基于SOFM神经网络的军用灰色涂层腐蚀行为研究

目的探究军用车辆有机涂层在全浸泡条件下的腐蚀行为特征,寻找评价涂层腐蚀防护性能的有效方法。方法利用电化学阻谱技术,对某型军用灰色涂层在全浸泡条件下的腐蚀行为进行了研究,分析了其阻抗谱及低频阻抗模值0.1 Hz|Z|、高频相位角10 k Hzq、相对介电常数re三种特征参数的变化规律。以三种特征参数作为评价指标,利用SOFM自组织神经网络对涂层性能的变化过程进行研究。结果灰色涂层在全浸泡下的腐蚀过程大致经历三个阶段。良好阶段:高频相位角q位于70°附近,低频阻抗模值|Z|均大于10~7。防护性能下降但仍具有保护作用阶段:高频相位角q下降至50°附近,低频阻抗模值|Z|下降至10~6附近。防护性能丧失阶段:高频相位角q全部低于50°,低频阻抗模值|Z|已经低于10~5。SOFM自组织神经网络对涂层的分类结果与对阻抗谱特征分析的结果保持一致。结论通过实例分析,证明自组织神经网络SOFM方法可实现对涂层性能状态的快速判断。     

Erosion-oxidation behaviour of pack-aluminized 9% chromium steel under fluidized-bed conditions at elevated temperature

Pack-aluminized 9% chromium steel specimens were exposed to angular silica sand particles in a fluidized-bed erosion-oxidation rig for 200 h. The exposures were conducted in air at temperatures of 550 °C to 700 °C for particle impact angles of 30° and 90°, at speeds of 7.0-9.2 m s⁻¹. Subsequently, the mean thickness changes of the specimens were determined and the specimens were examined and analyzed by scanning electron microscopy and X-ray diffraction. The specimens experience only slight thickness changes for 30° angle impacts but significant material loss for 90° angle impacts, typical of a brittle erosion process. Under 30° angle impacts, the coatings were mostly retained on the substrate surface and slightly deformed. Thin oxide scales were detected on the surface at all test temperatures. Under 90° angle impacts, thickness losses increased with increase in speed and temperature up to 650 °C, resulting in complete loss of the coating in the test period. A porous, cracked, but continuous, oxide scale was observed on the surface of the exposed substrate. At 700 °C, the coating was partially retained on the substrate, with the residual coating thickness decreasing with increase in speed. Explanations for these observations are presented, the interactions between the erosion and oxidation processes for the specimens are discussed and the degradation mechanisms for the coatings under the test conditions are described in this paper.     

The effect of heat treatment temperature on the structure and barrier performance of a zirconia coating electrodeposited by pulse current

Zirconia coatings were prepared by heat-treating the electrodeposited zirconium hydroxide produced by pulse current on 316L stainless steel. The results showed that a coating with amorphous structure obtained after heat treating at 200 °C, reveals no corrosion barrier performance. Heat treating at 400 °C resulted in zirconia coating with nanocrystalline/amorphous structure, which reveals a strong corrosion barrier performance. This coating shows a high value of pore resistance and Warburg behaviour in electrochemical impedance spectroscopy. As the temperature was raised to 600 °C, an entirely nanocrystalline structure was found. However, this coating revealed muddy shape cracks, with inferior corrosion barrier performance.     

Phase transformation in κ- and γ-Al2O3 coatings on cutting tool inserts

The phase transformation in as-deposited, metastable κ- and γ-alumina coatings on cutting inserts has been studied by a combination of X-ray diffraction, scanning electron microscopy and cathodoluminescence. In the case of κ-alumina, mechanical forces during metal cutting lower the transformation temperature of the metastable phases to the thermodynamically stable α-alumina phase from 1050 °C to 930 °C. This is the reason why that coating has a similar performance during metal cutting when compared to the stable α-alumina phase. The transformation temperature in γ-alumina coated cutting tools is found to be as low as 950-975 °C. Cathodoluminescence (CL) has been demonstrated as a possible method for differentiation between the various alumina phases. Specifically, both κ- and γ-alumina films have revealed a strong room temperature CL with different peak energies depending on the phase. CL of the metastable alumina coatings annealed at the transformation temperatures corresponds to the stable α-alumina phase.     

