Date of Award

Winter 1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Director

Desmond C. Cook

Committee Member

A. C. Van Orden

Committee Member

H. E. Townsend

Committee Member

G. R. Hoy

Committee Member

G. E. Copeland

Abstract

The study of corrosion behavior was performed using three different analytical techniques, which provided information on the formation, development and layering of iron oxides on the corrosion products as a function of atmospheric conditions, exposure time and type of steel. In particular, the protective layer formed on weathering steel was investigated as a function of different amounts of alloying elements in the steel, atmospheric conditions and exposure times. Combined together, the results provided a better understanding of the atmospheric corrosion behavior of steel, and formed a part of database of the atmospheric corrosion characteristics.

Accurate characterization of the iron oxides often formed on steel surfaces was one of the aims for this research. Seven iron oxides, goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), δ-FeOOH, hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4), which are often found in corrosion products present on steel, were characterized by Mossbauer spectroscopy, Raman spectrometry and x-ray diffraction analysis. Complete identification of the corrosion products formed on steel required more than one and most often three of the analytical techniques. In particular, the Mossbauer characterization of the iron oxides included measuring the relative recoilless fraction (F-value), which was defined as the ratio of recoil-free fractions of two different materials. The relative recoilless fractions of the iron oxides allowed the conversion of Mossbauer subspectral areas to the relative atomic, molecular, or weight fraction of each present in a mixed iron oxide sample.

The characterization of the iron oxides was used to study the atmospheric corrosion behavior of weathering, copper bearing and carbon steels as a function of environmental condition, exposure time and type of steel. Goethite, akaganeite, lepidocrocite, maghemite and magnetite were identified in the corrosion products formed on the steel coupons. The formation of superparamagnetic goethite showed a correlation with the corrosion rate for seven types of steels. After long term exposure, superparamagnetic goethite on weathering steel, was the final iron oxide formed in the corrosion products. The conosion products typically formed in two layers. The protective layer, the inner layer, was formed by goethite and superparamagnetic maghemite. Increased amounts silicon and smaller amounts of phosphorus in the steel substrate increased the relative fraction of superparamagnetic goethite, in marine and rural environments. However, different amounts of nickel did not affect the formation of the iron oxides even after long term exposure. Increasing the silicon content in steel would be helpful in enhancing corrosion resistance for the long term lifetime of steel structures under atmospheric conditions.

Comments

Additional dissertation committee member: L. Vuskovic

DOI

10.25777/2m2b-1793

ISBN

9780591623406

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