What is Stainless Steel Passivation?
Why is it necessary to passivate stainless steel?
Stainless steel is a naturally corrosion resistant alloy. The primary component of stainless steel which provides corrosion resistance is chromium, which in the presence of oxygen forms a corrosion-resistant (a.k.a. passive) layer on the surface of the stainless steel. This chromium oxide layer protects the metal below it from interacting with the environment around it to form corrosion. While the chromium typically present on the surface will naturally create a passive layer, there is a significant amount of iron inherent to the alloy (about 60-70%, depending on the grade of stainless) which limits the corrosion resistance. It is also common for additional iron contamination to be introduced to the surface during manufacturing processes, such as via iron dust in the shop air or contact with carbon steel tools, which further decreases corrosion resistance. The solution to these issues is passivation.
What is passivation?
Simply put, passivation is the intentional removal of iron from the stainless steel surface. Removal of surface iron and iron contamination results in a stainless steel surface that is low in iron and high in chromium. With more chromium at the surface a thicker chromium oxide layer can be achieved, which results in significantly improved corrosion resistance due to lack of iron available to react with the environment.
How can I passivate stainless steel safely and economically?
Passivation of stainless steel can be performed using nitric acid or citric acid. However, there are many reasons that it is better to use citric acid. Many tests have shown that citric acid passivation is more effective than nitric because it only removes iron while leaving all of the chromium, nickel, and other “good” components intact. By contrast, nitric acid will also remove some chromium and nickel along with the iron. Citric acid passivation is also much safer and better for the environment. Nitric acid is extremely toxic to humans, animals and the environment. For anyone who considers worker safety to be of paramount importance, citric acid passivation is the obvious choice.
CitriSurf for Passivation of Stainless Steel
We manufacture and sell a full line of Citrisurf® products for passivation of stainless steel. Passivating stainless steel with CitriSurf is safe, easy, economical, and environmentally friendly. Contact us today for a product recommendation that is tailored to your particular application, or view our product list.
ASTM A967 Compliance for Post-Fabrication Passivation
Proper passivation of stainless steel begins with compliance to ASTM A967, the recognised standard for post-fabrication chemical passivation. ASTM A967 specifies seven validated treatment methods (four nitric acid based and three citric acid based), each with defined concentrations, temperatures, and immersion times. Choosing an ASTM A967 compliant method is what distinguishes a defensible passivation process from informal surface cleaning.
For fabricators and OEMs the order of priorities is clear. First, ensure post-fabrication passivation meets ASTM A967, with surface preparation guided by the related ASTM A380 standard. Second, plan for in-service maintenance and repassivation, which protect that initial passive layer over the asset’s working life.
Why Citric Acid Passivation is Now the Preferred Choice
Long-lasting Corrosion Protection
Citric acid passivation creates a thicker, more robust chromium oxide layer on stainless steel than traditional nitric acid methods. Independent testing has shown that citric acid passivation passes ASTM A967 corrosion tests with extended durability, meaning longer intervals between repassivation cycles and lower lifetime maintenance costs.
Sustainable Plant-based Passivation
Citric acid is a non-toxic, biodegradable, plant-based chemistry, in contrast to nitric acid which generates hazardous nitrogen oxide fumes and toxic waste. Adopting citric acid passivation reduces your facility’s environmental footprint, simplifies waste disposal, and removes the specialised PPE and ventilation requirements associated with nitric acid handling.
Industries that Rely on Stainless Steel Passivation
Food and Beverage Processing
Food-contact and CIP-compatible passivation is one of the most demanding applications for stainless steel. Food processors, dairies, breweries, and beverage manufacturers must maintain process surfaces that resist corrosion from acidic foods, sanitisers, and caustic cleaning chemistry, while passing food-grade hygiene standards. ASTM A967 passivation on stainless tanks, piping, fittings, and process equipment is the baseline requirement for any new fabrication entering food or beverage service.
For ongoing in-line maintenance, low-foaming citric acid formulations are designed specifically for CIP (clean-in-place) applications. CitriSurf 3050 is blended for low-foam performance in CIP circuits and tank-bath applications, making it compatible with existing sanitation cycles without requiring system modifications. Plant operators can run passivation as a scheduled CIP step rather than a separate shutdown event.
