Stainless Steel Passivation
Talking about the passivation of stainless steels, we refer to the main property for which this type of alloy is chosen in a specific application. The passivation layer of a stainless steel is what makes it resistant to corrosion.
The passivation of stainless steels is a topic that would require volumes of metallurgy, electrochemistry and corrosion and an equal amount of real cases from all over the globe, where the passivation (or rather, the lack of passivation) is the main cause of damages and failures.
The corrosion resistance of stainless steels is therefore due to an invisible protective film formed mainly by Chromium oxides and hydroxides of the type Cr2O3 and Cr(OH)3. From here on talking about passivation, we will refer to the “chromium oxide” in its most common form: Cr2O3.
The minimum content of chromium, to consider a steel alloy as ?stainless? (due to the passivation), is standardized in the EN 10088-1 to 10.5% by weight (and a maximum of 1.2% carbon).
The specific characteristics of the passivation protective layer are:
-?????????? Compactness
-?????????? Insolubility
– ????????? Adhesion to the metal substrate
These main properties (of course not the only ones) make the film protective for the stainless steel surface, conferring a good passivation and protecting it from attacks by external agents. This process does not happen, for example, in the case of the iron oxides on carbon steels alloys: what is commonly called “rust” is nothing but a number of different oxides and hydroxides which, not respecting the above characteristics, confer no passivation. Those oxides, in fact, are porous allowing the flow of aggressive and oxidizing agents (even the simple oxygen in the air) to the metal substrate.
They are not insoluble, so in contact with water or simply with moisture, they dissolve.
On the contrary, when it comes to passivation of stainless steel, it refers to an oxide perfectly adherent and compact. This passivation layer, therefore, besides being a physical barrier against the passage from the outside to the metal substrate of aggressive agents, also leads to an increase in the electrochemical potential of the metal surface, in other words, the passivation ennobles stainless steel.
