Why Machined Stainless Steel Needs Passivation

How Machining Affects Corrosion Resistance

While machining doesn’t necessarily change the internal composition of stainless steel, it can compromise the protective surface layer in several ways:

• Free iron contamination: Cutting tools and metalworking fluids can introduce iron particles that adhere to the surface.

• Surface deformation: Mechanical stress from tooling can smear or work-harden the outer layer of the metal.

• Oxide scale or discoloration: Heat from high-speed machining may discolor the part or create localized oxidation.

All of these factors leave stainless steel more vulnerable to corrosion, particularly in harsh or high-performance environments.

The Role of Citric Acid Passivation

Passivation is a chemical process that removes free iron and surface contaminants, allowing a new passive layer of chromium oxide to form naturally. At New England Electropolishing, we use citric acid passivation—a safer, more environmentally responsible alternative to traditional nitric acid methods to acheive the passivation of stainless steel w ith citric acid.

Benefits of citric acid passivation:

• Removes free iron without etching or altering the base metal

• Promotes the formation of a uniform, corrosion-resistant oxide layer

• Meets ASTM A967 and other industry standards

• Non-toxic, biodegradable, and safer for workers and facilities

This process is ideal for stainless steel parts used in industries such as medical, aerospace, food processing, and semiconductor manufacturing, where cleanliness and corrosion resistance are critical.

Achieving Passivation Through Electropolishing

While citric acid passivation is highly effective, electropolishing provides another powerful way to achieve passivation—while also enhancing surface finish and performance.

Electropolishing is an electrochemical process that removes a thin, controlled layer of material from the metal surface. This not only eliminates embedded contaminants and free iron but also:

• Smooths microscopic peaks and surface roughness

• Improves cleanability and reduces biofilm accumulation

• Enhances brightness and appearance

• Creates an ultra-passive surface with superior corrosion resistance

For many precision-machined parts—especially those with complex geometries or tight surface requirements—electropolishing offers both functional and aesthetic advantages. It’s particularly beneficial in applications where minimal surface roughness and long-term durability are essential.

Why Post-Machining Passivation Matters

Skipping passivation after machining may save time upfront, but it significantly increases the risk of premature corrosion, part failure, and costly rework. By treating your stainless steel parts with citric acid passivation or electropolishing, you restore the metal’s natural ability to resist corrosion—and ensure it performs as expected in demanding conditions.

Conclusion

Machining stainless steel can impair its protective oxide layer, leaving parts vulnerable to corrosion. To preserve their integrity, parts should undergo a post-machining treatment such as citric acid passivation or electropolishing. Both methods remove surface contamination and promote the formation of a robust passive layer, extending the life and reliability of your stainless steel components.

At New England Electropolishing, we specialize in precision surface finishing solutions that meet the highest industry standards such as the passivation of stainless steel and stainless steel passivation services. Whether you need superior passivation, smoother surfaces, or a combination of both, we’re here to help your parts perform at their best.