What is the Passivation of Stainless Steel?

Untreated stainless steel has a chemically reactive surface and can rust over time. However, the process of passivation — chemically washing the body of a stainless steel part — protects the surface and results in a longer-lasting item that better resists corrosion and rusting. Passivation removes free iron from stainless steel surfaces, which is highly advantageous for newly machined and manufactured parts. Some of the benefits of passivation include:

  • Resistance to the elements: The iron in stainless steel is the reactive component in the alloy. Chromium and nickel (the other metals in the alloy) are much less reactive to air, moisture, and water. Because the acid wash removes the surface iron, only the passive chromium and nickel oxide layer get exposed to air.
  • Reapplication: Stainless steel parts can undergo the passivation process multiple times without weakening or adverse reactions, allowing operators to deal with iron exposure from physical impact or damage. Regular passivation processes extend the lifespan of the item.
  • Reduced maintenance costs: Passivation reduces the risk of broken parts, contamination from rust, and unscheduled downtime.

The Passivation Process

Passivation is a three-step process that can happen immediately following fabrication and machining. 

 

  1. Technicians thoroughly clean the stainless steel part to remove grease, dirt, debris, and shavings. This step ensures that the acid wash reaches every inch of the surface to dissolve any unwanted free iron. It also reduces the risk of flash attacks—dark spots and deposits that stain the passivated surface.
  2. The parts are placed in a passivating bath of either nitric acid or citric acid. The components sit in the bath until all of the iron in the surface layers has dissolved.
  3. Testing verifies the removal of all iron from the surface, after which passivation is complete. Different grades of stainless steel may undergo slightly different testing methods. The most common test involves swabbing a test part with copper sulfate to see if plating copper develops.

Considerations for Tank Passivation

Passivation experts can carefully balance different factors to tailor the passivation process for each batch of stainless steel products. Some of the unique characteristics that can change between batches include:

  • The chemical composition of the passivation bath: The bath will use nitric or citric acid, depending on the stainless steel grade, the application specifications, and the environmental concerns of the company.
  • Submersion duration: Different parts need to be submerged in the acid for different lengths of time to ensure all iron gets dissolved.

These factors and others will play a role in the efficacy of the free iron removal process and the final surface quality, which affects the service life and aesthetic appeal of the final product. 

Choose Highland Equipment for High-Quality Stainless Steel Passivation

At Highland Equipment, we specialize in thorough passivation processes that result in high-quality, aesthetically pleasing stainless steel components. We offer a full suite of passivation surfaces to meet the finishing needs of numerous product types. Contact us or request a quote to see how our passivation services can support your project. 

 

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