For applications such as water-cooled furnace components in which one surface is exposed to extremely high temperatures and the other is cooled by water or air, repeated expansion and contraction of the surface occurs.
Then the water-cooled component due to the changes in the operation temperature is subject to cyclic tensile and compressive stresses.
Often, due to temperature gradient between the inner and outer layers, cracking occurs on the surface. The cracked surface is then subjected to oxidation and other forms of corrosion that permits access of hot gases to deeper layers of the component. The cracking gradually progresses into the mass until the component fails.
Calorizing imparts resistance to oxidation and sulphidation from furnace atmospheres, due to the aluminum-rich surface layers created by the calorizing process. The aluminum oxidizes preferentially to extremely stable alumina that retards the formation of other oxides on the sidewall of open cracks, thus reducing the rate of propagation of the cracks. Additionally, both the alumina and the aluminum-rich alloy surface layers of the calorized components exhibit low thermal conductivity.