E. Beltowska-Lehman

Electrodeposition of Zn-Ni protective coatings from sulfate-acetate baths

Abstract The electrodeposition conditions for Zn–Ni alloys from sulfate–acetate electrolytes have been studied with the view of preparing protective coatings. The influence of electrolyte composition (different [Ni(II)]/[Zn(II)] ratios, pH, buffer), cathode current density, cathode potential and hydrodynamic conditions on the composition of coatings, cathode current efficiency and corrosion resistance were determined. For all of the conditions examined, strong inhibition of nickel reduction with simultaneous increase in the rate of zinc discharge characteristic of an anomalous system, have been observed. The Zn(II) discharge becomes diffusion-controlled at more negative cathode potentials, whereas the partial nickel current densities are independent of electrode rotation speed. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer. An increase in pH above 3.3 causes a significant catalysis of Zn–Ni deposition with a simultaneous decrease of the nickel in coatings. This effect may be related to the formation and increasing concentration of Zn(II) and Ni(II) acetate complexes in this condition. The Zn–Ni coatings obtained (5–18% Ni) characterise improved corrosion resistance in comparison to Zn layers deposited under the same conditions.

Effect of complex formation on the diffusion coefficient of CuII in citrate solution containing NiII and MoVI

An attempt has been made to account for the differences in the values of the observed diffusion coefficient of CuII at a transition from a separate reduction to a codischarge with Ni and Mo. For this purpose experiments were carried out in a wide range of pH for various electrolyte compositions, combining them with the analysis of species occurring in these solutions and with the analysis of different models of mass transport to the cathode surface. The observed results indicate that in the range pH = 6–8, the changes in the values of the limiting currents of Cu occurring in the presence of nickel ions are associated with the formation of a inert heteronuclear complex [CuNi(cit)2]4−. The obtained results also help to account for the differences in the value of the diffusion coefficient of CuII obtained in such electrolytes by other researchers. They also form a basis for the preparation of optimum bath compositions and for further explanation of the electrodeposition mechanism of NiCu and NiCuMo alloys.

Influence of structural factor on corrosion rate of functional Zn–Ni coatings

Abstract The electrocrystallization processes of Zn–Ni coatings from weakly acid sulfate solutions with the addition of citrate and acetate complexing compounds have been compared. The deposition kinetics and the influence of electrolyte composition on the Zn–Ni alloy composition and on some physicochemical properties (corrosion resistance, texture, phase composition, morphology) were determined. It was found that the properties of coatings obtained under the same operating conditions in electrolytes with the same [Ni(II)]/[Zn(II)] concentration ratio and pH but containing different complexing agents were different. The composition of the deposits and their texture as well as phase structure were found to depend on the complexing species present. Coatings deposited from acetate solutions are richer in Ni than those from citrate solutions. The corrosion resistance of a Zn–Ni alloy depends in practice on the percentage of Ni, but in sulfate–citrate electrolytes, layers containing less Ni were obtained with the same corrosion resistance as Ni-rich coatings deposited from acetate solution. The corrosion current decreases more rapidly with increasing Ni content. This effect is connected mainly with phase composition. In citrate solutions practically single-phase (hexagonal η) deposits were obtained which provide better protection with a lower corrosion rate in comparison with alloys consisting of three phases (hexagonal η, monoclinic δ, regular γ) deposited from acetate baths.

Electrodeposition of protective Ni–Cu–Mo coatings from complex citrate solutions

Abstract Ni–Cu–Mo ternary alloy deposition was investigated in order to obtain protective corrosion-resistant coatings. The present work is aimed at the determination of the influence of co-discharge kinetics on certain physicochemical properties of Ni–Cu–Mo coatings electrodeposited from complex ammonia–citrate baths. A schematic diagram of partial polarisation curves as well as the coating composition as a function of the cathode polarisation is presented. Based on the dominant species in the baths examined and the polarisation behaviour, the main reduction reactions in four ranges of cathode potential have been defined. A strong correlation between alloy composition and cathode potential was observed, due to the Ni–Cu–Mo electrocrystallisation kinetics being dominated by the induced Ni–Mo co-deposition. Bright compact metallic layers with a regular nodular morphology were obtained at more negative potentials where the nickel discharge is the predominant reaction. The X-ray diffraction patterns of all electrodeposited Ni–Cu–Mo coatings show the fcc structure of a Ni–Cu solid solution. The coatings show corrosion resistance similar to bulk commercial Monel alloy.

Pulse electrodeposition of NiCuMo alloys

Abstract Pulse plating of NiCuMo alloy has been investigated using a series of planned experiments. A domain of pulse parameters has been defined where the coating quality is optimal. The current efficieny and the Mo content can be larger than in d.c. deposition especially at low average current density. Only the copper content in the deposited alloy can be described by a linear function of the pulse parameters (average current density, frequency and duty cycle). Owing to complex kinetics and interactions during the codischarge, the molybdenum and nickel contents do not obey simple analytical dependences on pulse parameters.

