Passivation is a type of chemical reaction which gives stainless steel its famous resistance to rust. The process changes the outer layer of steel so that it cannot react with oxygen to form rust, but it leaves the internal layers unchanged.
In doing so, it can protect the metal from corrosion without compromising its integrity.
How Does It Work?
Chemical reactions are physical things, and they require atoms to come into contact with each other to happen. When steel rusts, the iron atoms in the steel come into contact with oxygen atoms while in the presence of water.
The iron and oxygen atoms combine to create rust molecules, which damages the steel.
Passivation creates an external layer that prevents the iron and oxygen from coming into contact with each other. It occurs when the chromium in the stainless steel reacts with the air to create chromium oxide.
The layer of chromium oxide will remain present until it is removed, either by a chemical reaction or from simple abrasion. When that happens, the newly-exposed chromium will react with the air to create a new layer.
As long as this type of layer can form, the iron will not have access to the oxygen that it needs to rust.
This process can only work when there is a complete external layer of chromium on the outside of the steel. If the outer layer is a mixture of chromium and iron, then some of the metal will be protected by chromium oxide while the rest rusts.
Workers ensure that this layer is present by exposing the metal to an acid which removes the top layer of iron, but leaves the chromium in place.
Preparing a piece of metal for passivating requires two steps. The first requires the removal of any rust from the surface. There are usually small flakes on fresh steel, called mill scale, but this is also necessary before refreshing the passivation on old steel. The rust can be removed through mechanical abrasion, but some producers prefer to use nitric or hydrofluoric acid.
The second treatment removes iron from the surface of the steel. Historically, most steel producers preferred to use a nitric acid treatment, but many modern manufacturers prefer to use citric acid.
The citric acid treatment only removes iron, and it doesn’t introduce any pollutants to the mixture or produce dangerous waste, which has made it much more popular in recent years.
Passivation is powerful, but it does have limits. Anything that grinds or scrapes the chromium layer can remove it, which exposes the internal layers to the air.
Any of the chromium in those layers will slowly react to form a new chromium layer, but some iron will also be exposed. That iron can rust, and in doing so it will expose more iron to the air. In the long term, this can destroy the steel.
Fixing this problem is as easy as repeating the passivation process. This should happen as soon as possible when the metal starts to show signs of rust.
The more often the steel is exposed to things that can remove the chromium oxide, such as bleach, high heat, and abrasion, the more often the process will be necessary. This does take some effort, but it is much cheaper and easier than replacing a rusted tool or appliance.