Steam treatment or black oxidation is a process which uses the porosity of sintered components. Components are treated in dry steam at approx. 550° C, where the steam reacts with the surface of the component. 

The reaction does not just take place on the outer surface but also on the inner surface of the open pores, i.e. the pores opening up onto the surrounding environment.

Thus steam treatment is a possible pre-treatment before e.g. carbonitriding or certain forms of surface treatment which require the pore system to be closed. 

Steam treatment does not just block the pores, but it also increases the corrosion resistance and alters the mechanical properties, thus increasing the density, the hardness and the tensile strength of the components, while improving tribological properties (wear) – in some cases dramatically.