Microstructure and corrosion behavior of electrodeposited nickel prepared from a sulphamate bath

Electrodeposited nickel was prepared from a sulphamate bath at different current densities ranging from 0.01 A cm^− 2 to 0.1 A cm^− 2. Based on the analysis of the microstructure, the corrosion behavior of the electrodeposited nickel in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). All the electrodeposits display active-passive-transpassive behavior in potentiodynamic polarization process. The electrodeposits with the best corrosion resistance are obtained at 0.05 A cm^− 2. As for other electrodeposits, the corrosion potential and breakdown potential decrease with increasing current density used to prepare electrodeposits. However, the variation of both corrosion current density and passive current density is opposite to that of the corrosion potential. The changes in the charge-transfer resistance determined from the impedance spectra are consistent with the results determined from potentiodynamic measurements.     

电沉积Co-Ni合金镀层结构及硬度的研究

采用氨基磺酸体系电解液电沉积Co-Ni合金，研究了电解液中钴、镍金属离子浓度与合金镍层中钴含量的关系。利用扫描电子显微镜和X射线衍射分析测定了不同钴含量沉积层的微观形貌和晶体结构，同时研究了合金沉积层显微硬度和合金成份的关系及热处理的影响。实验结果表明：电解液中Co^2 /(Co^2 Ni^2 ）在0.1-0.5的范围内时，钴离子的优先共沉积的趋势最强。SEM观察结果和XRD分析测试表明，随着钴镍合金沉积层中钴含量的不断增加，低钴含量合金层粗大的颗粒状结晶逐渐转变为细致、均匀的三角形状结晶，最终又形成中等大小的颗粒状结晶。同时合金层中钴含量的逐渐增加，结构由fcc镍固溶体过渡为fcc的钴固溶体，最后转变为hcp的钴固溶体。并且在形成两个钴固溶体的钴含量范围内（20％－50％和60％－80％），所对应的钴镍合金层的硬度值远大于低钴含量区所具有fcc镍固溶体结构的合金层硬度。     

二元配合物中电镀锌镍合金的结构与抗腐蚀性

为深入了解碱性锌酸盐体系的工艺参数对锌镍合金镀层结构和耐蚀性的影响,在以四乙烯五胺(TEPA)为镍离子主络合剂、三乙醇胺(TEA)为辅助络合剂的碱性镀液中电沉积制备了锌镍合金镀层,利用电子能谱、X射线衍射、极化曲线和电化学阻抗谱等方法表征镀层的组成结构和在氯化钠溶液中的耐腐蚀性.结果表明:镀层含镍原子数分数11.54%~20.12%,为γ-Ni2Zn11+纯Zn两相结构(低含镍原子数分数时)或单一γ相结构(较高含镍原子数分数时),γ相晶粒在(600)方向上具有不同程度的择优取向性;随着镀液中镍原子数分数的提高,镀层的腐蚀电位正移,阻抗增加,耐蚀性提高;当电流密度为2 A/dm2时,镀层的腐蚀电位和电荷传递电阻最高,耐蚀性最好.     

Enhanced tensile ductility in bulk nanocrystalline nickel electrodeposited by sulfamate bath

Electrodeposition is one of the fabrication techniques to produce nanocrystalline materials. In this paper, bulk nanocrystalline Ni (nc-Ni) was electrodeposited by using a sulfamate bath which generated low residual stress. In order to enhance tensile property of bulk nc-Ni, we investigated influences of glossing agents and bath condition on tensile properties, as these are reported to have an influence on surface condition, grain size and microhardness. It was found that saccharin contents and current density have significant effects on tensile properties of bulk nc-Ni. Moreover, we successfully obtained bulk nc-Ni displaying tensile ductility of over 10%. In particular, bulk nc-Ni from sulfamate bath with 5.0 g/l saccharin exhibited superior plastic deformation and good tensile strength (UTS = 1.2 GPa, ε_f = 15%). We were able to develop the relationship between tensile strength and ductility to a higher level on nc-Ni.     

Microstructural characterization of cast nickel aluminium bronze

The morphology and chemical analysis of the complex phases present in cast nickel aluminium bronze, of nominal composition 10% aluminium, 5% nickel and 5% iron, have been investigated using optical and electron microscopy techniques and energy dispersive analysis. It has been shown that , and four forms of can exist in the ascast microstructure of this alloy. Heat treatment can lead to the precipitation of a further phase which differs in morphology and chemical composition to those present in ascast structures.     

Characterization of zinc-nickel alloy electrodeposits obtained from sulphamate bath containing substituted aldehydes

Zinc alloy offers superior sacrificial protection to steel as the alloy dissolves more slowly than pure zinc. The degree of protection and the rate of dissolution depend on the alloying metal and its composition. Zinc-nickel alloy may also serve as at less toxic substitute for cadmium. In this paper the physico-chemical characterization of zinc-nickel electrodeposits obtained from sulphamate bath containing substituted aldehydes was carried out using hardness testing, X-ray diffraction, and corrosion resistance measurements. The corrosion behaviour of these samples in a 3.5% NaCl solution was examined. The decrease inI _corr and high charge transfer resistance indicated the improved corrosion resistance of these deposits.     

Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films

Based on previous work that identified an electrodeposited composite, multi-layer structure as a viable method of producing eutectic Au/Sn alloys for solder applications, a study of individual phase formation was undertaken. The AuSn phase, because of its higher deposition current (>2.0 mA cm⁻²), has a much faster deposition rate than Au₅Sn, which is deposited at <1.0 mA cm⁻². AuSn formation is growth controlled, while Au₅Sn formation is nucleation controlled. The AuSn forms a continuous layer within 60 s with a grain size of 50–75 nm. Because of the high deposition current, the dominant formation mechanism is two-dimensional nucleation, resulting in a relatively rough surface finish. Au₅Sn, on the other hand, forms a continuous layer within 600 s with an average grain size of 200 nm. Because of the significantly lower deposition current, the dominant formation mechanism is lateral spreading instead of two-dimensional nucleation. The result is a very smooth finish on the deposit surface.     

Microstructural, compositional and magnetic characterization of electrodeposited and annealed Co-Pt-based thin films

Near-equiatomic Co-Pt thin films with thicknesses of 520 nm were deposited from a single electrolyte onto glass-based Au-coated substrates using the electrodeposition method. The as-deposited Co-Pt-based films were annealed at temperatures from 500 ºC to 700 ºC for 1 h. The phase formation, microstructure and the magnetic properties were analyzed. It was found that with an increase of the annealing temperature the coercivity increases up to 1.18 T due to the transformation to the L1₀ phase. In contrast to the coercivity the saturation magnetization and the remanence were found to decrease upon annealing, due to the decrease of the Co concentration in the Co-Pt film because of the oxidation and the interdiffusion reactions between the Co-Pt thin film and the substrate.     

Abrasion and erosion wear of electrodeposited nickel ‐ silicon carbide nanocomposite coatings

A nickel matrix with silicon carbide nanoparticles composite coating is prepared by electrodeposition on non‐alloyed steel, assisted with an ultrasonic treatment of the galvanic bath. The abrasion and erosion wear resistance of the coating is determined, as well as its micro‐hardness. The structure and the composition of the nickel nanocomposite coating is defined using scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The corrosion behavior is investigated by the potentiodynamic polarization methods in the 3.5 wt% sodium chloride solution. The results show improved mechanical properties, wear and corrosion resistance as compared to the nickel metallic coating.     

Preliminary characterization of electrodeposited W2C coatings for wear applications

The recent development of hard adherent W₂C coatings by the simultaneous electrochemical reduction of tungstate (WO₄^2-) and carbonate (CO₃^2-) ions in a molten fluoride bath is described. The elements tungsten and carbon thus formed on the cathode combine in a subsequent chemical step to form the carbide. The structure of the deposit depends on the length of the plating time as well as melt composition and electrochemical parameters such as current and voltage. In pin-on-disk-type wear tests some fragmentation of the coating and abrasive wear were observed.     

Distribution of sulphur and electrochemical properties of nickel sulphur coatings electrodeposited on the nickel foam as hydrogen evolution reaction cathodes

Nickel sulfur electrode was prepared on the nickel foam by electrodeposition method in a modified Watts bath into which thiourea, citric acid and a small amount of saccharin were added. The micrographs of various layers of nickel sulphur coatings were investigated by SEM. The distributions and mass contents of sulphur in nickel sulphur coatings were measured by EDX. The phase constitutions of the first layer of nickel sulphur coating were determined by XRD. The influence of current density on coating structures was studied. The hydrogen evolution potentials of various electrodes, including nickel sulphur coatings on foam and net nickel substrates, pure nickel foam and nickel net were measured. Results indicated that the distributions of sulphur contents in various layers are gradient and not homogeneous, the sizes of fine particles in various layers are not the same. XRD examinations show that the structure of the nickel sulphur coating is amorphous. With increase of current density the amorphous structures gradually increase and the crystal ones gradually decrease. In the range of sulphur contents from 18% to 18.8% the electrochemical activities of Ni–S coating electrodes are the highest. The hydrogen evolution reaction activity of amorphous Ni–S coatings on the nickel foam substrate is much higher than that on the nickel net substrate.     

Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings

High cooling rates during laser cladding of stainless steels may alter the microstructure and phase constitution of the claddings and consequently change their functional properties. In this research, solidification structures and solid state phase transformation products in single and multi layer AISI 431 martensitic stainless steel coatings deposited by laser cladding at different processing speeds are investigated by optical microscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM), ternary phase diagram, Schaeffler and TTT diagrams. The results of this study show how partitionless solidification and higher solidification rates alter the microstructure and phase constitution of martensitic stainless steel laser deposited coatings. In addition, it is shown that while different cladding speeds have no effect on austenite–martensite orientation relationship in the coatings, increasing the cladding speed has resulted in a reduction of hardness in deposited coatings which is in contrast to the common idea about obtaining higher hardness values at higher cladding speeds.     

