New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Abstract

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.

The precipitation of β-FeC₂O₄.2H₂O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.

Acicular shaping of the β-FeC₂O₄.2H₂O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.     

Bright zinc–nickel alloy deposition from alkaline non-cyanide bath

Abstract

Zinc–nickel alloy deposition with polyvinyl alcohol and piperonal as bath additives has been investigated. The effects of additive concentration, triethanolamine concentration and other parametric variables on the deposit and solution properties have been studied. Polarisation studies were carried out under different conditions to understand the effect of triethanolamine and the brighteners on the co-deposition. Results indicated that the additives lead to bright alloy deposition containing ～10% nickel. The deposit produced at the optimum conditions with the additives has a nanograined structure with a γ phase.     

Nickel or cobalt doped zinc oxide varistors

Abstract

Zinc oxide containing either Ni or Co dopants in various proportions has been investigated. Only zincite phase was found by XRD analysis. Incorporation of Ni or Co into the ZnO structure resulted in a marked shift in the XRD peaks. Remarkable microstructural uniformity was obtained by the addition of 1 mol-% of either Ni or Co oxides. Densification was achieved through atomic diffusion along grain boundaries. All samples showed non-linear I–V dependence. Ni concentration had a non-monotonic effect on breakdown voltage and the non-linear coefficient decreased with increasing Ni content. In contrast, breakdown voltage and non-linear coefficient increased with increasing CoO content.     

Adhesion of electroless nickel plating on polished silicon

This work reports a study of the adhesion of electroless nickel plating on polished silicon as a function of several plating variables, namely substrate resistivity, activator composition, plating duration, and plating bath temperature. It is shown that an adhesion strength of ～100 kg/cm² or more can be obtained, irrespective of the silicon doping, for plating used in semiconductor devices. However, for this purpose, an appropriate silicon activation step is essential, except when the silicon is heavily N-type doped.     

New experience in field of application of corrosion resistance protection in flue gas desulphurisation plant absorbers

Abstract

Absorbers form the main part of the flue gas desulphurisation plants for 360 MW units at the Belchatow Power Station in Poland. The plant working cycle and service life are dependent on the grades of the corrosion resistance, protection materials used as an absorber lining in individual zones. The objective of manufacturers activities in this field has been to extend the service life of the flue gas desulphurisation plant. This has been implemented by testing 'wallpapering' technology in the absorber cylindrical part, using plate sheets of Hastelloy nickel alloys, 0·5 to 1·6 mm in diameter. Experiments involving Hastelloy nickel alloys, grades C-22 and C-2000, among others, were used to evaluation the resistance of the base material, 'wallpaper' plate sheet, welded joints and heat affected zones (HAZ) towards corrosion. The corrosion tests were executed for three different conditions of corrosive environment: (a) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 10 wt-%K₂SO₄; a testing temperature of 70°C; and a testing time of up to 1000 h; (b) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 6 wt-%K₂SO₄ and 4 wt-%NaCl; a testing temperature of 70°C; and a testing time of up to 1000 h; (c) simulated waste incineration environment, produced during thermal utilisation of wastes with the following chemical composition: N₂–9%O₂–0·08%SO₂–0·2%HCl; testing temperatures of 450 and 600°C; and a testing time was up to 1150 h. For the purpose of comparison, corrosion resistance tests were carried out on boiler steel types T22, E911, HCM12 and TP347H. This paper includes results of the evaluation of welded joints structure and HAZ, joint corrosion resistance characteristics, corrosion product test results, as well as requirements and recommendations with regard to fabrication of welded joints, the preparation degree of lining plate join surfaces, and requirements in field of weld joints.     

