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This review discusses the role of iron oxides and rust in tribology. Properties of interest are composition, formula, crystalline structure, magnetic properties, color and hardness. Five anhydrous iron oxides, and 10 hydrated iron oxides known as rusts are described. One problem invalves reddish brown material on a steel surface, or discoloring an oil or grease. The article gives guidelines for identifying rust, which can occur on a single steel surface exposed to water and air, (and accelerated by chloride ions) and fretting corrosion debris which occurs between two vibrating steel surfaces. X-ray diffraction is the most important tool for the identification of crystalline compounds. Scanning electron micrographs show short sliding marks under fretting corrosion, and pitting under rust.
KEY WORDS
Iron Oxides; Rust; Fretting Corrosion
INTRODUCTION
The following review of iron oxides and rust discusses their occurrence and significance in tribology. Table 1 lists five anhydrous iron oxides and ten forms of rust (hydrated iron oxides) taken from mineralogy literature. The table includes the formula, crystalline structure, color, magnetism, and hardness of fifteen compounds.
Tribologists conducting failure analysis, are often called upon to determine whether reddish brown deposits on steel, or discoloring an oil or grease, are fretting corrosion or rust. Correct identification is important because the cause and remedy for these two forms of corrosion are difterent. Fretting corrosion between two vibrating surfaces requires mechanical changes, anti-wear additives or palliative coatings. Fretting corrosion debris in an anti-scuff oil may promote polishing wear. Rust, the corrosion of one steel surface by water and air, requires the protection of the part from the environment, or the use of a better rust inhibitor in the lubricant. Some rusts are hard enough to abraded softer metals.
ANHYDROUS IRON OXIDES
Wustite
Occurrence-Wustite, FeO, is formed on ferrous metals at high temperatures such as 300C in a plentiful supply of oxygen. In the steel industry it is called mill scale.
Significance-When gears, bearings, or other lubricated components fail and overheat, Wustite is found in the black films. Plastic deformation from hot flow, and carbon deposits from burned oil are further evidence of overheating.
Magnetite
Occurrence-Ancient people discovered the magnetic properties of the mineral magnetite and called it lodestone. Magnetite, Fe304, exists on the surface of all ferrous metals in...