Electro polishing is electro-plating in reverse. Instead of depositing a coating of another material on a surface, the action of electro polishing is to remove a surface layer, typically 20-40 micro-metres in depth in the case of stainless steel.
In the majority of cases, electro polished stainless steel surfaces are bright and highly reflective. This results from the removal of an often contaminated surface layer and the electrochemical action of micro-smoothing. The advantageous features resulting from such a bright and smooth surface are many. The passive oxide layer, which is essential to prevent stainless steel from corroding, cannot be improved upon following electro polishing. By reducing the total surface area, a result of micro-smoothing, products are less likely to adhere to an electro polished finish. In the same way, surfaces can be cleaned and kept clean more readily. Friction is also reduced and the mechanism of preferentially removing surface high spots, makes electro polishing suitable for eliminating fine burrs. The highly reflective bright finish is ideally suitable for a number of decorative applications, particularly where the shape of the item requiring polishing is extremely complex.
Stainless steels are designed to naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen to form the chromium rich oxide surface layer, on which the corrosion resistance of these alloys depends.
Naturally occurring conditions such as air or aerated water will do this and so under many exposure conditions stainless steels will naturally self-passivate.
Passivation treatments are sometimes specified, but it is important to consider whether this is strictly necessary or not.
Stainless steels cannot be passivated unless the steel surface is clean and free from contamination and scale from welding operations.
Scale may need to be removed first by 'pickling' (or mechanical abrasion) and although the surface of freshly pickled stainless steel will normally be immediately passivated once the pickling acid has been washed off, it is important not to regard these two treatments as the same.
Pickling usually involves nitric / hydrofluoric acid mixtures, whereas, traditionally passivation has been done using only nitric acid.
Nitric acid alone can be used to remove light surface iron contamination after which the acid facilitates the passivation of the cleaned steel surface.