Effect of the electrode supply mode on the chemical composition gradient of permalloy thin films

Abstract This study was aimed at investigating the production of uniform permalloy films by electrodeposition from acid sulphate baths. It was found that non-stationary electrolysis (pulsed and programmed) and a constant voltage supply favoured stability of the film chemical composition. With a pulsed current supply the film thickness corresponding to a fixed composition was one-half that obtained with a d.c. supply, the pulse shapes and frequencies being of minor significance. With a programmed current supply the composition was established four times as quickly as with a constant current supply. Under constant voltage conditions the uniformity of the films was improved by decrease in the cathode potential. For relatively low potential values the chemical composition of the film can be stabilized at a thickness which is ten times smaller than with a d.c. supply.

Structure Characterization of Ni/Al2O3 Composite Coatings Prepared by Electrodeposition

In the present study, the electrodeposition of composites consisted of metal matrix (nickel) and inert particles (hard nano-sized Al2O3 oxide) has been carried out in a Watt’s type bath of pH 4, at room temperature in a system with the steel rotating disk electrode. The influence of dispersed Al2O3 powder on structure characteristics (morphology, phase composition, texture, residual stresses) of Ni/Al2O3 coatings has been investigated. The crystallographic texture of Ni and Ni/Al2O3 coatings deposited on the steel substrates was analyzed by XRD technique based on the back-reflection pole figures. The “sin2ψ” X-ray diffraction method was used to determine the residual stress as a function of X-ray penetration depth. The influence of Al2O3 particles on the value of the Ni coating microhardness was also analyzed.

X-ray diffraction grazing-incidence methods applied for gradient-free residual stress profile measurements in electrodeposited Ni coatings

The present work investigates the possibility of bias introduced in grazing-incidence-angle X-ray diffraction techniques applied to residual stress measurements. In these studies, monotextured nanocrystalline nickel coatings obtained by electrodeposition were examined as the model reference samples. Selected Ni coatings exhibited well developed and simple gradient-free residual stress states that were visible using conventional sin2ψ measurements with varying X-ray penetration depths. These results were verified against the stress state picture obtained by two variants of grazing-incidence X-ray methods: multi-reflection (different hkl) and constant angle of incidence (single hkl). The outcome of both grazing techniques consistently excluded stress gradients in the samples, which agreed with conventional sin2ψ measurement results. However, only the results of the constant angle of incidence technique agreed with those obtained by the sin2ψ method in terms of calculated residual stress level, suggesting this approach could be applied in further studies of graded material coatings. All analysed coatings yielded uniformly distributed tensile residual stress related to gradual structure development in electrodeposited Ni coatings studied by electron microscopy techniques.

Microstructural characterisation of electrodeposited coatings of metal matrix composite with alumina nanoparticles

In the present work a nanocrystalline Ni-W metallic matrix was used to fabricate Ni-W/Al2O3 composite coatings. The MMC (metal matrix composite) coatings with inert α-Al2O3 particles (30 - 90 nm) were electrodeposited from aqueous electrolytes under direct current (DC) and controlled hydrodynamic conditions in a system with a rotating disk electrode (RDE). The chemical composition and microstructure of electrodeposited composites mainly control their functional properties; however, the particles must be uniformly dispersed to exhibit the dispersion-hardening effect. In order to increase the alumina particles incorporation as well as to promote the uniform distribution of the ceramic phase in a matrix, outer ultrasonic field was applied during electrodeposition. The influence of embedded alumina nanoparticles on structural characteristics (morphology, phase composition, residual stresses) of the resulting Ni-W/Al2O3 coatings was investigated in order to obtain a nanocomposite with high hardness and relatively low residual stresses. Surface and cross-section morphology and the chemical composition of deposits was examined in the scanning electron microscope, the EDS technique was used. Microstructure and phase composition were determined by transmission electron microscopy and X-ray diffraction. Based on microstructural and micromechanical properties of the coatings, the optimum conditions for obtaining crack-free homogeneous Ni-W/Al2O3 composite coatings have been determined.

Microstructure and Deposition Relations in Alumina Particle Strengthened Ni-W Matrix Composites

The nanostructured Ni-W/Al2O3 composite coatings were prepared by electrodeposition technique on ferritic steel substrates from aqueous electrolyte solutions containing ultrafine alumina particles in suspension. The effects of plating parameters like current density, inert particle concentration in plating bath and ultrasonic field frequency on the incorporation of α-Al2O3 particles (TM-DAR Taimicron) into an Ni-W matrix were investigated. The MMC coatings microstructure, phase and chemical composition were studied by means of scanning (E-SEM FEI XL-30) and transmission (TECNAI G2 SuperTWIN) electron microscopies, as well as XRD measurements (Bruker D8 Discover). SEM and TEM observations of composite cross-section microstructure showed that the presence of ultrasounds considerably reduces the particles agglomeration and enables a uniform distribution of particles in the Ni-W matrix. The electron diffraction pattern analysis revealed that the composite metallic matrix consists of an α-Ni(W) solid solution. The matrix was characterized by quasifibrous, nanocrystalline grains of an average size about 10 nm.