High temperature soldering of SiC particulate aluminium matrix composites (series 2000) using Zn–Al filler alloys

Abstract

The present paper evaluates the solderability of three discontinuously reinforced aluminium matrix composites. The tested composites were an aluminium alloy of the 2000 series (AA2014) reinforced with different percentages of silicon carbide particles (6, 13, and 20 vol.-% respectively). A similar study was carried out on the unreinforced aluminium alloy for comparative purposes. Three low temperature filler alloys of the Zn–Al system were used for soldering. Drop formation tests were performed to evaluate the wettability of the molten filler alloys and sample joints (single overlap specimens) were produced to determine solder penetration in the joint clearance. Microstructural studies of the joints were carried out to determine the effects of the solid reinforcement particles on molten pool behaviour and solidification mechanisms.     

TEM study of nanopores and the embrittlement of CVD nickel foam

Abstract

Ni foam, manufactured by depositing Ni onto polyurethane foam using the carbonyl CVD process, exhibits brittleness which has been correlated with the presence of a high number-density of 'nanopores'. These nanopores, which were typically less than ~25 nm in diameter, tended to be associated with sulphur, which is present in the form of H₂S as a catalyst during the deposition process. The as deposited foam also shows significant hardening, ascribed to the high density of nanopores. This hardening may also help to reduce the ductility compared to well annealed nickel foam. High temperature annealing restores the foam to the high ductility typical of high purity nickel by coarsening the nanopores and removing the sulphur. The failure of the pores to sinter during high temperature annealing indicates the presence of stable gas molecules, the pressure from which balances the surface tension forces that drive sintering.     

Corrosion resistance of Ni/Al2O3 coatings in NaCl solution

Abstract

This paper presents results of a research on the corrosion resistance of composite Ni/Al₂O₃ coatings electrochemically deposited from Watts baths containing different amounts (0, 20, 100 g dm^?3) of Al₂O₃ particles. Potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 3% solution of NaCl. The coatings with about 6 wt-% of corundum, deposited from a bath with 100 g dm^?3 of a powder, showed the best protective properties. The rate of corrosion of such coatings after 7 days of exposure in the NaCl solution was over two times slower than that of coatings containing 2 wt-% of Al₂O₃ and six times slower than that of a standard nickel coating. Two equivalent circuits: one consisting of two RC circuits and the other one made up of three RC circuits were used for the analysis of the impedance spectra. Regardless of the presence and amount of the Al₂O₃ particles in the nickel coating, during first day of exposure in the NaCl solution a layer of nickel oxides and hydroxides forms on the surface of the coatings increasing their corrosion resistance. In the case of coatings with 6 wt-% of Al₂O₃, the passive layer is least vulnerable to the aggressive action of Cl^? ions.     

Solidification paths in modified Inconel 625 weld overlay material

Abstract

Inconel 625 is commonly used for overlay welding to protect the base metal against high temperature corrosion. The efficiency of corrosion protection depends on effective mixing of the overlay weld with the base metal and the subsequent segregation of alloy elements during solidification. Metallographic analysis of solidified samples of Inconel 625 with addition of selected elements is compared with thermodynamic modelling of segregation during solidification. The influence of changes in the melt chemistry on the formation of intermetallic phases during solidification is shown. In particular, focus is put on how the composition of the dendrite core is affected by modifications to the alloy. It has previously been shown that when the overlay material corrodes, the corrosion take place in the dendrite core. Therefore, the discussion will be directed towards explaining the extent to which the variations in chemical compositions influence the composition of the dendrite core of the weld overlay.     

Microstructure,mechanical properties and tribological behaviour of PM Fe-Cu-Zn alloys containing solid lubricants

Abstract

Fe-Cu-Zn alloys containing solid lubricants of graphite and talc produced via cold pressing and sintering technology are investigated. The influence of composition and sintering temperature on open porosity, density, strength and hardness of these alloys is studied. The microstructure is correlated with the mechanical properties of the sintered materials. The tensile tests showed that the peak strength was observed for samples sintered at ≤1000°C. Above this temperature the bending and tensile strength values decrease. Microstructural analysis of sintered materials revealed three phases in the structure: α-Fe, α-brass and talc. Results indicate that the microstructure of sintered samples is sensitive to process variables such as brass and talc contents and sintering temperature.