scope:

Purpose-This engineering practice covers determination of preliminary information on the relative growth, scaling, and microstructural characteristics of an oxide on the surface of a pure metal or alloy under isothermal conditions ill still air. This test does not necessarily apply to testing of coated specimens.

Application-This procedure may be applied to ally pure metal, alloy, or groups thereof that exhibit the formation of a surface oxide structure in still air at the temperature of interest, usually above about 540°C (1004°F). Direct comparison of material at a constant temperature or the effect of temperature on a given material may be investigated. Oxidation is a dynamic time- and temperature-dependent process. The relative resistances of materials to oxidation at constant temperature should, therefore, be determined over at least three time periods.

Limitations:

Materials usually exhibit one of several basic reactions to a high-temperature oxidizing environment. They may form a protective oxide layer which protects them indefinitely. They may form a protective oxide layer which persist, for some finite time after which "scale breakaway" occurs and a scaling rate develops. They may also form a nonprotective oxide which allows rapid oxygen penetration to the metal and subsequent rapid deterioration by internal oxidation, which may render the material brittle and unusable without much observable surface or mass change. Some oxides may be liquid and thereby flux any protective oxides from the surface. Another reaction may be vaporization of the scale or one or more of the reactants. The correct interpretation of this test is thus dependent upon both mass change and microstructural depth of attack data. One should not be used without the other. For materials that form high vapor pressure oxides one must also collect the vaporized oxide by some method if it is required to complete the material balance. Specific methods to do this are beyond the scope of this practice.²

Materials that develop very adherent and protective oxide layers at the test temperature of interest may not be comparable in any practical amount of time because the time to scale breakaway is so long. Some alloys also react very differently to the relatively mild conditions of static oxidation than they do to the cyclic temperatures often found in service where differential thermal expansion may cause accelerated scale breakaway. This general test should thus not be used to predict the quantitative reactions of materials for specific. high-temperature applications.

This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

² For a discussion of oxide vaporization at high temperatures, see Tedman, Jr., C. S., "The Effect of Oxide Volatilization on the Oxidation Kinetics of Cr and Fe-Cr Alloys," Journal of the Electrochemical Society, Vol 113, 1966, pp. 766-768.