Non-equilibrium phase composition of ZnNi alloys prepared by electrocrystallization
|Marek S. Michalec 1, Zbigniew Świątek 2, Piotr Ozga 2|
1. Jagiellonian University, Faculty of Chemistry, Ingardena 3, Kraków 30-060, Poland
Electrodeposited zinc coatings are widely used for corrosion protection of steel. Apart from chemical composition, the corrosion resistance of Zn–Ni coatings depends mainly on their microstructure, phase composition and structural parameters (texture and crystal lattice imperfections). The zinc–nickel coatings were electrodeposited on steel substrates from sulfate solutions with the addition of sodium citrate Na3Hcit (cit=C6H4O7) and ammonium sulfate. The influence of electrolyte composition (different [Ni(II)]/[Zn(II)] ratios, pH, buffer), cathode current density and hydrodynamic conditions on the chemical and phase compositions of coatings were determined.
From X-ray diffraction studies, it is evident that the composition of the phase occurring in as-deposited Zn-Ni alloys varies greatly with the deposition conditions and do not corresponds to those obtained at thermodynamic equilibrium. Moreover, various Zn-Ni phases appear with the same mean Ni content. For the deposits with Ni content up to 13% in Zn-Ni alloys electrodeposited in presence of acetate ions, the meta-stable and distorted h-phase crystallizing hexagonally is dominating, whereas alloys with the highest Ni content consist mainly of the cubic g-phase. On the contrary in citrate solution practically single-phase (hexagonal h) deposits were obtained in whole range of Ni content. Deposited alloys are not in thermodynamic equilibrium and such phase situation changes with time, leading the whole system to the equilibrium state.
The aim of this paper is the structural characterization of the electrodeposited Zn-Ni phases in stable and meta-stable state. The investigations were carried out on the as-deposited, 4-years natural aged and annealed alloys. Structural changes of the hexagonal Ni-rich h-phase and Zn-rich cubic g-phase caused by crystallization and/or recrystallization processes are analyzed. The phase diagram of electrodeposited Zn-Ni alloys is proposed.
Presentation: Poster at E-MRS Fall Meeting 2007, Symposium D, by Marek S. Michalec
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-11 11:22 Revised: 2009-06-07 00:44