Masters Theses

Date of Award

6-1984

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

E. E. Stansbury

Abstract

The general corrosion and electrochemical behavior of zinc and iron-zinc couples was explained in water and aqueous solutions con-taining HCO-3, NO-3, CO=3, and Cl- environments within the temperature range 30-60°C under stagnant, aerated and deaerated conditions.

Bicarbonates, nitrates and carbonates cause the potential of the zinc to increase. This results in a potential reversal of iron-zinc couples which is accelerated by higher temperature (60°C) and is promoted by the presence of oxygen. The increasing potential of the zinc is due to a very compact film of zinc corrosion products. The film does not appear to be associated with the typical transition to a passive state. It is proposed that the effect may relate to the semiconducting properties of the film and the progressively de-creasing anode/cathode area ratio as it forms.

For zinc in chloride solutions, a potential reversal does not occur. The galvanic current remains high and constant. The electrode potential of the iron is depressed well below its equilibrium value and the surface remains bright and free from corrosion product throughout the test period of the weeks.

The morphology of the corrosion product film and the substrate were investigated with a scanning electron microscope. The surface morphology was very dependent on the environmental conditions. the bicarbonate solutions, most of the oxide film, before removing In the corrosion product, revealed a dense cellular type structure with protrusions. However, in chloride solutions, the corrosion deposit Upon appears to be flaky, and does not show any cellular structure. removing the corrosion product formed, in both bicarbonate and chloride solutions, the highly localized and crystallographic nature of attack was observed.

X-ray analysis was used to examine the corrosion product. A moist film examined immediately following removal from the environment, results in a broad diffraction pattern indicating amorphous or poorly developed crystalline material. Dried films showed well defined diffraction peaks for zinc oxide.

Polarization measurements were undertaken in an attempt to pro-vide insight into the corrosion mechanisms responsible for the weight loss, potential-time behavior and surface morphology observations. Although schematic polarization curves could be developed which were consistent with the experimentally measured polarization curves, a satisfactory understanding of the corrosion behavior, particularly the cathodic reactions supporting the corrosion was not developed. Rather these measurements served to demonstrate that the corrosion mechanisms are quite complex and the present results shed some light on the information available in the literature.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS