Passivating Stainless Steel
It’s no secret, stainless steel isn't quite stainless. Parts of the alloy are, but it can be subject to rust, discoloration, and other slightly unappealing visual properties. Food and beverage grade stainless steel can be made from plenty of elements: chromium, nickel, molybdenum, silicon, aluminum, iron, and carbon. But for right now, let’s focus on iron and chromium.
This brings us to passivation. This is the establishment of an oxide barrier to provide a protective layer to the stainless steel from corrosion.
During the manufacturing and plant installation process, many things happen on the microscopic scale to the stainless steel. Scratches, bumps, pieces of tools break off; all etc. These tiny flaws expose or place a small piece of iron on the surface of the steel. When exposed to oxygen, the iron reacts to become iron oxide/rust. This is commonly seen as a discoloration called "rouging" where the rust changes the color of the stainless to a red/copper color. These small amounts of rust can be detrimental to the long life of the equipment and free iron can contribute to a medicinal and copper off flavors in beverages.
The first step of passivation is to remove those impurities and small pieces of iron. A degreaser used to clean any surface oils left over from manufacturing is critical, otherwise, a thin surface film of oil could remain and prevent a proper passivation. After cleaning, and acid solution is applied to remove any iron particles from the surface and create a new layer of chromium oxide. The most common acid for this purpose has been nitric acid, but citric acid (the same in your lemons and oranges) is becoming a better alternative for environmental and disposal reasons.
Finally, a protective layer must be created to prevent rust from forming in the future. The equipment must be left to dry with plenty of air for several days, a minimum of 24 hours for nitric acid and 72 for citric acid. This allows the chromium to oxidize. Where iron oxide changes color and starts to break down, chromium oxide does not and will protect the steel underneath it. So whenever you are getting a new piece of stainless steel equipment, it is a very good idea to clean off any oils and dirt, wash with an acid, and then allow to air dry completely before use.
If you need more information, or you are having difficulty finding citric acid in Canada, the company I recommend reaching out to is:
4511 Prime Parkway
McHenry IL 60050