Surface treatment

Surface treatment refers to a modification or change of the surface of the component with the object of giving it properties which cannot be obtained using the base material alone. 

Thus surface treatment embraces an extremely wide spectrum of processes from coating through chemical (e.g. steam treatment) and thermo-chemical treatment (e.g. carbonitriding and nitrocarburizing) to heat treatment or mechanical deformation of the surface. 

In principle, all surface treatments applied to solid steel may be applied to components manufactured through powder compaction, with the reservation that the pore system of powder compacted steel may considerably affect treatment. 

It may therefore be necessary to close the pore system in a suitable manner before actual surface treatment. Plastic impregnation or steam treatment are examples of possible treatments. 

In everyday life the term surface treatment is usually used about actual coating methods carried out with the object of improving the component’s corrosion resistance. 

Electro-galvanizing is a surface treatment process which involves steel components being given a corrosion resistant layer of zinc through galvanic precipitation. The zinc coating is then chromated to protect the zinc against corrosion and preserve the appearance of the zinc surface for some time.

Zinc is a base metal compared to steel and provides - during atmospheric use – cathodic protection of the steel even where this may be exposed as a consequence of damage to the zinc layer. 

Electro-galvanization may be regarded as a "standard process” in the area of corrosion protection coatings.

Chromium/nickel plating is often performed where a decorative surface coating is required, one which also provides some corrosion protection. However, compared to steel such coatings are noble and if there are pores or cracks in the coating, the steel surface will be exposed to galvanic corrosion in damp conditions.

Delta-MKS and Dacromet are examples of more recent types of surface coatings which provide considerably improved corrosion protection than classic electro-galvanization at the same or thinner layer thicknesses.

These involve comparable systems which comprise a base coat consisting of zinc and aluminium flakes held together by an inorganic binding agent and an organic top coat intended to protect the base coat. 

Coatings do not involve galvanization but the processes are carried out as drum, dip or spraying processes, which ensure an extremely high and uniform penetrability even in the deepest apertures etc. 

Besides providing particularly good corrosion protection these types of coating also provide the surface with improved slide properties, since the coefficient of friction is considerably reduced.

Zinc-Iron is yet another example of a more recent type of surface coating which exploits the fact that it is possible to improve the corrosion properties of zinc through the addition of small amounts (0.4-0.6%) of iron.

Zinc-Iron is alkali precipitated on the steel components. 

The improved corrosion protection compared to classic galvanization primarily stems from the fact that iron reacts with the chromate layer and contributes to better adhesion and thus improved corrosion protection.

It is important to emphasise that regardless of surface treatment type, one will not normally expect the same corrosion resistance in the treated sinter steel as surface treatment would give to the solid material.

This is among other things due to the fact that the sinter steel because of its porous structure has a considerably larger active surface which may be attacked by corrosion.