composition and element concentration. The thermogravimetric technique was used to establish the kinetics of corrosion. The bare alloy underwent severe hot corrosion attack. The Ni–20Cr coating shows excellent hot corrosion resistance with negligible spallation, whereas Stellite-6 coating reveals less hot corrosion resistance and more spallation. The hot corrosion resistance of Ni–20Cr coating has been attributed to the formation of oxides of chromium, nickel and spinel of nickel chromium. The oxides of silicon, chromium, cobalt and spinels of cobalt–chromium and nickel–chromium have contributed for hot corrosion resistance of Stellite-6 coatings.     

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

The objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na₂SO₄–60%V₂O₅ dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm² in case of Al–Cr coating and Rs. 1.86 per mm² in case of Ti–Al coating.     

The hydrophobicity of Ni–Fe alloy electrodeposits obtained at different current densities

A simple one-step method was used to fabricate a hydrophobic surface by electrodepositing Ni–Fe coating on low carbon steel at various current densities ranging from 2 to 6 A?dm^?2. The as prepared coatings were hydrophilic showing a water contact angle (CA) of zero. After 17 days exposure of the coatings in ambient air, the wettability of the Ni–Fe coatings changed to hydrophobic. Increasing the current density of electrodeposition from 2 to 6 A?dm^?2 led to a reduction in the CA. The hydrophobic behavior of the Ni–Fe coatings after storage in air is related to two factors including the formation of airborne hydrocarbon compounds and the micro/nano pinecone-like structure of the Ni–Fe coating. Polarization tests and electrochemical impedance spectroscopy measurements indicated that the hydrophobic surfaces had higher corrosion resistance in comparison to the as-deposited coatings.     

Effect of pre-oxidation on increasing resistance of Fe–Al–Ni–Cr–Co–Mn high entropy alloys to molten Al attack

Abstract

The corrosion behaviours of two high entropy alloys (HEAs, namely Fe–Al–Ni–Cr–Co and Fe–Al–Ni–Cr–Co–Mn) in molten Al, each with and without pre-oxidation treatment, were investigated. Both bare alloys lost their thickness owing to dissolution, and various intermetallic compounds formed on their substrate surfaces during immersion in molten Al. Pre-oxidation treatment gave rise to the formation of an effective oxide barrier on each alloy surface against molten Al attack. The characteristics of pre-formed oxides in resisting molten Al attack were examined using electron backscatter diffraction (EBSD) analysis.     

Performance of high-velocity oxyfuel-sprayed coatings on an fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C Part I: Characterization of the coatings

This article demonstrates the successful formulation of NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings on an Fe-based superalloy by a high-velocity oxyfuel (HVOF) process for hot corrosion applications. The microstructure, porosity, coating thickness, phase formation, and microhardness properties of the coatings have been characterized using the combined techniques of optical microscopy, x-ray diffraction, scanning electron microscopy/energy-dispersive x-ray analysis. A microhardness tester was used to determine the hardness of the coatings. The coatings in general exhibit characteristic splat-like, layered morphologies due to the deposition and resolidification of successive molten or semimolten powder particles. The NiCrBSi, Cr₃C₂-NiCr, and Ni-20Cr coatings have shown a nickel-base face-centered cubic (fcc) structure as a principal phase, whereas Stellite-6 coating has an fcc Co-rich metallic matrix. Oxides/spinel oxides are formed in small fraction as intersplat lamellae or globules oriented parallel to the substrate surface. Coatings possess some unmelted/partially melted particles, inclusions, and porosity less than 2%. The microhardness of the coatings is found to be higher than the superalloys. The Cr₃C₂-NiCr coating has indicated a maximum microhardness of 990 Hv, while a Ni-20Cr coating has shown a minimum value of about 600 Hv. This article is focused on the characterization of HVOF coatings. The hot corrosion behavior of these coatings in a molten salt (Na₂SO₄-60%V₂O₅) environment at 900 °C under cyclic conditions is being presented as part II included in this issue.     