The result is a passive layer that holds up against food acids, sodium hypochlorite sanitisers, peracetic acid, hot caustic, and repeated cleaning cycles, extending the service life of expensive stainless assets and reducing contamination risk to food and beverage product.
Explore the CitriSurf range →Medical and Pharmaceutical
Sterile environments and patient-contact equipment demand maximum corrosion resistance and chemical inertness. Passivation removes free iron from medical devices, surgical instruments, and pharmaceutical processing equipment.
Aerospace and Defence
Aircraft components, fasteners, and structural assemblies operate under extreme thermal cycling, vibration, and humidity. Passivation to ASTM A967 is a contractual requirement on most aerospace stainless steel parts.
Chemical and Petrochemical
Reactors, heat exchangers, and piping handle aggressive chemistries and elevated temperatures. Passivation establishes the baseline corrosion resistance needed for safe long-term operation.
Semiconductor and Electronics
Wafer fabs, cleanrooms, and ultra-pure water systems cannot tolerate iron particulate contamination. Passivation of stainless surfaces is part of standard installation and qualification procedures.
Marine and Offshore
Saltwater, chloride exposure, and marine atmospheres are the harshest environments for stainless steel. Passivation provides essential protection for shipboard components, offshore platforms, and coastal infrastructure.
Automotive and Heavy Industry
Exhaust systems, fuel components, and structural stainless parts benefit from passivation that resists road salt, fuel chemistry, and thermal cycling.
The Passivation Process: Step by Step
Effective passivation follows a defined sequence. Skipping or rushing any step compromises the resulting passive layer and the corrosion performance of the finished part.
Cleaning and Degreasing
The stainless surface must be free of oils, cutting fluids, organic residues, and embedded particulates from fabrication. ASTM A380 provides guidance on appropriate cleaning methods including alkaline cleaners, solvent cleaning, and mechanical preparation.
Acid Treatment
The cleaned part is immersed in or contacted with a passivation solution. The acid (citric or nitric) selectively dissolves free iron and iron compounds from the stainless surface without attacking the chromium and nickel content of the alloy. Concentration, temperature, and immersion time are specified by the ASTM A967 method chosen.
Rinsing
Thorough rinsing with deionised or potable water removes residual acid and dissolved iron from the surface. Inadequate rinsing can leave deposits that cause spotting or localised corrosion.
Drying and Inspection
The part is dried and inspected. Verification testing per ASTM A967, ASTM A380, or customer specification confirms that free iron has been removed and the passive layer has formed.
Treatment times vary by ASTM A967 method, ranging from approximately 30 minutes for high-temperature citric methods to several hours for ambient-temperature treatments.
Frequently Asked Questions
What is the difference between non-toxic citric acid passivation and traditional nitric acid passivation?
Both methods are recognised under ASTM A967 and both achieve the same fundamental result: removal of free iron and formation of a passive chromium oxide layer. The difference is in handling and lifecycle. Nitric acid generates hazardous nitrogen oxide fumes, requires specialised ventilation, neutralisation, and waste disposal, and presents serious safety risks. Citric acid is biodegradable, non-fuming, and safe for in-house operation without specialised infrastructure. Independent testing shows citric acid passivation produces a thicker, more durable passive layer than equivalent nitric methods.
Can I perform stainless steel passivation in-house, or do I need a specialist service?
Both options are valid. For routine maintenance, in-process passivation, or one-off jobs, in-house treatment with a citric acid product such as CitriSurf is straightforward, safe, and cost-effective. For large fabrications, awkward geometries, or commissioning of major installations, an onsite mobile passivation service brings the process to the equipment and ensures defensible ASTM A967 compliance with full documentation.
How do I know if my stainless steel needs passivation?
New fabrications always require passivation as part of post-fabrication processing to ASTM A967. In-service equipment should be repassivated if you observe surface rust spotting, after major welding repairs, after exposure to chlorides or aggressive chemistry, or on a scheduled basis as part of a preventive maintenance program. A simple test using copper sulphate or ferroxyl indicator can confirm whether free iron is present on a stainless surface.