Grain refinement and boundary misorientation transition by annealing in the laser rapid solidified 6FeNiCoCrAlTiSi multicomponent ferrous alloy coating

A 6FeNiCoCrAlTiSi multicomponent ferrous alloy coating with simple BCC solid solution phase has been prepared by laser cladding on a low carbon steel substrate. Grain boundary misorientation transition and abnormal microstructure refinement in the coating after annealing at 500 °C have been studied by electron backscatter diffraction (EBSD) observation. Before annealing, the microstructure of the coating is mainly composed of directionally solidified columnar grains with low angle grain boundaries distributed as networks. After annealing, the main phase and microhardness of the coating almost remain unchanged. However, the columnar grains transform to equiaxed refined grains with the grain boundary misorientation transition from low to high angles. This result supports such an argument that the reheating process in post-stage of solidification plays a key role on the grain refinement during rapid solidification process.     

Effect of sintering temperatures on corrosion and wear properties of sol-gel alumina coatings on surface pre-treated mild steel

Sol-gel alumina coatings were developed on the surface pre-treated (zinc-phosphated) mild steel substrate and subsequently sintered at 300 °C, 400 °C and 500 °C. The alumina sol was synthesised using aluminium iso-propoxide as a precursor material. FTIR of the boehmite (AlOOH) gel sintered at above-mentioned temperatures was employed to identify the presence of various functional groups. The microstructural features and the phase analysis of the sol-gel coated specimens were carried out using SEM and XRD respectively. The corrosion resistance of the sol-gel alumina coatings was evaluated by electrochemical measurement in 3.5% NaCl solution at room temperature. The abrasive wear behaviour of the sol-gel coated specimens was measured in two body (high stress) conditions. The experimental results revealed that the sol-gel coated specimen sintered at 400 °C has superior wear and corrosion resistance properties as compared to the sol-gel coated specimen sintered at 300 °C. However, the sol-gel coated specimen sintered at 500 °C has exhibited a very poor corrosion and wear resistance properties. Poor performance of the sol-gel coatings sintered at 500 °C could be explained to be due to (i) the presence of numerous cracks (ii) absence of organic groups in the coating.     

Study of the corrosion and microstructure with annealing conditions of a β-quenched HANA-4 alloy

The variation of microstructure and corrosion characteristics with the applied annealing conditions of a HANA-4 (Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr) alloy were studied by utilizing transmission electron microscopy and a corrosion test at 360 °C in a water environment. The samples were annealed at temperature ranges from 540 to 660 °C up to 16 h after β quenching at 1050 °C. The corrosion behaviour with the annealing conditions was divided into two groups following the second phase characteristics. The suitable annealing temperature to obtain good corrosion resistance in the HANA-4 alloy ranged from 570 to 600 °C.     

The role of microstructure in the mechanism of high velocity erosion of Cr3C2-NiCr thermal spray coatings: Part 2 — Heat treated coatings

In Part 1 of this two part series the variation in erosion mechanisms as a function of as-sprayed coating microstructure was presented. The oxidation resistance of Cr₃C₂-NiCr coatings means that they are used in high temperature applications where WC-Co based systems are no longer suitable. High temperature exposure has been shown to generate microstructural development in these coatings, leading to variations in coating hardness. In this work the effect of such coating development on the high velocity erosion response is investigated. The HVAF and HVOF coatings of Part 1 were heat treated for up to 30 days at 900 °C to generate a range of coating microstructures up to steady state. Erosion was performed under the same conditions as in Part 1. Heat treatment increased the ductility of the NiCr phase, enabling ductile erosion deformation to occur. Intersplat sintering reduced the significance of splat based erosion mechanisms and forced mass loss to become dictated by the phase microstructure. Such developments improved the quantified erosion resistance of both coating systems relative to the as-sprayed conditions. The coating microhardness was shown to be a poor indicator of erosion response across the range of coating microstructures investigated.     

Stability of electroplated titanium diboride coatings in high-temperature corrosive media

The corrosion behaviour of molybdenum and steel materials, protected by electrochemically plated TiB₂ coatings, in contact with liquid aluminium alloys and liquid glass in air has been studied. The corrosion performance tests followed by characterization of the treated samples by optical microscopy, SEM, elemental EDX analysis, have shown high corrosion resistance of the coatings to the liquid metal. However, the coating was not stable in contact with molten glass in an oxidative atmosphere at temperatures higher as 750-800 °C. Dissolution of the corrosion products in the melt facilitates the destruction of the coating.     

