Formation of oxides (scale) on metal surfaces.
The reaction of iron and its accompanying elements with atmospheric oxygen or oxidizing gases results in scaling, forming a scale layer consisting of oxidation products from the iron, silicon and accompanying and alloy elements.
As this scale layer is not dense or firmly adherent, further oxidation processes occur – except with high silicon concentrations (> 5%) –, i.e., as can be seen from Fig. 1, the scaling continues rapidly at first and then at a slower pace. Ferritic nodular graphite cast iron therefore exhibits a slightly better oxidation resistance than pearlitic qualities.
Parts used at elevated temperatures and additional temperature changes, which represents particularly high material stresses, are not only subject to an accelerated scaling rate, but the part is additionally stressed by tensions occurring as a result of the temperature differences. These tensions are most pronounced at the surface and lead, together with oxidation, to the formation of a network of firing cracks (see Firing cracks).
The scaling resistance can be improved by adding alloy elements. These elements result in an improved adherence of the external scale layer, also reducing permeability.
The most beneficial influence is achieved with silicon. Even common silicon concentrations of less than 3% produce a noticeably good effect. The more the silicon concentration is increased, the better is the scaling resistance of the material, in particular reducing the scaling rate. However, this has an extremely negative effect on the mechanical properties.
Nickel and copper barely show a detectable influence on the scaling behavior in low-alloyed ferritic nodular graphite cast iron. The same applies to molybdenum, even though scale resistance is improved when adding increasing amounts of this element of up to 3%. Compared to high silicon concentrations, this influence is rather low.
Zinc has a positive influence, but does not confer total scaling resistance to the iron. Very good effects on scaling resistance can be achieved with aluminum which is, however, rarely used in industrial nodular graphite cast iron due to its interfering effect on nodule formation, its poor casting properties and its propensity to gas absorption and the formation of oxide films (see Oxide inclusion).