Chemical surface treatment – what is it?

A chain is only as strong as its weakest link and this is most certainly also the case when it comes to stainless steel. Most corrosion forms attack the weaknesses of the steel and one of the secrets to any kind of processing of stainless steel is, therefore, to eliminate potential weaknesses – or at least ensure that they are not going to be critical. Unfortunately, most machining processes (cutting, welding, bending, chip-breaking machining, etc.) will leave their mark in the shape of impairments regarding corrosion resistance. However, a subsequent chemical surface treatment can often neutralise these impairments and thereby re-establish the original good corrosion resistance.

 

Pickling is the most important process and takes place in a strong mixture of nitric acid and hydrofluoric acid. During the process, the steel is corroded slowly and tempering etc. is especially dissolved from the welding process. The end result is a beautiful, matte surface without impairments in terms of corrosion resistance. The roughness of the steel is increased during the pickling, which can be critical for food/pharma applications.

 

Passivation is carried out in a pure nitric acid (20-25%) and has the purpose of strengthening the natural protecting film of chromic oxide of the steel. On the other hand, tempering as well as other things from the welding process is not dissolved. Passivation does not influence the roughness of the steel to a degree worth mentioning.

 

Decontamination is carried out by treating the steel in “weak acids” such as citric acid, phosphoric acid, formic acid or similar. The acid is suitably mild to a degree where the steel is not affected at all. The process is especially used to dissolve potential impurities (e.g. rouge) from the surface of the steel. Industrial CIP (“Cleaning In Place”) is often carried out by use of decontamination.

 

Electropolishing is an electrochemical process and requires an external power source. The liquid is often strong sulphuric acid/phosphoric acid and the polishing effect occurs by the steel being gradually dissolved. The result is an extremely shiny and resistant surface with ultra-low surface roughness and without any weaknesses. Electropolishing is particularly applied in the pharma industry.