Stainless steels are considered to be self-passivate. The conditions required include being exposed to adequate oxygen. Having the appropriate amount of oxygen permits the development of chromium.
The corrosion resistance lies in the steel’s ability to resist corrosion hinges on the steel’s ability to respond to oxygen appropriately. Aerated water also enhances the stainless steel’s ability to self-passivate.
Overview of the Passivation Process
Any time there is passivation of stainless steel, the surface is inevitably compromised. The natural chromium oxide film is altered, exposing the steel to imperfections and contamination. Passivation refers to process of coating a metal surface.
The coating process is essential to combating signs of reactivity. You don’t want the iron to be contaminated because this causes the formation of rust. The goal is to strip the surface iron while leaving behind remnants of chromium and nickel. The process permits the formation of an oxide layer. The layer serves as a barrier to corrosion.
Removing Iron Contamination
Nitric acid can enhance the passivation process. The steel must be clean and free of scale in order to complete the process. No contamination may exist prior to the passivation process.
A combination of nitric and hydrofluoric acid mixtures can be applied to the surface in order to complete the pickling process. The pickling process is done to remove the scale from the surface. Nitric acid applications are capable of thoroughly removing signs of iron contamination.
Why does passivation have to be performed in the first place?
The process has to remove the film of the chromium oxide to preserve the integrity of the steel. The reason the damage happens in the first place is because there is heat, chemical or mechanical acts that have caused the problem to occur.
The corrosion resistance is affected by the extent of the damage to the surface. Damage can be done to the surface in a variety of ways. It isn’t uncommon for the passivation process to have to be completed on several occasions to keep the surface properly protected.
Nitric Acid During the Process
In a commercial setting, a similar process takes place. The piece must first be dipped in nitric acid. The acid is capable of removing the contaminants from the surface. The nitric acid sanitizes the surface of the metal.
It also re-oxidizes the steel in a matter of minutes. The presence of the chromium on the surface becomes prevalent when the entire process has been completed.
What Can’t Passivation Accomplish?
Passivation is usually performed following the welding process. The passivation process is influenced by the caliber of the welding performed on the piece. The process isn’t as effective if there aren’t any signs of discoloration. The process can’t eliminate all issues associated with the weld.
Welding is the reason why steel has to be passivated. Any time some form of fabrication occurs, the stainless steel becomes vulnerable to surface contamination. The passivation process restores the chrome to iron ratio to make the iron resistant to corrosion.