Things You Should Know About Citric Acid Passivation

What is passivation?

Before understanding what passivation is, let us first learn about stainless steel. Stainless steel is a combination of various metals, primarily iron, nickel, chromium, and in some cases manganese and molybdenum along with other materials in small quantities. It is this unique mixture that allows the special attribute of corrosion resistance by forming an outer protective passive oxide layer. Stainless steel surfaces are generally made highly resistant to corrosion and they must be properly cleaned prior to use. Even with 300 series austenitic stainless steel, once a corrosion site has started, it will only get worse, being continuous and self-catalyzing. In the corrosive environment, the growth of the corrosion site accelerates very quickly! Thus, proper cleaning and passivation of surfaces before use is essential to achieve maximum resistance to corrosion.

Passivation of a particular metal is a process that is carried out to get rid of the impending contaminants from the said metal. It is aimed at making it safer and more functional for everyone irrespective of their job responsibilities.

When it comes to passivation of stainless steel, the industries often adopt their techniques that they either develop or follow as they are prominent among the users. The first step of passivation aims at removing the contaminants that largely include iron and iron compounds from the higher exterior of the metals. It is an indispensable step that prevents the metal from corroded. This first step is generally termed as ‘Pickling’.

The entire process chemically eradicates free irons and a passive oxide “film” layer which further enhances corrosion resistance. When exposed to oxygen(air), the stainless steel, going through passivation, forms a chemically inactive or inert chromium oxide surface.

Per common passivation specifications ASTM A967 & A380 – “the eradication of external iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, mostly by a treatment with an acid solution that removes the surface contamination, but does not significantly affect the stainless steel itself.”

Further, ASTM A380 explains passivation as “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.”

It is a non-electrolytic technique that uses citric or nitric acid for removing free iron and forming an inactive, protective oxide layer that consecutively makes the stainless steel more rust-resistance due to lack of iron to react with the atmosphere.

Contrarily, nitric acid is extremely corrosive by nature, represents significant personal dangers, and threatens property as well. It is also an oxidizer which increases risk and expense. Heated, its risk is aggravated. After use, expensive waste treatment is essential prior to disposal and can expose the user to long-term liability under regulatory agencies.

Citric acid provides the maximum protection by providing:

1. Quick removal of free iron from the surface

2. Improved removal of free iron from the surface

3. Lower cost

4. Low hazard chemistry

5. Environmentally safe chemistry

6. Passivated surfaces that pass all salt spray, immersion and high humidity tests

However, the actual question is whether these two techniques give equivalent outcomes.

If we witness the outcomes of the classic nitric acid process and the modern citric acid process, we will find that results are quite comparable and, in many cases the citric acid technique renders better results when compared to corrosion tests of different types.

In practical, citric acid is a gentle yet efficacious manager for passivation. It is recyclable and hardly comes up with any toxic gases that can harm not only you in person but also the environment, directly or indirectly. Unlike the nitric acid passivation process, citric acid process is not a chromium oxidizer and that is why it never involves any difficult procedure. Due to its clearer and cleaner effect, citric acid passivation is extremely effective for the passivation of smaller steel parts. The industry leaders believe that this process is not only helpful but also it is risk-free for the users.

While on the other hand, the citric acid passivation is comparatively less expensive and so, it goes well with the budget of various industries or users. The process is time-saving as well. As per a recent research, 99% of citric acid consumption in the USA is due to this industrial use of the chemical.

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