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Why Is Passivating Stainless Steel Necessary?

The identification of chromium around the turn of the 19th century led to it becoming the key ingredient in an iron-based alloy used widely today. In the mid-1800s, it was found that chromium, when combined with iron, caused the metal to be both more formable and durable. This made it an ideal additive for steel products, though at the time the proportion of chromium added was less than 5 percent. In the early 1900s, it was found that higher proportions of chromium were added also helped prevent the iron from rusting.

This eventually led to the 18/8 stainless steel formula containing 18 percent chromium and 8 percent nickel. This alloy is still used for cooking equipment and cutlery and is included in the 300 series of stainless steels. Many of the traits for which stainless steel is best known are due to passivation, which creates an ultrathin layer that further protects stainless steel from rusting and other corrosion. Though only 1-3 nanometers thick, passivation provides enough protection when conditions are right to enhance this anticorrosive trait. Due to its enhancement of corrosion resistance, manufacturers of components or equipment need to understand the basics of passivating stainless steel.

Understanding Stainless Steel Passivation

Passivating stainless steel makes the surface passive, enhancing the alloy’s immunity to corrosive environmental elements. The process of passivation is a chemical treatment that can also be used on various alloys, metals, and metalloids. For most metal alloys, including stainless steel, chromium is used due to its excellent corrosion resistance.

Besides passivating stainless steel, passivation is often used with materials that include: 

  • Aluminum
  • Copper
  • Nickel
  • Silicon
  • Titanium
  • Zinc
  • Other types of steel

Passivation occurs once the alloy or metal is bathed in citric or nitric acid. The chromium binds with oxygen to create a non-reactive or passive layer of chromium oxide on the surface. This chemically inert coating prevents chemical reactions that lead to corrosion. This passive layer forms naturally in alloys with chromium content from 10.5 to 12 percent and higher, though passivating stainless steel equipment should be done chemically to provide immediate protection.

Purpose of Passivating Stainless Steel

The exact ratios of iron, nickel, and chromium determine a stainless steel’s grade. When required to resist corrosive agents at extreme levels, molybdenum is added to the chromium to enhance its protective qualities. Often stainless steel components have free iron from the machining process on their surfaces, and the acid bath treatment that passivates stainless steel also cleans these and other contaminants away. Passivating stainless steel makes the alloy more versatile, enhancing its capabilities to perform well in various applications.

Passivation Process for Stainless Steel

Before passivating stainless steel, however, a critical first step involves cleaning the surfaces to eliminate contaminants that might compromise the process. Besides free iron, other substances like coolant, grease, or other materials left over from the production process can negatively affect passivation. Stainless steel compromised by these contaminants will tend to corrode more readily.

When chemically passivating stainless steel, the acid is applied to all equipment surfaces. The most commonly used are nitric or citric acid. This acid reacts with iron, removing it from the surface. If it hasn’t been removed thoroughly enough, spots can develop in localized areas on the surface, which then grow over time.

Nitric acid reacts with the chromium in stainless steel, creating a passive layer of chromium oxide that prevents corrosion. Though nitric acid is typically used for passivating stainless steel, it has downsides. It’s both dangerous to handle and toxic and capable of contributing to explosions. The use and disposal of nitric acid are also strictly regulated. Because of these dangers, citric acid is increasingly used when passivating stainless steel.

Reasons for utilizing citric acid instead of nitric acid include:

  • Biodegradability makes its disposal less challenging
  • Capable of passivating more types of stainless steel alloys
  • Lower toxicity and less hazardous
  • Used as a food additive, so is well-suited for applications that involve processing food and beverages

In addition to these advantages of passivating stainless steel with citric acid, this process promotes a thicker layer of chromium oxide. Unlike other passivation processes, it doesn’t remove chromium or nickel from the alloys, resulting in thinner layers. However, it’s not the citric acid that does the actual passivation. Citric acid simply prepares surfaces to passivate spontaneously.

Advantages of Passivating Stainless Steel

Manufacturing stainless steel components often results in embedded free iron or other foreign material on the parts’ surfaces. These contaminants shorten components’ lifecycles, allowing corrosion to develop over time. Passivating stainless steel helps restore the corrosion resistance of the alloy.

Passivation offers these four main benefits:

  • Decreases chance of contaminating the product
  • Improves corrosion resistance
  • Reduces frequency of maintenance needed
  • Removes contamination from surfaces left over from machining processes

Passivating stainless steel results in better corrosion resistance but doesn’t prevent surface deficiencies like burrs, heat tints, micro-cracks, and oxide scale. To deal with these imperfections, other finishing techniques should be used.

Standards for Passivating Stainless Steel

Generally, standards for stainless steel passivation are created by ASTM International, a global organization that sets standards regarding materials, processes, products, and services. Stakeholders that include consumer groups, government agencies, manufacturers, professional societies, and trade associations develop and continue improving the various standards for passivating stainless steel. The ASTM has a specific committee to deal with steel, stainless steel, and associated alloys. This committee assists with research, collecting data, and incorporating their knowledge into classifications, guides, procedures, specifications, terminology, and testing methodology on these standards.

Standards for passivating stainless steel include: 

  • A380/A380M-17: Stipulates standards regarding what precautions and recommendations should be taken when cleaning, descaling, and passivating stainless steel assemblies, components, machinery, and systems.
  • ASTM A967/A967M-17: Establishes standards for immersion treatment passivation with citric or nitric acid solutions, along with electropolishing; it states the appropriate steps that should be taken during the process, including what tests should be conducted on the surface and how the passivated stainless steel surface should appear. This standard was developed from the Department of Defense’s original standard, QQP-35, which was withdrawn in the mid-1990s in favor of ASTM A967.
  • ASTM B600: Specification for descaling and cleaning titanium alloy surfaces, particularly within the medical industry.
  • AMS 2700F: Specifies the requirements for removing surface contaminants, such as free iron, from the surface of stainless steel parts.

Using these standards, stainless steel components going through the passivation process will be made more corrosion-resistant and cleaner.

Request a Passivation Quote from RP Abrasives

Because many of our customers required passivation services, RP Abrasives began offering full-service finishing, including for passivating stainless steel. RP Abrasives is certified under ISO 9001:2015 for quality management in stainless steel passivation, which includes ASTM and AMS (Aerospace Material Specifications) standards that utilize citric acid. Using citric acid passivation techniques fits our company’s environmental goals to supply services that minimize our environmental impact.

Based in New Hampshire, RP Abrasives offers services to customers throughout the United States. However, most of our customers are in Connecticut, Maine, Massachusetts, New York, Rhode Island, Vermont, and our home state of New Hampshire. If you need a component passivated, we invite you to call us at (603) 335-2132, contact us online, or request a free quote.