A simplified guide of the process of passivation using nitric acid.
Passivation can be defined as a metal finishing process that serves to prevent corrosion. For stainless steel, the process of passivation uses nitric acid to eliminate free iron from the surface.
The chemical treatment that is done produces a protective layer of oxide. This layer doesn’t react with air so you can rest assured that there won’t be any kind of corrosion.
The Origin Of Passivation
In the midst of the 19th century, a chemist known as Christian Friedrich Schonbein made a discovery on the effectiveness of passivation. This was after he dipped iron into concentrated nitric acid and realized that the iron had minimal to no reactivity in comparison to iron that wasn’t exposed to the concentrated nitric acid.
How Nitric Acid Passivation Works
Stainless steel, an iron-based alloy is composed of iron, chromium, and nickel. Its corrosion-resisting properties come from the chromium content. When chromium is exposed to air, it forms a thin chromium oxide film that covers the surface of the stainless steel, thereby protecting the iron beneath from rusting.
The main reason why passivation is done is to optimize and boost the creation of chromium oxide.
Immersing stainless steel into an acid bath dissolves the free iron off of the surface but the chromium remains intact. The acid chemical eliminates the free iron. What is left is a uniform surface that has more chromium content.
Once the stainless steel is exposed to air after going through the acid bath, it forms a chromic oxide layer within 24 to 48 hours. The high concentration of chromium on the surface empowers the creation of a more protective thicker chromium layer. Eliminating the free iron off of the surface cuts off all the avenues of corrosion.
The established passive layer creates a surface that doesn’t react chemically and protects the stainless steel from getting rust.
When Is Passivation Done?
Passivation as a process is done after grinding, cutting, welding, or any other mechanical operation that alters stainless steel. In normal conditions, stainless steel will naturally resist corrosion. This, however, doesn’t rule out the need for passivation.
Things such as foreign materials found in a manufacturing environment, sulfides that are added to stainless steel to boost machinability, or particles of iron derived from cutting tools that stick on the stainless steel surface can all hinder the chromium film layer from forming and prevent corrosion.
All these contaminants must be eliminated to the level of the surface grain boundaries in order to restore a uniform corrosion-resistant surface. Passivation serves to correct such problems.
The most common passivation specification, astm a967, serves to give instructions on the correct process to follow when passivating stainless steel, titanium, among other materials.
The phases followed as per the specifications include:
- Cleaning – This involves eliminating any contaminants like oils and grease from the surface.
- Passivating – This involves performing the chemical treatment process by immersing the stainless steel into an acid bath, more specifically nitric acid.
- Testing – This involves testing out the stainless steel surface that has been passivated to determine whether the process done has been effective.
That is basically a simplified guide of the process of passivation using nitric acid.