G.D. Wilcox

Electrodeposited zinc alloy coatings

Abstract This review examines the range of zinc alloy electrodeposited coatings that are available as finishes for continuous steel strip and also possible replacements for electroplated cadmium. Each system is described in terms of available electrolytes, physical properties and corrosion resistance. It is apparent that zinc-nickel, zinc-iron and zinc-cobalt are the most widely utilized, although zinc-manganese, which appears to have excellent corrosion resistance, is in all probability unavailable commercially. Zinc-tin is also discussed, being recognized as another possible replacement for cadmium.

The rotating cylinder electrode: its continued development and application

The application of rotating cylinder electrodes (RCEs) in electrochemistry has been reviewed for the period 1982–1995. Among the applications highlighted are the novel design of cell geometries and reactors for a range of electrochemical processes, voltammetry and analysis, electrodeposition and corrosion. This range amply indicates the widespread acceptance of the RCE for studies in a number of interdisciplinary fields.

Methods for the reduction of shape change and dendritic growth in zinc-based secondary cells

Abstract The widespread adoption of alkaline zinc-based secondary systems has been hindered by two inherent problems associated with the charge reaction. These are commonly referred to as shape change and dendrite growth. This review examines the attempts that have been made to lessen their effects and, hence, ultimately arrive at a working, cycleable secondary zinc-based battery.

Preparation of nickel coated mica as a conductive filler

Electroless plating was used to prepare nickel coated mica fillers. To optimise the conductivity of the filler the nickel coating needed to exceed a certain weight percentage, depending upon the particle size of the mica, and cover the surface of the mica particles. Treatment of the filler in hydrogen improved its conductivity considerably. The fillers were incorporated into an ABS resin to prepare composites as potential electromagnetic interference shielding materials. Increasing the particle size of the mica reduced the critical filler loading required to produce electrically conductive composites. Reducing sample thickness caused a decrease in resistance, due to changes in filler orientation.

Behaviour of molybdate-passivated zinc coated steel exposed to corrosive chloride environments

The behaviour of molybdate conversion coatings on zinc coated mild steel in corrosive chloride environments was investigated using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and neutral salt fog exposure. It was found that the presence of a simple molybdate coating initially increased the rate of corrosion of zinc. Molybdenum species were initially present in the conversion coating in either the +V or +VI oxidation states. Exposure to neutral salt fog reduced molybdenum to either Mo(IV) or Mo(III). This reduction of molybdenum, an additional cathodic process, may result in the activation of zinc observed in these studies. For molybdate-passivated surfaces in the early stages of exposure to neutral salt fog, corrosion products were found to be less voluminous than those observed on untreated surfaces. This may be due to the presence of inhibiting Mo(IV) or Mo(III) species in the corrosion product layers. However, after 24 h exposure to salt fog, no molybdenum could be detected. This implies that the lower oxidation state molybdenum species formed are soluble. However, surfaces passivated from molybdate solutions appear to forestall the onset of red rust, during immersion in chloride solutions and exposure to salt fog, by approximately 12 to 24 h. This behaviour may be attributable to corrosion inhibition by Mo(III) and Mo(IV) species while they are present at the surface.

The development of passivation coatings by cathodic reduction in sodium molybdate solutions

Abstract The incidence of marked arrests (inflections) on cathodic polarization curves for substrate metals of zinc, tinplate and platinum in sodium molybdate-based solutions has been examined. On tinplate, a reaction mechanism based upon mechanical degradation of the film, as opposed to a direct reduction process, has been proposed, correlating electrochemical and ESCA-type data.

The production of compositionally modulated alloys by simulated high speed electrodeposition from a single solution

Simulation of high speed electrodeposition, using a rotating cylinder electrode, has been carried out in order to obtain an operating window for applied current density and cathode electrode movement as a means of agitation. The electrolyte used was based on an acid sulphate, zinc-nickel alloy. Electroplating was carried out at room temperature in the range of current density from 0.5–130 A dm−2 and at cathode speeds of 0–1500 rpm on 99.5% purity iron tube. By assessing the deposit characteristics in terms of appearance, composition, crystal structure and corrosion resistance and correlating these results, it is possible to predict the conditions for production of compositionally modulated alloys from the simple zinc-nickel non-complexing electrolyte system.

Development of a Bath for Electrodeposition of Zinc-Cobalt Compositionally Modulated Alloy Multilayered Coatings

The production of compositionally modulated alloy multilayers (CMAM) of Zn-Co alloys from an acid sulfate bath has been extensively studied and the electrodeposition parameters and conditions optimized to yield layers of high and low cobalt contents. It is shown that a change of deposition mechanism (from anomalous to equilibrium codeposition) occurs and is associated with the formation of Co 3+ ions in solution and not to the formation of an electrode film of Co(OH) 2 as traditionally believed. Spectroscopic studies (UV-visible) showed that Co 3+ ions are formed on the anode surface due to oxidation of Co 2+ ions in the bath, and these trivalent ions prevent high-cobalt-containing Zn-Co alloy coatings. Using a two-compartment cell was found to be very effective in preventing the formation of trivalent cobalt ions in the catholyte portion of the bath and thus changing the codeposition mechanism from anomalous to equilibrium and consequently promoting high levels of cobalt in the deposits. This technique also increased the cathode current efficiency to 98%. By using a computer-aided pulse plating unit and designing different waveforms, various types of Zn-Co CMAM coatings of different thickness and number of layers were electrodeposited.

Compositionally modulated zinc alloy coatings for corrosion protection

SummaryAn investigation has been undertaken to attempt to electrodeposit layered coatings of zinc, nickel and zinc-nickel alloy. A dual electrolyte approach has been applied with sequential layers being electrodeposited from the two baths. Coated steel substrates have been sectioned and examined using scanning electron microscopy and the layered nature of the deposits confirmed. Electrochemical and salt fog corrosion tests have been undertaken to ascertain the corrosion protection capabilities of various layered coatings. Preliminary data, particularly from the salt fog trials, suggests that some of the layered structures have considerably better protective qualities than similar thickness coatings of zinc and zinc-nickel alloy.

Passivation studies using group VIA anions: Part 5: Cathodic treatment of zinc

AbstractThe cathodic polarization of zinc has been examined in aerated and still group VIA oxyanion solutions at 0·1 and 1·0M concentrations. The occurrence of inflections on certain curves has been noted and discussed with respect to present theories on such phenomena. The corrosion resistance of coatings formed from selected solutions has also been examined in a 24 h salt spray test.