Nitric acid combines with the chromium to generate the chromium oxide passive layer, which prevents corrodents from accessing the iron beneath.
FREMONT, CA: As stainless steels like 304 and 316 are alloys with some critical components, they are ‘stainless’ and resist corrosion, as many sanitary processors know. Some people are even aware that stainless steel production equipment creates an inner layer that keeps the metal from rusting. The fact that this protective barrier is barely one to three nanometers thick is difficult to comprehend. It is only a few atoms deep, but it is enough to provide vital protection if the conditions are appropriate and stable.
Passivation is a corrosion-prevention metal finishing technique. The passivation process in stainless steel removes free iron from the surface by using nitric acid or citric acid. The chemical treatment creates a protective oxide layer that makes it less probable for air to react chemically and cause corrosion. Stainless steel that has been passivated resists rust.
The Passivation Process
Two stages occur during chemical passivation; however, whether two steps are required depends on how the process is carried out. The first step is to apply an acid to the interior of the device. The acid reacts with the iron on the surface, removing it. If it is not entirely eliminated from the start, it can leave behind localized corrosion sites that will increase over time. Nitric acid combines with the chromium to generate the chromium oxide passive layer, which prevents corrodents from accessing the iron beneath.
For a variety of reasons, citric acid is gaining ground on nitric acid in terms of passivation, including:
- It can passivate a broader range of stainless steel alloys, allowing it to be employed in systems with several alloys.
- It is significantly less toxic and harmful, as well as biodegradable, making disposal much easier.
- It is well-suited for food and beverage processing usage because it is utilized as a food additive and is on the FDA’s GRAS (Generally Recognized as Safe) list.
Some passivation techniques thin the passive layer by eliminating nickel and chromium from alloys as well as iron. Citric acid reduces the risk of this damaging removal by producing a thicker oxide layer.
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