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How Do You Passivate with Citric Acid?

Citric passivation involves treating metal surfaces in an acid solution to make them better resist corrosion. As an iron-based alloy, stainless steel contains iron, nickel and chromium, with corrosion resistance due primarily to its chromium content. When exposed to oxygen in the air, chromium forms a thin layer of chromium oxide, which coats stainless steel surfaces to keep them from rusting.

Passivation augments and optimizes this layer, further protecting stainless steel surfaces from corrosion, which is why many stainless steel components undergo a passivation process after machining. Immersing stainless steel in an acid bath dissolves free iron on the surface; it doesn’t affect the chromium. Rather, passivation creates a layer on the surface with higher chromium content that further helps reduce the chance of corrosion.

Advantages of Passivation for Stainless Steel

Some of the primary benefits of passivation include:

  • Creates corrosion-resistant layer
  • Extends the lifecycle of stainless steel parts
  • Reduces maintenance needs
  • Removes contamination from product surfaces

Corrosion reduces the structural integrity and strength of stainless steel parts. Even though stainless steel resists corrosion due to its chromium content, without passivation metal surfaces often begin to pit, while also developing rust within cavities. This corrosion can spread through simple contact from a piece of rusty metal. This corrosion can then spread further, even getting into gaps between nuts and bolts.

Citric Passivation vs. Nitric Passivation

The two passivation methods used for stainless steel involve either nitric acid or citric acid.  Though nitric acid baths have been used for much longer, studies have shown citric passivation to be more effective and safer. Developed from citrus fruits like lemons, limes and oranges, citric acid is more environmentally-friendly than nitric acid. When correctly done, citric acid passivation cleans and brightens most stainless steel surfaces exceptionally well.

Compared to nitric passivation, citric passivation provides significantly increased safety while reducing costs. Used in many foods and beverages, this type of acid is also a bio-friendly additive in many cleaning products. With nitric passivation, companies must invest in safety devices to protect employees from nitric acid exposure. Additionally, citric acid doesn’t cause corrosion, so requires no specialty equipment to handle it, unlike nitric acid.

Additionally, there’s no regulatory oversight needed for manufacturers using citric passivation, as it doesn’t produce toxic waste material. With nitric passivation, nitrogen oxide vapors are produced, which result in detrimental effects to the planet’s ozone layer, while also causing acid rain and smog. Citric passivation doesn’t negatively affect the atmosphere.

Citric passivation is more efficient at removing free iron from stainless steel surfaces than nitric passivation, so requires less concentrated solutions. In addition, this type of passivation is more selective, removing only the free iron from the surface. Unlike nitric passivation which removes nickel and cobalt from the surface along with the free iron, citric only removes the free iron. Typically, these acid solutions used in passivation baths range from 4-10 percent by weight, whereas standard nitric acid solutions are 20-50 percent by volume. The time needed in an acid bath is lower with citric passivation as well, with production speeds up to five times as fast in some cases.

Benefits of Citric Passivation

Considering how much easier citric acid is to use for the passivation of stainless steel, it makes sense to offer a brief list of its key advantages.

Significant benefits of this type of passivation include:

  • Free iron is removed more quickly from metal surfaces using this process.
  • Its environmentally safe chemistry makes it easier to dispose of and use.
  • Processing results are excellent for all stainless steel grades.
  • The process adheres to industry standards, passing copper sulfate, high humidity, immersion or salt spray tests.
  • Using this technique only removes iron, while preserving other metals in alloys.
  • With no corrosive or toxic fumes, it makes workplaces less hazardous.

Applications for Citric Passivation

Citric passivation of stainless steel is used in the aerospace, automotive, defense, transportation and other industries to make precision components.

Other applications include:

  • Architectural features like catwalks, facades and handrails
  • Medical and dental instruments
  • Orthopedic implants
  • Sculptures and other metal artwork
  • Semiconductors and other electronics
  • Surgical implements

Citric Passivation Process

All passivation of stainless steel parts occurs after they’ve already been fabricated via cutting, grinding, welding or other machining techniques. The basic steps in the passivation process are simply cleaning, passivating and testing. Though stainless steel naturally forms an oxidized layer to inhibit corrosion, several adverse aspects of production can inhibit this from properly developing. This can include iron particles from cutting tools, foreign matter adhering to parts during production and sulfides utilized to improve machinability of parts. Passivation provides a means to restore corrosion-resistance to stainless steel surfaces uniformly.

The exact process of citric passivation for stainless steel depends on its machinability properties, how much chromium is in the alloy and other surface treatments employed to the part.

RP Abrasives: Citric Passivation for Safety & Environment

As an ecologically responsible company, RP Abrasives takes great pride in providing first-rate citric passivation services. Using citric acid-based solutions, we effectively passivate stainless steel and titanium, without damaging the environment. Through the use of citric acid passivation techniques, RP Abrasives improves metal components corrosion resistance substantially.

RP Abrasives offers citric passivation services that include:

  • Cleaning of parts before passivation
  • Large components
  • Part cleaning before passivation
  • Small batch sizes
  • Testing passivated parts with copper sulfate

RP Abrasives offers a variety of finishing services for metal surfaces, including citric acid passivation for stainless steel. Our company is versatile, able to deal with single complex parts, expensive prototypes, and largescale production. We invite you to contact us today to learn more about citric passivation or our other processes and services.