What is the passivation process from jaiachuancasting's blog

As defined by MIL-STD-753C, a passivation process is a final treatment/cleaning process used to remove iron from the surface of corrosion-resistant steel components, resulting in a more uniform passivated surface formation for enhanced corrosion resistance.

Stainless steel differs from other metals in that the composition of the metal actually changes when you get close to the surface. During passivation, free iron is removed from the surface into solution, leaving behind a higher chromium content. A good chromium to iron ratio is generally considered to be 1.5 to 1 or higher.

What is passivation?

Passivation is a chemical treatment of stainless steel and other alloys that increases the corrosion resistance of the treated surface.

Passivating equipment and systems has many benefits:

Passivation removes surface contamination

Passivation improves corrosion resistance

Passivation reduces the risk of product contamination

Passivation allows you to extend system maintenance intervals

Composition of stainless steel

To understand the passivation of stainless steel, the key is to look at the 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 makes stainless steel corrosion resistant. Steelmakers often add molybdenum to enhance the protective properties of chromium in highly corrosive or high temperature applications.

In addition to the chemical composition of the metal, the different layers that make up stainless steel also vary in composition. On the surface is a passivating layer, sometimes called a passivation film, which is responsible for providing corrosion resistance. It is a very thin layer of highly stable metal atoms that does not corrode or rust easily. Only a few atomic layers thick, the ratio of chromium to iron (Cr/Fe) is at least 1.5 to 1. Chromium combines with oxygen to form a chemically inert "passive" surface.

Below the passivation layer is a transition region with a higher nickel concentration. Like the passivation film, the thickness is only 3 to 4 atomic layers. The nickel in this section protects the passivation film by preventing a chemical reaction with the iron in the layer below. It also acts as a protective barrier for the base metals that make up the bulk of stainless steel. The percentages of chromium, nickel and iron vary depending on the intended use of the article.

Why do you need passivation?

Although passivation occurs naturally in corrosion-resistant and chromium-rich alloys, under the right conditions, new stainless steel vessels or components will require passivation before being placed in service. Fabrication, machining, and welding leave behind contaminants such as metal oxides, inclusions, fabrication debris, and tramp iron that impair the metal's natural resistance to corrosion.