Definition - What does Thermal Emittance mean?
Thermal emittance refers to the ratio of the degree of radiant deflection of heat of a given object to that of a standard reference black body. It describes the degree to which a material can radiate the heat it absorbs.
Corrosionpedia explains Thermal Emittance
Thermal emittance is a dimensionless quantity that is measured on a scale of 0 (a perfect reflector) to 1 (a perfect emitter). Most substances fall somewhere within this numerical range. Thermal emittance has a direct correlation between the light absorbed by a surface and the resulting corrosion. A perfect emitter substance is opaque and emits thermal radiation, thus absorbing any and all light that is projected onto it.
Empirical investigations have shown that a relationship between light and electrochemical cells affect the corrosion rates of metallic surfaces such as carbon steel. Metal objects that are in contact with water and free ions have a higher corrosion rate when exposed to sunlight through a blue filter than when exposed to sunlight through a red filter. Ultraviolet light reacts with metallic surfaces in a manner that accelerates corrosion.