Yar-Ming Wang

Voltammetric Evaluation of Zinc Electrowinning Solution Containing Nickel

The effect of nickel impurity in zinc sulfate electrowinning solutions was studied using voltammetry techniques. The reactions occurring at various points on the polarization curve are proposed and comparisons made for both pure electrolyte and those containing nickel. A nickel‐activated hydrogen evolution peak was found to occur at low current densities during the anodic sweep portion of the curve, the height of which was found to be proportional to the nickel concentration in solution. The influence of certain operating parameters such as zinc ion concentration, potential scan limit, and scan rate on the nickel activated peak was also determined.

Influence of Surface Pretreatment on Coating Morphology and Corrosion Performance of Cerium-Based Conversion Coatings on AZ91D Alloy

Abstract The corrosion protection of cerium-based conversion coatings (CeCC) formed on AZ91D magnesium alloy has been studied using potentiodynamic polarization measurements, electrochemical impeda...

An Electrochemical Technique for Characterizing Metal-Lubricant Interfacial Reactions

An AC impedance technique, in combination with an electrical property cell, was employed to measure the electrical resistance and capacitance of lubricants. A comparison of the combined surface coating plus bulk fluid resistance and capacitance values measured for different additives blended in mineral oil, demonstrated that different metal-lubricant interaction mechanisms occur. The electrical measurements indicated that additives such as the mixed alkyl acid orthophosphate (acid phosphate) reacted with the cast iron electrodes to firm reaction films, whereas oleic acid was absorbed on the electrodes. Although the implied reactivity from electrical measurement for N-oleyl-1-1,3 diaminopropane (fatty amine) is comparable to that for acid phosphate, no reaction film for the former was detected by elemental analysis. Presented at the 41st Annual Meeting in Toronto, Ontario, Canada May 12–15, 1986

A Comparative Study on the Corrosion Resistance of Cerium-Based Conversion Coatings on AZ91D and AZ31B Magnesium Alloys

In the present work a comparative study has been carried out on the surface morphology, electrochemical properties and corrosion performance of cerium-based conversion coatings (CeCCs) on AZ91D and AZ31B magnesium alloys. The as-deposited coatings consisted of a two layer structure: a continuous Mg/Al oxide transition layer of ~ 50 nm thick and a CeCC layer of ~ 400 nm thick. Potentiodynamic polarization and impedance spectroscopy results using a 0.6 wt% NaCl and 0.6 wt% (NH4)2SO4 electrolyte showed that cerium-based conversion coatings enhanced the corrosion resistance of AZ91D and AZ31B magnesium alloys compared to polished uncoated samples. Good correlation of electrochemical test and ASTM B117 salt spray was realized.

Modeling of the current distribution in aluminum anodization

The growth of porous anodic Al2O3 films, formed potentiostatically in continuously stirred 15 wt.% H2SO4 electrolyte was studied as a function of the anodization voltage (14–18 V), bath temperature (15–25 °C) and anodization time (15–35 min). The variation of the anodic surface overpotential with the current density was measured experimentally. The film thickness at the more accessible portions of the anode was observed to increase with the anodization voltage and the bath temperature. However, the film thickness on the less accessible portions of the anode did not significantly change with the voltage or the bath temperature. This indicates that the anodization process at the more accessible regions is more strongly influenced by the surface processes than by the electric migration within the electrolyte. Furthermore, analysis confirms that the major portion of the film resistivity resides within a thin sub-layer that does not vary with the anodization time, and the growing anodic layer contributes only marginally to the overall film resistance. Computer aided design software was employed to simulate the current density distribution. For the range of process parameters studied, the electrochemical CAD software predicts accurately the measured thickness distribution along the anode.

AC Impedance Spectroscopy (ACIS) Analysis of a Polymer Dispersed Liquid Crystal (PDLC) Film

Abstract AC impedance spectroscopy is applied to monitoring the chemical-physical condition of polymer dispersed liquid crystals (PDLC). The results define an approach for non-destructive evaluation as the PDLC is designed into informational displays. Response. functions are linked to electrical properties of the constituents through mixture relations. Resistivity is established as being much more sensitive than permittivity to the PDLC state. Consideration is given to treating explicitly the large interphasal region found surrounding each liquid crystal droplet in the continuous matrix.

Corrosion and Electrochemical Behavior of In and In-Bi Alloys in HCl

Abstract The corrosion of In-Bi alloys in HCl was studied using x-ray, scanning electron microscopy, atomic absorption spectroscopy, and cyclic voltammetry techniques. In deaerated 3N HCl solutions, the selective dissolution of indium was the predominant mechanism in the corrosion of In-Bi alloys. The corrosion was always accompanied by a phase change on the surface, and the corrosion rates of In-Bi alloys were found to increase in the presence of oxygen. The corrosion potential values indicated that the corrosion of In-Bi alloys was cathodically controlled, and the occurrence of preferential or simultaneous dissolution in In-Bi alloys was also dependent on potential.

The Use of AC-DC-AC Methods in Assessing Corrosion Resistance Performance of Coating Systems for Magnesium Alloys

The potential utility of AC-DC-AC electrochemical methods in comparative measures of corrosion-resisting coating system performance for magnesium alloys under consideration for the USAMP “Magnesium Front End Research and Development” project was previously shown in this forum [1]. Additional studies of this approach using statistically-designed experiments have been conducted with focus on alloy types, pretreatment, topcoat material and topcoat thickness as the variables. Additionally, sample coupons made for these designed experiments were also subjected to a typical automotive cyclic corrosion test cycle (SAE J2334) as well as ASTM B117 for comparison of relative performance. Results of these studies are presented along with advantages and limitations of the proposed methodology.

The Influence of Galvanic Current on Cerium‐Based Conversion Coatings on Mg, AI, and Galvanized Steel Couples

The influence of galvanic current on cerium-based conversion coatings (CeCCs) for magnesium (AZ91, AZ31), aluminum (6016), and electro-galvanized steel (EGS) couples has been studied using zero resistance ammeter (ZRA) measurements in prohesion solution. The galvanic current measured between magnesium-aluminum, magnesium-galvanized steel, and aluminum-galvanized steel couples correlated with significant changes in coating morphology and deposition rate. The ZRA galvanic currents (mA) were 0.02 for 6016-EGS, 0.38 for AZ91-EGS, 0.72 for AZ91-6016, 1.08 for AZ31-EGS, and 1.08 for AZ31-6016 couples. The corrosion performance of the coated couples was evaluated by ASTM Bl 17 neutral salt spray testing. Cerium conversion coated couples performed better in salt spray testing compared to uncoated couples. The correlation of galvanic current, cerium deposition, and corrosion performance will be discussed.

Effect of Thickness on the Morphology and Corrosion Behavior of Cerium‐Based Conversion Coatings on AZ31B Magnesium Alloy

Cerium-based conversion coatings (CeCCs) were deposited onto AZ31B magnesium alloy substrates using a spontaneous reaction of CeCl3, H2O2 and gelatin in a water-based solution. The coating thickness was adjusted by controlling the immersion time in the deposition solution. Prior to deposition, the AZ31B substrates were treated using an acid pickling in nitric acid and then an alkaline cleaning in sodium metasilicate pentahydrate. After deposition, the coated samples were immersed in a phosphate bath that converted cerium oxide/hydroxide into cerium phosphate. Electrochemical impedance spectroscopy, potentiodynamic polarization and neutral salt spray testing studies indicated that ~100 nm thick CeCC had better corrosion performance than ~400 nm coatings. Characterization of the CeCCs by transmission electron microscopy (TEM) revealed a three layer structure with different compositions.