Passivation is a term referred to the acid-cleaning process that is needed to remove free iron on the surface of stainless steel equipment. It is an important step in removing any foreign substance on the surface and, afterward, creating a chromium-rich surface that can withstand corrosion. This helps in extending the lifespan of the equipment or part.
Different Methods of Passivation
There are different methods used to passivate. Among these are:
As the name suggests, the entire metal is submerged in a tank containing an acid solution. This is commonly used for small equipment or parts that can be taken apart quickly.
- Spray Application
This passivation method is generally used on parts that cannot be submerged in tanks. This involves spraying the acidic solution on the surface manually to remove any substances or free iron.
A machine’s internal components also need to be passivated too. This is to ensure that they will continue running even after being used frequently. One way to achieve this is through the circulation method. In this method, the acidic solution will run through the inner parts of the machine.
During the passivation process, an acidic solution is needed to remove the foreign substances and free iron. There are two options used here and these are:
- Nitric Acid
This is the primary chemical method when passivating stainless steel surfaces. This is a strong mineral acid that can easily dissolve any iron compounds on the surface as well as other trace metals. Aside from that, nitric acid is a strong oxidizer. This means that it can also activate the chromium oxide layer on the stainless steel. To successfully passivate stainless steel using nitric acid, high temperatures and extended times are needed.
- Citric Acid
Citric acid can also be used in the passivation process. It is as effective in removing iron and other compounds like nitric acid. In addition, it is also biodegradable and can be used as a food ingredient. However, it cannot oxidize like nitric acid. If this acid solution is used, you will need to rely on natural air oxidation to create a layer of protection.
Importance of Passivation
Some may think that passivation is just for cosmetic purposes. However, having to deal with rusting machinery is not only frustrating but will cost you a lot of money. This is why passivation is a necessary process in the manufacturing industry. Through it, you are adding a layer of protection against corrosion and ensuring that the machine will not break down in an untimely manner.
Passivation processes follow industry standards. The most prevalent are ASTM A380, ASTM A967, as well as AMS 2700. These standards list acceptable passivation processes. The vendor and customer can choose their method of passivation. As stated before, the method used can either be nitric acid or citric acid. Both these acids will be able to remove rust and free iron on the surface of stainless steel equipment and machine without compromising the chromium.