Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Aravamudhan Raman

Abstract

Formation and transformation characteristics of different kinds of magnetite were studied extensively in this work. Magnetites containing various amounts of cation deficiencies were characterized using magnetization data, lattice parameters, and Infrared Absorption Spectrophotometry. It is shown that $\alpha$-Fe$\sb2$O$\sb3$ nucleates in $\gamma$-Fe$\sb2$O$\sb3$ during the oxidation process of magnetite. Relative defect contents of several laboratory synthesized as well as field samples were obtained and correlated to their IR patterns. Stability criteria of magnetite formed under atmospheric conditions were probed into utilizing laboratory fog tests and it was concluded that in wet/dry cyclic testing continuous wetting or wetting in excess of 90% of the cyclic time produces magnetite in the oxide formed on steels, whereas drying in each cycle in excess of about 15% of the cyclic period produces defective magnetite as the dominant iron oxide phase present in the rust. It was concluded that magnetite is not stable in an oxidative environment and it transforms (oxidizes) to defective magnetite or to $\gamma$-Fe$\sb2$O$\sb3$, but appeared not to $\gamma$-FeOOH. Thick magnetite electrodes obtained from bridge corrosion products were studied in potentiodynamic polarization experiments. During oxidation magnetite was found to passivate at pH levels of 4.0, 7.0, and 10.0 and passivation breakdown was found to occur at a potential around +1200 mV (SCE). Finally, coating of a steel with a phosphate layer using 50% phosphoric acid was found to result in a reduction of oxidative weight gain by 56% and 16% at 750$\sp\circ$C and 925$\sp\circ$C, respectively, for a 100 hour oxidation. The samples formed scales containing FeO, Fe$\sb3$O$\sb4$, and $\alpha$-Fe$\sb2$O$\sb3$ in successive layers from the metal side, all of them showing preferred orientation (PO) in the untreated cases, about 70% FeO with (111) PO, about 20% Fe$\sb3$O$\sb4$ with (400) and (333) PO's, and about 10% $\alpha$-Fe$\sb2$O$\sb3$ with (006) and (10,10) PO's. Presence of a subzone in the spinel region having $\gamma$-Fe$\sb2$O$\sb3$, atop Fe$\sb3$O$\sp4$, immediately below $\alpha$-Fe$\sb2$O$\sb3$, could also be documented. Application of the results obtained to practical situations which would require stabilization of magnetite and prevention of its phase transformation through passivation are outlined.

Pages

151

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