Doctoral Dissertations

Abstract

"The purpose of this investigation was to study the effect of oxygen and nitrogen (in the alpha solid solution range) on the kinetics and electrochemistry of titanium dissolution in hydrofluoric acid. Dissolution rates, activation energies, difference effects, and potential measurements were determined with TiOXNY sintered alloys and sheet metal. Values of x ranged from 0.01 to 0.126 and values of x. from 0.001 to 0.230.

Dissolution rates were determined using a gas evolution technique. Temperatures were varied from 5 to 45°C. The hydrogen evolution rates were converted to TiOXNY dissolution rates by means of the following stoichiometric relationship based upon the simple overall reaction established experimentally by analytical methods:
TiOxNy + 3HF = TiF3 + xH20 + yNH3 + (1.5-x-l.5y) H2

The following equations represent the type of reactions which may take place at the surface of the metal:
Ti + 6HF = 3H+ + TiF3-6 + 1.5 H2
Ti + 3H+ = Ti3+ + 1.5 H2 (electrochemical)
Ti203 + 12HF = 6H+ + 2TiF3-6 + 3H20 (dissolved oxide)
TiNx + 3HF = TiF3 + xNH3 + 1.5(l-x) H2 (dissolved nitrogen)
TiF3 + 3HF = 3H+ + TiF-6
TiF3 + 3HF = TiF4 · 2HF + 0.5H2

Rates increased slightly (about 10%) with an increase in oxygen content over the range indicated for the samples. A dramatic decrease in reaction rates (about 70%) was noted for increased nitrogen content in the range indicated, however.

Activation energies increased with both increased nitrogen and increased oxygen concentrations in the samples. The increase was more pronounced (nearly doubled) for the nitrogen. There was an approximate increase of five per cent for the oxygen.

The reaction rate was dependent upon the concentration of the hydrofluoric acid. The order of the reaction with respect to hydrofluoric acid normality was found to be 0.8±.1. It was the same for both sintered samples over the range of compositions given and the sheet metal. The reaction rates of the sintered samples were approximately ten per cent greater than for the sheet metal.

The difference effect constant increased slightly with increased oxygen content of the samples. The polarization would appear to increase from sixty to ninety per cent. An increase in nitrogen content causes a very noticeable change in the difference effect constant from a positive value to a negative value. Such a range of conditions represent a change where the surface is approximately sixty per cent polarizable to one where the fraction of the freely corroding rate attributed to the 'chunk-effect' is approximately ten per cent.

The difference effect constant for the sintered samples became more negative with an increase in hydrofluoric acid concentrations.

Potential measurements indicated that electrode potentials increased with an increase in current density for samples with proportionately greater amounts of oxygen. Proportionately greater amounts of nitrogen gave results which indicated a decrease in electrode potential with increased current density.

The results are explained in a consistent manner by the presence of various films upon the surface of the metal. Although these films are of a complex nature, they probably result from ammonia produced in the dissolution and Ti2O3 known to be present in the metal. The dissolution process is believed to be controlled by the diffusion of the corrosive elements through these films. As the amount of nitrogen in the sample is increased the diffusion process becomes more difficult due to the nature of the films.

As would be expected, the dissolution process under carefully preserved reducing conditions was found to differ from that where oxidizing effects were present.

The exact nature of the reactions occurring on the metal surface cannot be determined from this study, although they are believed to be electrochemical" -- Abstract p ii-iv.

Department(s)

Chemical and Biochemical Engineering

Degree Name

Ph. D. in Chemical Engineering

Publisher

Missouri School of Mines and Metallurgy

Publication Date

1963

Pagination

382 pages

Rights

© 1963 Arliss V. Martin, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Thesis Number

T 1539

Print OCLC #

5955275

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