Stainless Steel Passivation Process
Stainless passivated steel in a food factory
The corrosion resistance of stainless steels and other metals is strictly dependent on their surface conditions and in particular by the presence or less on their surface a layer of oxides said passive layer. The changes that occur on the stainless steel surface are defined as phenomena of passivation. If these phenomena lead to the cancellation of the corrosion rate, then it can be said that lead in passivation conditions the stainless steel.
Re-passivatin process of the stainless steel
The passive layer of the stainless steel has a chemical composition which is particularly different from the basic composition of the same:
- Around 65%Cr + Chromium oxide
- Around35%Fe + Iron oxide
Molybdenum and Nickel have a very low percentages within the passive layer.
Not always the intervention of the passivation phenomena lead to passive conditions. In the case of stainless steels, the colored oxide films which are observed during the welding phase or black scales that are formed during the hot rolling are less protective of the corm of oxide film that forms on the metal surface.
Normally, the protective oxide layer is often about 1.5-2.5 nm, and is easily visible through special and expensive microscopes (TEM). The term passivation derives from the fact that chromium has a strong affinity with oxygen. When the steel is in contact with an oxygen-rich environment, the chrome is very reactive and tends to form very stable oxides and hydroxides. These compounds are protective because they suppress unwanted reactions that can lead to the stainless steel corrosion. Thus, the stainless steel corrosion resistance derives from the fact that it has a percentage of chromium equal or greater than about 18%. Thus, the stainless steel has the opportunity to spread locally some chromium particles on the surface to form the oxides that enhance the corrosion resistance of stainless steel. The passive layer that forms on the surface of stainless steel is equipped with electronic conductivity then it can generate the chemical oxidation-reduction processes with oxygen that can stop the corrosive circuit.
The percentage of chromium and other substances present in the steel is one of the parameters which influence the quality of the passive layer. A steel of the AISI 200 series will have a lower corrosion resistance compared to an AISI 304 because, having a lower concentration of Nickel, does not have the ability to reform quickly the passive layer after, for example, an abrasion processes and / or pickling.
Another key parameter to determine the quality of the passive layer is stainless steel surface finishing. Very often to increase the corrosion resistance, the stainless steel is electrochemical polished. This process ensures:
- The passive layer grain microstructure is smooth and homogeneous
- Decreasing the roughness that prevents the adhesion of contaminants on the steel surface
- An increase of the migration of chromium atoms on the surface
- An increase of the passive layer thickness due to a better chemical interaction with oxygen in the environment.
Stainless Steel electropolished
Electropolished stainless steel: Passivation layer structure
If the surface finish has been subject to mechanical abrasion (satin finish):
- The microstructure is not homogeneous
- It presents the contaminated abrasive substances that interlock on the surface, becoming points for pitting corrosion
- Reduction of the passive layer thickness.
Pitting corrosion after abrasion
Passivated structure of abrased stainless steel
The construction of the passive layer also depends on the thermodynamic characteristics (temperature, oxidizing environment, etc ….) that allow to adjust the passive layer in order to obtain a stable and durable layer over time.
During standard operation, the quality of the passive layer is independent of:
- Clean air;
- Pure water;
- Passivation in concentrated nitric acid at 5% -30%.
These factors listed above govern the time of passivation of stainless steel:
- Clean air: around 48-96 hours;
- Pure water: around 6-15 hours;
- Passivation in concentrated nitric acid at 5% -30%: around 30-120 minutes.
Finally the quality of the passive layer is determined by the percentage of alloying substances inside the steel and the thermodynamic conditions of the environment, promoting a compact and chemically stable layer. The passivation time is determined by the different environments in which the steel is exposed. The electropolished stainless steel is the best operation to obtain a homogeneous surface, free from contamination and passivated.
Electropolishig and passivation of AISI 304 component