One often wonders what pickling stainless steel is for: isn’t it already stainless and corrosion-resistant?
In reality, the answer is no: the passivation layer loses its natural properties as a result of various processes, including welding, and a pickling treatment is necessary to recreate that protective coating. Let’s try to clarify this aspect of steel more clearly.
Stainless steel pickling treatment
The main advantages of pickling stainless steel are:
- Removal of contaminants such as welding oxides
- Surface restoration of the thermally altered area
- Preparation of the material for the subsequent passivation treatment
- Benefit from the future formation of the protective oxide film
In order to maximise the corrosion resistance of the pickled stainless steel, the process continues with a passivation process: the natural characteristics will be restored, removing all the impurities present on the surface again.
Like welding, pickling and passivation treatments are also regulated by strict standards: these are the ASTM A380 and ASTM A967 standards.
Surface treatments on steel: electrolytic pickling
Electrolytic or electrochemical pickling is one of the most commonly used surface treatments on steel. It can be performed in three different ways:
- Anodic: while surface oxides are dissolved, the steel surface is passivated. To be totally passivated, the metal must no longer be electrochemically active; on it, the oxygen will begin to oxidize, which will cause the removal of the fragments still present.
- Cathodic: during this process, the hydrogen generated favours the removal of the slag present. Compared to anodic pickling, the metal is not passivated and becomes more fragile, precisely because of the gases.
- Alternating current: the two previous methods are applied alternately over time, at a constant frequency. To do this, the material is passed through different tanks (anodic and cathodic) or alternating current is applied directly in the same tank.
This type of treatment, in addition to ensuring a superior aesthetic quality, more respects the metal and avoids the formation of imperfections that, if underestimated, can lead to the rupture of the structure itself.