How Do Nitric Acid Passivation, Pickling, and Electropolishing Work?

In Wire Forms, Stainless Steel Baskets

Marlin Steel on October 1, 2019

Marlin Steel Basket 105A-31 Electropolished Stainless Steel Basket

Passivation is an important process for helping to maximize the corrosion resistance of stainless steel alloys. There are many different variations on the process of passivation, but the basic goal is the same—to remove impurities and enhance the performance of the steel alloy’s protective oxide layer.

Two of the most common variations of the passivation process are nitric acid passivation and pickling.

A separate process known as electropolishing shares some characteristics with passivation, but has a few extra effects and benefits.

What is Nitric Acid Passivation?

Nitric acid-based passivation is one of the most basic passivation techniques out there. Here, the stainless steel wire or sheet metal is dipped in a bath of oxidizing nitric acid. This mild acid bath removes some free iron molecules and other foreign matter from the surface of the steel while encouraging the development of the passive oxide layer.

However, this process does not remove heat tint, weld burns, or oxide scale from the surface of the steel.

What is Pickling?

Pickling is very similar to the basic passivation process in that a bath of acidic chemicals is used to strip the surface of the steel. However, where the basic passivation process uses mild nitric acid, pickling uses a more intensive chemical mixture, such as hydrofluoric acid.

This stronger acid strips the surface layer of the steel, removing impurities, weld burns, and free iron molecules—leaving an even, satin-like finish.

What Should You Know about Passivation Processes Like Nitric Acid and Pickling?

These brass screws have undergone a form of passivation to improve their surface condition and enhance their corrosion resistance.These two processes cost about the same to carry out, and can help remove impurities and improve the strength of stainless steel’s passive oxide layer.

On a side note, it’s important to know the exact alloy of stainless steel being processed prior to picking your passivating process. This is because using the wrong solution on your stainless steel could cause damage to the steel rather than enhancing it.

For example, compare 304 vs 430 stainless steel for passivation processes. Each of these allows have different levels of resistance to nitric acid and other corrosives, which means that one might need a stronger nitric acid bath than the other to ensure proper passivation in the stainless steel. To note, austenitic stainless steels (like grade 304 stainless steel), tend to have higher chromium content than martensitic stainless steels (like grade 430 stainless steel), making the austenitic alloy more resistant to corrosion and pitting.

Electropolishing Stainless Steel

Like the passivation processes listed above, electropolishing involves the use of a chemical bath. However, unlike the two processes above, electropolishing uses an electrolyte bath and an electrical current to dissolve the surface layer of the metal—leaving a microscopically smooth, shiny surface.

The smoothness of the new surface makes it almost perfectly non-stick because the electropolishing process removes nearly all of the microscopic flaws that debris would normally stick to. This has the added benefit of making the electropolished part easier to clean and sanitize.

Benefits of Electropolishing Stainless Steel

  • Deburring Parts. By actively stripping the surface layer of a part, electropolishing removes many of the burrs and sharps that other finishing processes may have missed.

  • Decreased Part Fatigue. Electropolishing eliminates surface fissures and minimizes the fatigue splits that occur on a microscopic level during the manufacturing process. This helps reduce part fatigue and create a stronger, longer-lasting part.

  • Reduced Galling/Seizing. Because of the microscopic smoothness provided by electropolishing, this process is often used by manufacturers on threads of material to prevent galling and seizing in delicate mechanisms.

  • Uniform Appearance. Electropolishing removes many of the weld and burn marks from steel that milder passivation processes would leave behind. This provides an aesthetic improvement for some manufactured parts.

  • Ease of Control. Electropolishing can be applied to a variety of stainless steel alloys without requiring much customization for the setup. This makes electropolishing a better option for manufacturing setups that process numerous kinds of stainless steel alloys—such as when the manufacturer often has to choose between 304 vs 430 stainless steel for different applications.

The biggest drawback of the electropolishing process is that it can cost a bit more up front over a standard passivation process. Also, electropolishing doesn’t significantly alter the protective oxide layer of steel the same way that passivation can.

However, electropolishing remains the preferred treatment option when the surface condition of the part is the primary concern—largely because it creates a superior surface compared to passivation. For example, both the food and pharmaceutical manufacturing industries tend to prefer electropolished stainless steel surfaces because the microscopically-smooth surface is incredibly easy to clean and sterilize compared to a standard 2B metal finish.

With the right finishing process, the performance of a custom wire basket or other metal form can be greatly enhanced.

Need help choosing the right surface finish for your custom wire baskets? Reach out to the Marlin Steel team for help and advice. Marlin’s engineers have years of experience in helping manufacturers create the perfect solutions for their specific challenges.

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Author: Marlin Steel
Marlin Steel
Marlin Steel is a leading manufacturer of custom‐engineered products from steel wire and sheet metal. Its industrial material handling containers serve many industries including aerospace, defense, medical and automotive.

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