TABLE OF CONTENTS Oxidation is an environmental phenomenon in which metals and alloys (and other materials) exposed to oxygen or oxygen-containing gases at elevated temperatures convert some or all of the metallic elements into their oxides. The oxide can form as a protective scale if it remains adherent, and reduces further oxidation, or may continually spall off, exposing fresh metal. The latter results in progressive metal loss. Additionally, internal oxidation may occur. The technological implications of oxidation lie in the loss of load-bearing capability of the original metal or alloy component, eventually resulting in component failure.
Oxidation is studied by controlled exposure of metals to oxidizing atmosphere such as O 2, CO 2, H 2O, NO 2, and combinations thereof, at high temperature, for various lengths of time. Continuous weight change may be monitored in microbalances, or metal thickness loss may be monitored microstructurally. Although all of the gases just listed induce oxidation in metals and alloys, the oxide composition, morphology, adherence, and spallation characteristics strongly depend on the gas composition.
Most metals oxidize readily because the free energy change A G associated with the reaction is negative. An example is aluminum oxidizing to form aluminum oxide:
?G = ? 251.8kcal/mole of oxygen at room temperature and 1 atmosphere (atm) pressure.
When a metal in standard state (pure solid at 298 K, 77 F) reacts with oxygen in standard state (as a gas at 1 atm pressure and 298 K) to form...