Role of zinc compounds on the formation, morphology, and adsorption characteristics of β-FeOOH rusts

To simulate the corrosion of galvanized steel in marine zone, β-FeOOH was prepared by aging the FeCl₃ solutions containing ZnCl₂ and zinc rusts such as ZnO and zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O:ZHC). Adding ZnCl₂, ZnO, and ZHC inhibited the crystallization and particle growth of β-FeOOH and the inhibitory effect was in order of ZHC ≈ ZnO > ZnCl₂. The adsorption of H₂O and CO₂ was suppressed by adding ZnCl₂, ZnO, and ZHC. These results imply that the rust formed on galvanized steel in marine environment is more compact, amorphous, and hydrophobic in nature which may lead to improve the corrosion resistance.     

Degradation behaviour of ZrO2-Ni/Al gradient coatings in molten Zn

In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO₂-Ni/Al gradient coatings were sprayed on the surface of the Fe-0.35-0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO₂-Ni/Al gradient coatings in molten zinc were studied. The ZrO₂-Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO₂-Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO₂ coatings. The ZrO₂-Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO₂-Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO₂-Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion.     

Comparison of corrosion resistance between Al2O3 and YSZ coatings against high temperature LiCl-Li2O molten salt

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li₂O molten salt under an oxidizing environment. Al₂O₃ and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al₂O₃ and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li₂O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al₂O₃ coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li₂O molten salt to form a porous reaction layer of LiAl, Li₅AlO₄ and LiAl₅O₈ for the Al₂O₃ coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.     

Die Rolle der Alkalisulfate bei der Hochtemperaturkorrosion

The role of alkali sulfates in high temperature corrosion Tests with steels (%,16 to 20 Cr, 13 to 20 Ni, 0 to 20 Co), Co alloys and Ni alloys (%, 20 to 30 Cr, 0 to 20 Co, 3 to 15 Fe) in sulfates and their mixtures with other salts at 600 to 900 ° C. In the presence of SO₃ (originating from the decomposition of alkali sulfates) chromates and heavy metal sulfates are formed which yield low melting eutectic mixtures with alkali sulfates. Addition of inert materials such as finally dispersed silica has a favorable effect because such materials adsorb molten phases. Enamelling can as well be taken into consideration. Chromium coatings must be perfectly pore-free in order to prevent pitting and penetration under the coating; the protection afforded by chromium coatings is restricted to temperatures below 750° C. Co alloys go through a corrosion maximum below 800° C and can be used at higher temperatures.     

Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.     

Preparation of Fe/Cr doped SiO2 thin film on 304 stainless steel substrate originated from corrosion

Abstract

A Fe/Cr doped SiO₂ thin film which can improve the anticorrosion characteristic of materials was prepared on SUS304 stainless steel substrates by a modified hydrothermal method that constitutes a two part preparation process. The corrosion of stainless steel and the dissolution of silica from Na–Ca glass are simultaneously stimulated by an aqueous lithium bromide solution, and then a Fe/Cr doped SiO₂ thin film is coated onto the substrate. The Fe/Cr doped SiO₂ film was characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electron probe X-ray microanalyser. The results showed that the corrosion products and the silica are combined to form a Fe/Cr doped SiO₂ thin film. Furthermore, a chemical bond is formed between the silicon coating and the metal that improves the compactness and the adhesive property of the film. The anticorrosion characteristics of the film were measured by the methods of weight loss and electrochemical potentiodynamic polarisation, indicating that the Fe/Cr doped SiO₂ thin film can improve the anticorrosion performance of stainless steels effectively.     

Wettability of Atmospheric Plasma Sprayed Fe, Ni, Cr and Their Mixture Coatings

Wetting behaviors of plasma sprayed Fe, Ni, Cr and their mixture coatings were investigated. The contact angle of water droplets on the surfaces increases with time when the surfaces are exposed to air, with the highest value greater than 150°. To the best of our knowledge, this is the first report that superhydrophobicity can be achieved by one step plasma spraying. It is found that the feedstock particle size distribution has a significant influence on the coating morphology. For the mixture coating, a micro/submicro dual scale roughness is obtained. The carbon content in the surface is found increasing with time. Results of X-ray diffraction, X-ray photoelectron spectroscope analysis and heat treatment in CO₂/Ar atmospheres suggest that the surface may contain some active ferrites which can decompose carbon dioxide into carbon. The origin of the superhydrophobicity is attributed to the accumulation of surface carbon materials and the dual scale roughness.     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.