Corrosion resistance of multilayer coatings of nanolayered Cr/Ni electrodeposited from Cr(III)-Ni(II) bath

In this study the corrosion resistance of chromium and nickel single layers and multilayer coatings of nanolayered Cr/Ni, electrodeposited from Cr(III)-Ni(II) baths on low carbon steel substrates, has been studied. The coatings were electrodeposited from a bath using pulse current and modulated agitation. The total thickness of single layer and multilayer coatings was fixed at 5 μm and multilayer coatings with different modulation wavelengths and Cr to Ni thickness ratio were electrodeposited. Corrosion behavior of coatings was then studied by using potentiodynamic polarization test and electrochemical impedance spectroscopy in 0.1 M H₂SO₄. The results showed that Cr and Ni single layers had low corrosion resistance due to the presence of surface cracks and pores, respectively. On the other hand, optimized 20 nm Cr/50 nm Ni multilayer deposition significantly improved corrosion resistance.     

Microstructural study of aluminide surface coatings on single crystal nickel base superalloy substrates

Abstract

Aluminide diffusion coatings are frequently employed to enhance the oxidation resistance of nickel base superalloys. However, there is a concern that the presence of an aluminide coating could influence the properties of the coated superalloy, especially in respect of fatigue behaviour. To understand the nature of the effects of surface coatings on the fatigue properties of superalloys, an understanding of microstructural development within both the coating and the coating/substrate interfacial zone during high temperature fatigue testing is necessary. This paper is concerned with microstructural changes in aluminide diffusion coatings on single crystal γ′ strengthened superalloy substrates during the course of high temperature fatigue testing. The ‘edge on’ transmission electron microscopy technique is employed to study cross-sections of two stage (aluminisation plus diffusion treatment) coated superalloy samples. The paper examines the degradation of the coating produced by phase transformations induced by loss of aluminium from the coating and/or aging of the coating. Aluminium removal both by interdiffusion with the substrate and by oxidation of the coating surface is considered. Microstructural development in the portion of the substrate influenced by interdiffusion with the coating is also discussed.

MST/1694     

Synthesis and characterization of nickel/barium hexa-aluminate composite coatings

Electrodeposition of nickel/barium hexa-aluminate (Ni/BHA) composite coatings has been carried out from a Watt’s bath on mild steel substrate. BHA powders with plate habit were synthesized by solution combustion synthesis followed by heat treatment to ensure complete conversion to the hexa-aluminate phase. Heat treated material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with X-ray analysis. The dispersion behaviour and stability of BHA suspensions with cationic and anionic surfactants at room temperature were studied by dynamic light scattering under different pH. The influence of BHA concentration in the electrolytic bath, deposition temperature, pH, current density and duty cycle on particle incorporation in the coatings were studied and conditions for maximum particle incorporation were established. Coatings with a roughness of about 0·4 μm were produced by using this technique. Effect of BHA content on microhardness was also investigated. A reasonably good thickness of the coatings was achieved in a given set of conditions.     

Microstructural development of an electrodeposited Ni layer

This study investigated the microstructural development of an electrodeposited Ni layer formed from an additive-free Watt's bath. The major texture component of the electrodeposited Ni layer is strong < 110> fiber. The electrodeposited Ni layer consists of epitaxial regions with a thickness of about 100 nm and fine columnar grains extending along the growth direction. The fine columnar grains contain a high density of twins parallel to the growth direction. These twins were formed at side {111} facets during the lateral growth of the electrodeposited Ni layer. We surmise that the twins formed in order to change the orientation so that the energy of boundary between the existing grain, on which twin related grains nucleated, could be reduced.     

Microstructural characterization of high velocity oxy-fuel sprayed coatings of Inconel 625

High velocity oxy-fuel (HVOF) thermal spraying is being increasingly used to deposit high quality surface coatings. In the present study HVOF spraying was used to deposit coatings of the Ni-based alloy Inconel 625 onto mild steel substrates and the structure of the sprayed coatings were related to the processing conditions employed. The microstructural characteristics of the deposits were investigated using X-ray diffraction together with optical, scanning electron and transmission electron microscopy. The as-sprayed microstructure was found to consist of Ni-based metallic regions together with oxides exhibiting the Cr2O3 and NiCr2O4 crystal structures. Transmission electron microscopy revealed that although the metallic regions were predominantly highly alloyed, Ni-rich grains (depleted in solute) were also present in all coatings. Three processing variables were examined: oxygen to fuel gas ratio, total gas flow rate in the gun and combustion chamber length. All were found to significantly influence the oxide contents of coatings as measured by X-ray diffraction methods. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructural characterization of nanocrystalline nickel produced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers are produced in pure Ni plates. The average crystallite size, root mean square (r.m.s.) microstrain, dislocation density, and stored elastic energy are determined by X-ray diffraction (XRD) line profile analysis. The average crystallite size obtained by XRD is compared with the grain size observed from transmission electron microscopy (TEM) image. The high-resolution TEM (HRTEM) micrograph confirms the presence of high density of dislocations obtained by XRD, and reveals that most of dislocations distribute at the subgrain boundaries with few inside the subgrains.