Introduction of Passivation
What is the Passivation Process?
As defined in MIL-STD-753C, the passivation process is the final treatment/cleaning process used to remove iron from the surface of corrosion resistant steel parts such that a more uniform formation of a passive surface is obtained thus enhancing corrosion resistance.Stainless steel is different from other metals in that as you get closer to the surface the composition of the metal actually changes. In the passivation process, free iron is removed from the surface into solution, leaving behind a higher chromium level. A good chrome to iron ratio is usually considered to be 1.5 to 1 or higher.
Passivation is a chemical treatment for stainless steel and other alloys that enhances the ability of the treated surfaces to resist corrosion.
There are many benefits of passivated equipment and systems:
· Passivation removes surface contamination
· Passivation increases corrosion resistance
· Passivation reduces the risk of product contamination
· Passivation allows you to extend system maintenance intervals
To understand passivation of stainless steel, it is critical to look at stainless steel itself. All stainless steels are alloys of iron, nickel, and chromium. Chromium makes up at least 10% of the metal. It is this element that gives stainless steel its resistance to corrosion. Often steel-makers add molybdenum to enhance chromium’s protective characteristics for highly corrosive or high-temperature applications.
Beyond the chemical makeup of the metal, the composition varies in the different layers that make up stainless steel. At the surface is the passive layer, sometimes referred to as the passive film, is responsible for providing corrosion resistance. It is a very thin layer of highly stable metal atoms that do not easily corrode or rust. It is only a few atomic layers in thickness where the ratio of chromium to iron (Cr/Fe) is at least 1.5-to-1. The chromium binds with oxygen to create a chemically inert, “passive” surface.