Development of electroless Ni–Zn–P/nano-TiO2 composite coatings and their properties

Electrodeposition Science and Technology in the Last Quarter of the Twentieth Century

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Ni-Zn and Ni-Zn-P alloys supported on Vulcan XC-72 are effective materials for the spontaneous deposition of palladium through redox transmetalation with Pd(IV) salts. The materials obtained, Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C, have been characterized by a variety of techniques. The analytical and spectroscopic data show that the surface of Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C contain very small, highly dispersed, and highly crystalline palladium clusters as well as single palladium sites, likely stabilized by interaction with oxygen atoms from Ni--O moieties. As a reference material, a nanostructured Pd/C material was prepared by reduction of an aqueous solution of PdCl(2)/HCl with ethylene glycol in the presence of Vulcan XC-72. In Pd/C, the Pd particles are larger, less dispersed, and much less crystalline. Glassy carbon electrodes coated with the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C materials, containing very low Pd loadings (22-25 microg cm(-2)), were studied for the oxidation of ethanol in alkaline media in half cells and provided excellent results in terms of both specific current (as high as 3600 A g(Pd)(-1) at room temperature) and onset potential (as low as -0.6 V vs Ag/AgCl/KCl(sat)).

Ethanol oxidation on electrocatalysts obtained by spontaneous deposition of palladium onto nickel-zinc materials.

On the electrodeposition of nickel-zinc alloys from a eutectic-based ionic liquid

The codeposition of Zn–Ni alloys from chloride bath has been studied by means of potentiostatic electrodeposition in the potential range −700 to −1100 mV vs Ag/AgCl, where both normal and anomalous codeposition occurs. Deposition of alloys of different composition, morphology and structure, depending on the cathodic potential, was found. Analysis of the partial current densities showed that the production of nickel rich alloys in the potential range −700 to −900 mV is due to the underpotential reduction of zinc, driven by nickel ion discharge. Morphological and microstructural analyses showed that these alloys have the face-centred-cubic structure of nickel (α phase) and that the addition of zinc in the nickel lattice causes internal stresses in the deposits, which are prevalently amorphous. At potentials more negative than −910 mV, corresponding to the equilibrium potential of the zinc rich γ phase deposition, the rate of deposition of the α phase decreases and the further increase in deposit zinc content leads to the formation of the γ phase, with a decrease in internal stress. In this range of potential, zinc and nickel reduction can occur separately, according to their respective exchange current densities.

Normal and anomalous codeposition of Zn–Ni alloys from chloride bath

Electroless Ni–Zn–P alloy coatings were obtained on an iron substrate from a sulfate bath at various pH values The effects of changes in bath pH on alloy composition, morphology, microstructure and corrosion resistance were studied Scanning electron microscopy was performed to observe the morphological change of the deposits with bath pH Coating crystallinity was investigated by grazing incidence asymmetric Bragg X-ray diffraction and transmission electron microscopy A transition from an amorphous to polycrystalline structure was observed on increasing the bath alkalinity, and thus decreasing the phosphorus content of the alloys A single crystalline phase corresponding to face-centred-cubic nickel was identified in the alloys obtained from a strong alkaline solution An increase in zinc percentage up to 23% in the deposits does not change the fcc nickel crystalline structure Corrosion potential and polarization resistance measurements indicated that the corrosion resistance of electroless Ni–Zn–P alloys depends strongly on the microstructure and chemical composition The deposits obtained at pH 90–95 and with 114–125% zinc and 118–112% phosphorous exhibited the best corrosion resistance

Microstructural characterization and corrosion resistance of Ni–Zn–P alloys electrolessly deposited from a sulphate bath

Electroless Ni–Zn–P alloy deposition from a sulphate bath, containing sodium hypophosphite as reducer, was investigated. To increase the plating rate, the deposition parameters were optimized. The effect of process parameters (T, pH and [Zn2+]) on the plating rate and deposit composition was examined and it was found that the presence of zinc in the bath has an inhibitory effect on the alloy deposition. As a consequence, the percentage of zinc in the electroless Ni–Zn–P alloys never reaches high values. Using cyclic voltammetry the electrodeposition mechanism of Ni–Zn–P alloys was investigated. It was observed that the zinc deposition inhibits the nickel discharge and, as a consequence, its catalytic activity on hypophosphite oxidation. It was also found that increase in temperature or pH leads to the deposition of nickel rich alloys.

G. Roventi

Papers

Normal and anomalous codeposition of Zn–Ni alloys from chloride bath

Microstructural characterization and corrosion resistance of Ni–Zn–P alloys electrolessly deposited from a sulphate bath

Ni–Zn–P alloy deposition from sulfate bath: inhibitory effect of zinc