Nitric vs. Citric Acid Passivation of Corrosion-Resistant Stainless Steel & Ti
Many often ask about the differences between nitric acid and citric acid for stainless steel, titanium, and other alloy parts. In the past, citric acid was avoided due to potential organic growth and molding issues. Today, citric acid has improved leaps and bounds with new formulations for biocides which prevent any organic growth in solution. See our complete write-up on the advancements of citric acid (specifically Citrisurf®). These new advancements in citric acid have allowed smaller manufacturers which had little to no experience with chemical handling and processing to bring their passivation needs in-house.
Today, many companies are taking a second look at citric acid due to its ease of chemical handling, disposal and overall safety for employees all of which are more challenging when going with nitric acid. Best Technology works closely with many companies in the aerospace and medical device industries and have successfully assisted these companies with converting them from nitric to citric acid. Often times, it is as simple as talking with the end customer and showing them the equivalency of the types of passivation.
Comparison of Nitric vs Citric acid:
Nitric Acid | Citric Acid | |
Safety | Hazardous handling required | Very safe to use as directed |
Air Breathing | Emits toxic gases | No toxic gases emitted |
Ease of use | Chemical handling safety equipment and extreme care required for most use | Minimum chemical handling safety equipment required |
Passivation | Excellent passivation of most grades of stainless | Excellent passivation of nearly all grades of stainless steel |
Environment | Environmentally hazardous | Environmentally friendly |
Cost | Low cost raw material, but high cost maintenance and disposal, high cost of a ventilation system for safe use | Lower overall cost to use: lower maintenance, safety and waste disposal costs, lower concentration of chemicals required |
Passivation Process Duration | 20 minutes to several hours | 5-20 minutes typical |
Process Temperature | Required for many grades, nitric acid very dangerous at elevated temperatures | Room temperature use for many grades, elevated temperatures expedite the process and is safe without ventilation |
Chemical Process Maintenance | Regular solution monitoring required (titration) | Regular solution monitoring required (titration or pH) |
Iron Oxide Removal | Slowly removes iron oxides | Readily removes iron oxides |
Equipment | Long-term corrosive degradation of non-stainless steel metal or polymer based equipment or components | No corrosive degradation of equipment |
Process stability | Must control time and temperature closely as danger of nitric oxide gas exists – proper ventillation required | Less prone to time and temperature variation, no hazardous vapors |
ASTM A967 | Meets requirements | Meets requirements |
AMS 2700 | Meets requirements | Meets requirements – when approved by cognizant engineering organization |
ASTM A380 | Meets requirements | Meets requirements as referenced in ASTM A967 |
AMS QQ-P-35 | Meets requirements as referenced to AMS 2700 and ASTM A967 | Now referenced to AMS 2700 and ASTM A967 |
Applicable documents and specifications:
Because of the long ago issues with citric acid, many industries developed their processes and specifications around using nitric acid. Specifically the medical device and aerospace industries have required many manufacturers to use nitric acid as the passivation solution for their stainless steel, titanium and other alloy parts. Best Technology has seen many long-time nitric acid passivation users start to transition to using citric acid.
ASTM A380 – Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C)
AMS 2700 – Passivation of Corrosion Resistant Steels
AMS-QQ-P-35 – Passivation Treatments for Corrosion-Resistant Steel
ASTM B600– Passivation for titanium and titanium alloys is now recognized in the ASTM standard.
BS (British Standard) EN 2516 – Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys
Military Specs and Standards
The following specs refer to QQ-P-35 for passivation of stainless steel and thus allow the use of ASTM A967 and AMS 2700:
- MIL-STD-808A (section 5.3.2.4.1, Table II. finish code numbers F-200, F-201, F-202, F-203, F-204, Table VIII. finish code number D-200) – “Finishes, Materials and Processes for Corrosion Prevention and Control in Support Equipment (S/S by MIL-HDBK-808)”
The following specs refer to ASTM A380 for passivation of stainless steel:
- MIL-DTL-14072E (Table IV. Finish E300) – “Finishes for Ground-Based Electronic Equipment”
The following specs refer to ASTM A967 and AMS 2700 for cleaning and passivation of stainless steel:
- MIL-S-5002D (section 3.8.6) – “Surface Treatments and Inorganic Coatings For Metal Surfaces of Weapons Systems”
- MIL-STD-171F (section 5.1.4.2, Table V. finish numbers 5.4.1 and 5.5.1) – ” Finishing of Metal and Wood Surfaces”
CitriSurf is a registered trademark of Stellar Solutions, Inc.
