Stainless Steel
Surface treatment of stainless steel
Every mechanical and thermal treatment of
stainless steel influences the corrosion
resistance. Stainless steel is widely employed in various sectors of industry, such as the food industry, the chemical and petrochemical industries, oil and gas recovery etc. Stainless steel is used in fact due to its specific corrosion resistant features. This corrosion resistance is the result of its natural capacity to form a protective oxide film. This oxide film is however extremely thin and therefore very vulnerable. Many of the treatments that the stainless steel undergoes in the construction and sometimes also in the transport phase may result in damage to
the oxide film. These will result in the stainless steel losing its corrosion resistance with the possibility of, depending on the application, (corrosion) damage arising.
The main causes for the onset of corrosion are:
Oxidation (discolouration) of the heated zone after welding and annealing after
heat treatment:
Application of heat, for example during welding, causes discolouration due to
oxidation with an excessively thick oxide film
and a chromium impoverished zone. These locations provide significantly reduced
resistance to local corrosion (chloride cavity corrosion).
Alien iron:
Numerous sources may be stipulated whereby alien iron contamination of the stainless
steel surface may occur. For example as a result of sanding, grinding, setting
with material (tools) made of carbon steel or tools that are also used for processing
carbon steel. Handling and transport of the material are also a frequent source
of contamination.
Pickling after the construction phase will restore the corrosion resistance and remove chromium impoverished zones and alien iron. It will restore the corrosion resistance to the basic material’s original level.
- Pickling and Passivating
- Electropolishing
- Mechnical Polishing
- Bead Blasting