Machines are the heart of any processing operation (food, pharmaceutical or chemical), so corrosion is a major concern for factory owners. Most machines are made with stainless steel, which is corrosion-resistant but not corrosion-proof. That’s why manufacturers perform passivation to protect their machinery from corrosion.
The idea is to make the stainless steel surface passive, i.e. less likely to react and corrode due to exposure to air or other substances. The exact process may differ from factory to factory. Here’s everything that you need to know about passivating in brief:
What Is Passivation?
Passivation is the process of turning a material passive so that it reacts or corrodes very slowly under the environment it exists in. In the processing industry, the process is a non-electrolytic process that makes stainless steel surface resistant to rust. Factories do this by using nitric acid or citric acid to remove iron and iron oxide from the surface to form a chromium oxide layer that doesn’t corrode easily.
Why Passivate Stainless Steel In Processing Equipment?
This process might seem unnecessary when you consider that stainless steel is fairly corrosion-resistant material. However, corrosion-resistant does not mean corrosion-proof. The alloy can corrode over time, so manufacturers passivate their machinery to increase its life.
Chromium is the metal in the stainless steel alloy that gives it the corrosion-resistant property it is known for. Manufacturers passivate their processing equipment to form a chromium oxide layer over its surface by exposing chromium to oxygen. The chromium oxide layer is corrosion resistant and prevents the metal below from reacting to the environment.
Iron is the most vulnerable metal in the alloy because it reacts to oxygen and forms iron oxide, which can contaminate whatever the machine processes. Iron makes up about 60-70 percent of the alloy depending on its grade. Manufacturers prevent rust by passivating the stainless steel surface to remove iron and iron oxide through nitric acid or citric acid.
Under ideal conditions, the alloy in the processing equipment should form a corrosion-resistant oxide layer naturally over time. But under normal conditions, the alloy cannot form this layer due to cutting tools used on the surface, sulfides added to it or contamination from the factory. So you need to passivate the surface for better resistance.
How Does It Work?
The workers cover the stainless steel surface with an acid (usually nitric or citric) and let it sit for some time. The acid removes the iron and its oxides from the surface, leaving chromium and nickel. These metals react with oxygen to form a corrosion-resistant layer that protects the metals below. More chromium on the surface means a thicker corrosion-resistant layer, which means better protection.
Should I Use Nitric Acid Or Citric Acid?
It’s common for manufacturers to use nitric or citric acid before putting up their capsule machine for sale. Most people prefer citric over nitric because many tests show that the former is more effective than the latter.
Nitric acid removes nickel along with iron, which can make the corrosion-resistant layer less effective. It also takes longer to passivate the surface and is harmful to humans.
In contrast, citric acid only removes iron and leaves the chromium and nickel layer intact. This forms an effective protective layer on the surface. The acid is safe for humans because it’s found in many food items, particularly fruits. So it’s easy to see why people prefer this acid when passivating their capsule machine for sale.
Do I Need Passivation?
You usually need to passivate your machines for most manufacturing processes but not for everything, You can perform a passivation test based on ASTM A380 specifications to know whether you need the process or not. If your machine passes the test, then it’s surface is passive and you don’t have to passivate it.
Types Of Passivation
There are many passivation techniques that a factory owner may use based on their requirements:
Tank Immersion: In this method, the workers wash, rinse, passivate and dry machine parts in a multi-tank system. The tank treats all surfaces of the parts with the same level of finish, giving you maximum corrosion-resistance.
Circulation: In this technique, a chemical solution circulates through a pipe system. It’s useful for processing equipment involved in carrying corrosive substances.
Gel Application: Here, workers treat the machine manually by applying pastes or gels to the surface. It’s perfect for welds or other areas that need precise treatment by hand.
Spray Application: Here, workers spray the acid on processing equipment on-site. They take proper precautions for safety and disposal.