A Review of Oxidation on Steel Surfaces in the Context of Fire Investigations 2012-01-0990
During the course of a fire and subsequent exposure to the environment, iron and low-carbon steels oxidize by two mechanisms: high temperature oxidation and atmospheric corrosion. Of particular interest to fire investigators are oxide properties and distribution that could be of use to better understand important characteristics of the fire such as the location the fire originated, the direction the fire traveled or even temperature versus time characteristics. This could be particularly valuable in cases where burn damage to combustible material, which is known to be an important indicator of fire origin, is so extensive that little if any material remains after the fire. However, there is little data in the literature that specifically addresses the utility of oxide properties in the context of fire investigations. In this paper, we review the literature associated with both the high-temperature oxidation and atmospheric corrosion mechanisms in the context of a fire and post-fire environment. This review illustrates that both mechanisms are very complex and coupled to an equally complex and often ill-defined combustion environment associated with fires. As a result, the contribution of oxide characteristics to fire investigations is currently limited to identifying regions at the fire scene that reached temperatures sufficient to damage the protective coatings or to form wustite (FeO), which occurs at about 570°C. While oxide thickness developed during high-temperature oxidation may be related to the fire environment, its significance in fire investigations will likely depend on numerous confounding factors, which could include complex lattice defect formation processes, a complex and variable combustion environment, the dependency on both exposure time and exposure temperature, and the mass and surface area of the steel in question. Although variations in oxide color are often observed after a fire, no clear physical link between color variations and important characteristics of the fire has yet to be demonstrated by data currently available in the literature. Instead, oxide color is dependent on oxide thickness, the type of oxides present, natural variations of color for the same oxide, and the type and concentration of combustion products or other contaminants in the lattice structure.