超音速等离子喷涂沉积可磨耗封严涂层研究

采用超音速等离子喷涂沉积Ni-C及NiCr-BN可磨耗封严涂层,通过实验对比研究两种涂层的结合强度、表面硬度及在不同冲蚀角下的冲蚀磨损性能.结果表明:NiCr-BN涂层中的润滑相尺寸比Ni-C涂层更为细小;NiCr-BN涂层的结合强度及表面硬度均高于Ni-C涂层;NiCr-BN涂层的抗冲蚀性能要优于Ni-C涂层.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

Corrosion behaviour of an aluminized nickel coating in a carbon dioxide capture process using aqueous monoethanolamine

Carbon steel A106 with an aluminized Ni coating was tested in a 5 mol/l (M) monoethanolamine (MEA) with a carbon loading of 0.2 mol/mol MEA at 80 °C using polarization resistance, Tafel, and electrochemical impedance spectroscopy (EIS) techniques. Surface morphology and elemental line profile of the coating were examined before and after a 900 °C heat-treatment in atmosphere environment. X-ray diffraction (XRD) results showed that the coating surface constitutes transformed from Ni₂Al₃ to Al-rich NiAl after heat-treatment. The electrochemical results show that the heat-treated coating displays outstanding corrosion performance with the conditions tested.     

Corrosion properties of surface-modified AZ91D magnesium alloy

Samples of AZ91D magnesium alloy were dipped into AlCl₃–NaCl molten salt at different temperatures between 250 °C and 400 °C for 28800 s. The thickness of the alloying layer is increased with the rise of the treatment temperatures. The coating was mainly composed of Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compounds. The corrosion resistance of the coating which is obtained at 300 °C for 28800 s is the best. When the treatment temperature is higher than 300 °C, some cracks developed in the alloying layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the AZ91D substrate and the alloying coating during the rapid cooling process.     

Scratch resistance of platinum-vanadium single and multilayer systems

The effects of annealing on coating morphology and scratch resistance have been studied in several single and multilayered Pt-V coated systems. The changes in coating morphology included the formation of distinctive “square shape precipitates”, increased surface roughness, coating thickening and cracking. The results show that scratch resistance was affected by coating thickness, the sequence of deposited layers and Pt-V phases induced by annealing. The scratch resistance was improved for all coated systems by annealing. However, there is an indication that a large volume fraction of PtV₃ formed at 900 °C/45 min significantly increased the critical load at which the coating failed. This result will be the base for further study on the development of targeted phases to improve the surface characteristics for specific coating applications.     

Effects of zinc bath temperature on the coatings of hot-dip galvanizing

The purpose of this work is to identify the influence of zinc bath temperature on the morphology and the thickness of hot-dip galvanizing coating. Hot-dip galvanizing samples were prepared at temperatures in the range of 450-530 °C in steps of 10 °C and at different immersing time. The samples were characterized by using scanning electron microscopy/energy dispersive X-ray analysis. When the galvanizing temperature ranges from 450 to 470 °C, the coating has a coherent and compact ζ layer on the top of a δ layer. ζ phase coexists with liquid zinc pocket around 480 °C. It was found that the coating thickness reaches maximum at 480 °C. When the temperature is above 500 °C, the coating changes to be primarily δ phase.     

钽表面微弧氧化陶瓷层的制备及耐磨性能研究

目的研究不同电压、电流和氧化时间下,钽表面微弧氧化陶瓷层的生长机理与耐磨性能的变化。方法通过微弧氧化技术在钽金属表面制备陶瓷层,并采用扫描电子显微镜观察陶瓷层的表面形貌,采用摩擦磨损试验仪对陶瓷层的摩擦学性能进行研究,探讨放电电压、放电频率、氧化时间对陶瓷层摩擦系数的影响。结果在电压400 V、频率1000 Hz、氧化时间20 min条件下获得的陶瓷层表面质量最好;在电压350 V,频率1000 Hz,氧化时间10 min条件下获得的陶瓷层耐摩擦性能最好。结论对钽金属表面进行微弧氧化处理,可以显著降低钽金属表面的摩擦系数,提高耐磨性能。     

The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures

Cast AZ80 alloy was subjected to conventional extrusion pressing at 250 °C, 300 °C, and 350 °C. In order to characterize the changes in their microstructure a thorough study was done through various microscopy analyses including Optical Microscope, SEM, and TEM.Corrosion performance of each condition was investigated in 3.5% NaCl solution saturated with Mg(OH)₂ (pH ≈ 10.5) using immersion and AC and DC polarization tests. The local potential difference on the surface resulting from different compositions of second phase particles to the matrix was investigated using scanning Kelvin probe force microscopy (SKPFM) technique.The results show grain refinement by a factor of about 15-20 and obvious evidence of dynamic recrystallization were identified leading to the formation of nano-sized grains after the extrusion process.The corrosion resistance of cast AZ80 alloy drastically decreases after the thermo-mechanical processes and the main factor is high dependence on different phase rearrangements before and after the extrusion process, especially β phase. For the extrusion conditions, different corrosion resistances are attributable especially to dislocation rearrangement results by grain growth after dynamic recrystallization.     

Sintering atmosphere and temperature effects on hydroxyapatite coated type 316L stainless steel

Electrophoretically deposited hydroxyapatite (HAP) coatings on type 316L SS was developed at the optimum coating parameters of 60 V and 3 min. Sintering of the coating enhances the metal-ceramic bond strength, but HAP structure is sensitive to temperature as it decomposes to other calcium phosphate phases. Sintering of HAP coatings in air at 900 °C for 1 h indicate the formation of a composite surface containing oxides of the alloy and decomposition products of HAP, mainly tricalcium phosphate. Open circuit potential-time measurements, potentiodynamic cyclic polarisation and electrochemical impedance experiments performed in Ringer’s solution indicate that the corrosion performance of HAP coatings were severely affected by the sintering atmosphere and temperature. Higher capacitance and low polarisation resistance values obtained from electrochemical impedance spectroscopic studies further indicate that the coatings are more prone to dissolution on comparison with the pristine type 316L SS. The sintering of the coatings in vacuum at 600, 800 and 900 °C for 1 h did not alter the phase purity of the coatings, and shifted the electrochemical parameters towards noble direction. Sintering of the coatings in vacuum lead to the formation of an adherent, stoichiometric HAP coating with enhanced corrosion resistance.     

Oxidation of a NiCrAlYSi overlayer with or without a diffusion barrier deposited by one-step arc ion plating

The coating systems of a NiCrAlYSi overlayer with or without a diffusion barrier (DB) were deposited by one-step arc ion plating (AIP). Isothermal and cyclic oxidations were performed at 1050 °C for 100 h and 100 cycles, respectively. The results indicate that the duplex coating system exhibits a more effective protection for the substrate, where thin and continuous scales are adhered to the overlayer surface, and very limited oxidation and interdiffusion attacks are detected. After exposure for 100 h or 100 cycles, the DB is stable and efficient. The oxidation resistance mechanism for the two coating systems is discussed.     

HVOF喷涂WC-10Co-4Cr涂层的砂浆冲蚀行为

以2种不同的HVOF喷涂系统和3种商用粉末在0Cr13Ni5Mo基材上喷涂6种WC-10Co-4Cr涂层,用自行研制的料浆罐冲蚀设备对涂层和基材的砂浆冲蚀行为进行了研究,分析了显微结构和显微硬度对涂层冲蚀性能的影响及涂层冲蚀后的表面形貌.结果表明,在文中试验范围内涂层和基材的冲蚀失重均随冲蚀时间线性增加;各攻角下,WC-10Co-4Cr涂层的耐冲蚀性远远优于0Cr13Ni5Mo不锈钢;除了分层和裂纹明显的涂层,HVOF热喷WC-10Co-4Cr涂层的冲蚀失重表现出对攻角不敏感的特征.涂层的冲蚀机制为粘结相的微切削和涂层片状剥落,低攻角下的剥落坑面积大而浅,高攻角时面积小而深,片状剥落的程度随涂层气孔率的增加而增加.