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Heat Treat (Mill) Scale Under the Microscope

Iron Heat Treat Scale

This very over exposed image is required to show the small amount of light that is actually transmitted.

Transmitted Brightfield Illumination


When iron is hot rolled or heat-treated a layer of iron oxide forms on the surface. The mechanical and termal properties of iron oxide are very different than the underlying metal. As a result the oxide film that forms on the flat sheet flakes off as flakes. These flakes are often black but may also be yellow, orange, or red with reflected light due to a coating of different oxides of iron.

Significance in the Environment:

Characteristic Features:

Mill scale tends to be flat to slightly wavy flakes with well defined edges. They tend to be opaque but may transmit some light if the light is intense enough, the flake is thin enough, and/or oxidation has gone beyond magnetite or maghemite. If light is transmitted it tends to be orange to red in color. Mill Scale tends to be black but may also be yellow, orange, or red with reflected light, depending on the oxidation state of the surface of the flake.

Associated Particles: