Pure zinc phase

The materials used were electrolytic-grade nickel (>99.98 mass % Ni) and cobalt (>99.98 mass % Co) plates and granules of chemically pure zinc (>99.94 mass % Zn). To ensure intimite contact between their constituents and favourable conditions for phase nucleation, the Ni-Zn and Co-Zn reaction couples were prepared by the interaction of solid nickel or cobalt and molten zinc, with their subsequent joint cooling until the solidification of the liquid phase. 

Zm (aq) + 2e = Zn(s) a zinc activity in pure zinc solid phase is equal to a unit. It follows from the text above that for a hydrogen electrode, the electrode potential can be written as in Eq. (8) ... 

No term for elemental zinc is included in the logarithmic term because it is a pure seeond phase (solid). Thus, the electrode potential varies linearly with the logarithm of the reciprocal of the zinc ion concentration. 

Zinc also forms one oxide ZnO and, therefore, a single phase, single-layered scale is expected when pure zinc is oxidized. However, ZnO is an n-type cation-excess... 

More than 10 years experience with thermally sprayed coatings of zinc-15% aluminum is now available. As a result of the two-phase structure (a zinc-rich and an aluminum-rich phase) of such coatings, the oxidation of the active zinc-rich phase occurs within the more inert aluminum-rich phrase. Oxidation products are encapsulated in the porous layer formed by the latter and do not build up a continuous surface layer as with pure zinc coatings. Consequently, no thickness or weight loss is observed even after several years of exposure in atmospheric field testing. 

Silver is much more soluble in zinc than in lead. In the Parkes process, zinc is added to a lead melt A silver-rich zinc phase forms on the surface and is drawn off When zinc is removed by distillation, raw silver remains. Very pure silver is obtained by a subsequent electrolytic process. 

In the hot-dip galvanising process (hot-galvanising), layer thicknesses of 0.040-0.150 mm are produced on the steel surface. This coating consists of an iron-zinc alloy layer at the phase limit to the steel and a layer of pure zinc over that. The protective effect of the coating is determined by the layer thickness of both layers. In the initial stress load phase, covering layers are formed by the reaction of the zinc coating with the seawater. After this, a stationary phase of corrosion is reached with mainly linear corrosion rates at an order of magnitude of 0.010-0.012 mm/a (0.39-0.47 mpy). 

In thermal-sprayed zinc coatings, layer thicknesses of 0.050-0.150 mm are the maximum reached. These layers consist mainly of pure zinc, with a certain amount of zinc oxide and pores depending on the layer thickness and spraying technique. The much lower protective effect of spray galvanisation at the same layer thickness is probably due to the lack of the iron-zinc alloy phase. 

Surface Finishes. Galvanized coatings are commonly characterized by surface spangles. In cross section, an Fe2Al (Zn) inhibition layer develops first, preventing any iron-zinc intermetallic phase formation. The overlay layer is made up of dendrites of pure zinc (t ) phase and appears as a poly-crystaUine structure. The three surface finishes commonly produced are ... 

Tin will protect copper from corrosion by neutral water. Pure tin is anodic to copper, and protects discontinuities by sacrificial corrosion. Both intermetallic phases are strongly cathodic to copper, and corrosion is stimulated at gaps in wholly alloyed coatings. An adequate thickness of tin is needed for long service, e.g. 25-50 xm. Another diffusion problem occurs with tin-plated brass. Zinc passes very quickly to the tin surface, where under conditions of damp storage zinc corrosion products produce a film... 

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