Passivation
Acid treatment that removes free iron from stainless steel surfaces and enhances the natural chromium oxide layer. Maximizes corrosion resistance per ASTM A967 / AMS 2700.
About Passivation
Passivation is a critical chemical treatment for stainless steel that maximizes its inherent corrosion resistance by removing surface contaminants and enhancing the natural chromium oxide passive layer. During machining, grinding, and handling, free iron particles from cutting tools, shop environment, and fixturing can become embedded in the stainless steel surface. These iron particles create galvanic cells that initiate localized corrosion (pitting and rust staining) even on high-grade stainless steels. Passivation removes these contaminants and restores the protective chromium-rich oxide layer.
Passivation is essential for stainless steel components used in medical devices, surgical instruments, food processing equipment, pharmaceutical manufacturing, chemical processing, semiconductor fabrication, and marine environments. In these demanding applications, even minor surface contamination can lead to corrosion, product contamination, or regulatory non-compliance. Passivation per ASTM A967 or AMS 2700 is a standard requirement in FDA-regulated medical device manufacturing and is specified across pharmaceutical and food industry standards.
The two primary passivation methods—citric acid and nitric acid—each offer distinct advantages. Citric acid passivation is environmentally friendly, faster, and effective across all common stainless steel grades. Nitric acid passivation is the traditional method with a longer track record in aerospace and military applications. Both methods produce equivalent corrosion resistance when properly specified and controlled. The process adds no material to the surface and has zero dimensional impact, making it suitable for the most precision-critical components.
Finish Properties
Available Types
Citric Acid Passivation
Modern, environmentally preferred method using 4-10% citric acid solution at 60-70°C for 20-30 minutes. Effective on all austenitic (300 series), ferritic (400 series), and precipitation-hardened stainless steel grades. Produces no NOx fumes, generates less hazardous waste, and is safer for operators. Increasingly specified as the default passivation method in new designs and standards.
Nitric Acid Passivation
Traditional method using 20-50% nitric acid solution at 50-60°C for 20-30 minutes. Well-established in aerospace (AMS 2700) and defense specifications. May include sodium dichromate addition for heavily contaminated or free-machining grades. Effective at removing heavy surface contamination and sulfide inclusions. Requires more stringent environmental controls due to NOx emissions and acid waste handling.
How It Works
Passivation begins with thorough cleaning of the stainless steel part to remove oils, greases, and organic contaminants that would prevent uniform acid contact. Parts are then immersed in a passivation bath—either citric acid (4-10% concentration, 60-70°C) or nitric acid (20-50% concentration, 50-60°C)—for a controlled period of 20-30 minutes. The acid dissolves free iron, iron oxide, and other metallic contaminants from the surface while leaving the chromium and nickel content intact. This selective dissolution enriches the surface with chromium, which then reacts with atmospheric oxygen to form a dense, continuous chromium oxide (Cr₂O₃) passive layer approximately 1-3 nm thick. After passivation, parts are thoroughly rinsed in clean water and dried. Verification testing per ASTM A967 (copper sulfate, salt spray, or high humidity tests) confirms successful passivation.
Design Guidelines for Passivation
Specify passivation for all stainless steel parts used in medical, food, pharmaceutical, or chemical applications—it is typically a regulatory requirement and best practice for corrosion performance.
Remove all carbon steel tooling marks, grinding swarf, and shop contamination before passivation—the acid treatment removes surface iron but cannot address deep contamination embedded during aggressive machining.
Avoid free-machining stainless steels (303, 416) for passivated parts when maximum corrosion resistance is required—their high sulfur content creates sulfide inclusions that compromise the passive layer integrity.
Specify the passivation method (citric or nitric) and ASTM A967 practice number in your drawing notes to ensure consistent processing across production batches.
Compatible Materials
Materials that can be finished with passivation.
Passivation FAQ
Passivation is a chemical cleaning process that removes surface contaminants and enhances the chromium oxide layer without changing the surface appearance or removing measurable material. Electropolishing is an electrochemical material removal process that dissolves 10-40 µm of surface material, producing a smooth, bright finish while simultaneously passivating the surface. Electropolishing provides both cosmetic improvement and passivation in one step but at higher cost and with dimensional change. Passivation alone is preferred when surface finish does not need to change.
ASTM A967 defines several verification tests. The copper sulfate test (immersion in acidified copper sulfate solution for 6 minutes) is the most common—a passing result shows no copper-colored deposits on the surface. Salt spray testing (per ASTM B117) exposes passivated parts to 5% NaCl fog for a specified duration. High-humidity testing in a controlled chamber is another option. The test method should be specified in the purchase order or drawing requirements.
Passivation produces minimal to no visible change in surface appearance. The part retains its original machined, polished, or bead-blasted texture. In some cases, passivation may slightly brighten the surface by removing a thin layer of surface oxide and contaminants, but this change is subtle. Passivation does not produce a mirror finish or color change—if cosmetic improvement is needed, electropolishing or mechanical polishing should be specified instead.
Passivation is a preventive treatment, not a corrective one. It removes microscopic free iron and light surface contamination but cannot remove visible rust, heavy heat tint, or weld discoloration. For heavily contaminated or discolored parts, a pre-treatment step such as acid pickling (stronger acid concentration), mechanical abrasion, or electropolishing is required before passivation to restore the surface to a clean, uniform condition.
Passivation is a one-time treatment applied during manufacturing. The chromium oxide passive layer is self-healing and regenerates naturally when exposed to oxygen. However, re-passivation may be required if the part is subjected to conditions that damage the passive layer, such as welding, grinding, prolonged exposure to chlorides, or contact with carbon steel tools during maintenance. Annual re-passivation is common for food processing and pharmaceutical equipment as part of routine maintenance protocols.
Ready for Passivation?
Upload your CAD file and specify your finish requirements for an instant quote.