In this study, HVOF spraying technology was employed to deposit the Ni–Mo alloy coating on nodular cast iron substrate. Post–spray annealing treatment was used to modify the microstructure and corrosion property of Ni–Mo coating. The microstructural evolution and corrosion resistance to HCl of both the as-sprayed and annealed Ni–Mo coatings were investigated. Results show that an unoxidized Ni–Mo coating with the porosity of 0.62% and bonding strength of 58.8 MPa can be achieved by HVOF. Electrochemical tests indicate that the influence of temperature on corrosion behavior of the as-sprayed Ni–Mo coating in 15wt.% HCl solution follows the order: 40 °C > 60 °C > 20 °C. With increasing the concentration of HCl solution, corrosion resistance of the as-sprayed Ni–Mo coating decreases. Post–spray annealing treatment has a significant effect on the microstructural evolution of Ni–Mo coating, including interfacial bonding quality, coalescence of micro-voids, phase transformation within coating and element diffusion reactions from substrate into coating. Annealing treatment accelerates the diffusion of Fe and C from nodular cast iron substrate into Ni–Mo coating with forming Mo2C and NiFe phases, which results in the enrichment of Mo. The 850 °C annealed Ni–Mo coating possesses the best corrosion resistance in 15wt.% HCl solution, which is due to the improved interfacial bonding and the low extent of Fe/C diffusion from substrate into coating, whereas the dramatical reduction of corrosion resistance of the 950 °C annealed case can be mainly attributed to the large-scale formation of NiFe and Mo2C phases within coating. 

HVOF sprayed Ni-Mo coatings improved by annealing treatment: Microstructure characterization, corrosion resistance to HCl and corrosion mechanisms

Abstract The active mechanisms in the precipitation of nickel powder by hydrogen reduction were investigated by means of mathematical models based on the moment form of the population balance equation (PBE). The objective of the work was to establish the mechanisms involved in powder formation and how these are affected by the presence of impurities. The effects of two major impurities were considered namely; iron, inherently present as ferrous sulphate, and a morphology modifier, a polyacrylic acid derivative used as an additive. Experiments were conducted on a laboratory and pilot-plant scale using a 0.5 and 75 L stainless steel autoclave, respectively. Nickel powder samples were collected from the autoclaves after each successive batch reduction (densification) and the particle size distribution (PSD) analysed using a laser diffraction technique. The PSD data was then transformed into moments and the experimental values were compared with those simulated using different models based on the moment form of the PBE. Five models were tested namely: (a) aggregation-only; (b) aggregation and growth; (c) nucleation and aggregation; (d) nucleation and growth and (e) aggregation and breakage. Under standard operating conditions, the process was best simulated by a size-independent aggregation and growth model in the early stages of the cycle, with breakage and growth becoming significant in later stages of the cycle when large particles have been formed. Crystallisation in the presence of Fe was characterised by a size-independent aggregation and growth model with varying degrees of nucleation depending on the Fe concentration and the available surface area. At modifier dosages of 0.25 and 5 vol% the process was best modelled by a size-independent aggregation and growth model coupled with a constant breakage frequency model. Based on the mathematical modelling results and evidence from scanning electron micrographs, the spherically shaped nickel powder particles were proposed to be formed through the formation of a pre-cursor by secondary aggregation followed by spherulitic growth. The degree of compactness of the spherulites was proposed to be determined by the number of active growth sites on the nickel particle surface. The morphology modifier was found to act as a growth inhibitor, decreasing the number of growth sites leading to more open spherulites. Iron was found to induce surface nucleation, thus, creating more growth sites on the particle surface and leading to more compact spherulites.

Nickel powder precipitation by high-pressure hydrogen reduction

A Novel Processing Route for Ni-doped Alumina Composites

Ultrafine Co powder with an average particle size below 0.1μm was prepared by hydrogen reduction from the basic Co(OH)2 slurry, which was obtained by mixing the solution of CoSO4 with NaOH. The pH of the slurry was controlled between 8 and 12. The filtration and washing of the slurry were not necessary before hydrogen reduction. The proposed reduction temperature was 170(200oC, concentration of catalyst PdCl2 2 mg/L, partial pressure of H2 was 2(3 MPa, and reduction time about 40 min. Temperature and concentration of PdCl2 showed obvious effects on the reaction, and pH of the end solution showed major effect on particle size of Co powder obtained.

Preparation of Ultrafine Co Powder from Basic Co(OH)_2 Slurry by Hydrogen Reduction

The nickel powder samples obtained at different reaction steps of the hydrogen reduction of Ni(OH)2 slurries were examined by XRD, TEM, SEM and other methods. The obvious differences between samplings from acidic medium with pH~3 and alkaline medium with pH12 were observed. The mechanism of hydrogen reduction of Ni(OH)2 slurry with pH3 was proved to be a reduction of Ni2+ ions dissociated from basic nickel sulfates like Ni(SO4)0.3.(OH)1.4 but for Ni(OH)2 slurry with pH12 the mechanism was confirmed to be a heterogenous reaction of Ni(OH)2 solid particles and activated hydrogen occurred at the surface of Ni(OH)2.

Haitao Yang

Papers

Research progress in preparation of metal powders by pressurized hydrogen reduction

A review of the design, properties, applications, and prospects of Ni-based composite powders