Passivation
What Is Passivation, and Why Do Data Center Cooling Loops Need It?
Passivation protects new piping after cleaning. What it is, where it fits between flush and fill, and why liquid cooling loops depend on it.
By Denham McCollum, COO, Prime Equipment Solutions | The Readiness Report | Mechanical Readiness 101
Key takeaways
Passivation is a chemical treatment, applied after cleaning, that helps form a protective oxide layer on the inside of new metal piping.
Cleaning and flushing remove debris. Passivation protects the clean metal that's left.
ASHRAE's data center cooling committee says a single unpassivated component "can jeopardize the operation of the system."
Bare, clean steel left in plain water can flash rust, and the risk grows the longer a system sits.
OCP guidance says to fill with the final fluid as soon as possible after the final flush, preferably within 24 hours.
Passivation is the step that protects a cooling loop's metal after it has been cleaned. Pre-commission cleaning strips new piping of mill scale, weld slag, oil and grease. That leaves bare metal that can corrode quickly. Passivation applies a chemical treatment that helps form a protective oxide film, so the system goes into service clean and stays that way.
What is passivation?
Water treatment company Solenis explains that "passivation follows pre-cleaning" and uses a chemical treatment that "promotes the formation of a protective oxide layer on metal surfaces."[1] For stainless steel, the industry standard is ASTM A967, which "covers several different types of chemical passivation treatments for stainless steel parts," including nitric acid, citric acid and electrochemical methods.[2]
How is passivation different from flushing and cleaning?
They do different jobs, in sequence:
Step | What it does |
|---|---|
Mechanical flushing | Removes loose debris at high velocity |
Chemical pre-cleaning | Removes "mill scale, welding slag, dirt, grease, and other debris"[1] |
Post-clean flush | Removes the loosened material and spent cleaner |
Passivation | Conditions the clean metal surface so it resists corrosion |
Fill and treat | Puts the final fluid in, with its own inhibitors |
Solenis lists common passivating agents as nitrites, molybdates and azoles, and notes that "water chemistry is closely monitored throughout the process."[1] The right chemistry depends on the materials in the system, such as carbon steel, stainless steel or copper.
Why does ASHRAE care so much about passivation in liquid-cooled data centers?
Because the damage is hidden and expensive. In its May 2026 bulletin on coolant integrity, ASHRAE Technical Committee 9.9 put its firmest language on stainless-steel passivation, according to StorageReview's summary: "An individual installed component that is NOT passivated can jeopardize the operation of the system."[3]
The bulletin warns that welding, grinding and field fabrication can damage stainless steel's protective oxide film. It calls for proper cleaning and re-passivation "per ASTM A380/A967 practice." Without it, free iron can be released and foul cold-plate microchannels. These problems "stay invisible at commissioning and surface as damage months later."[3]
ASTM A380 is the companion standard. It "covers recommendations and precautions for cleaning, descaling, and passivating of stainless steel parts," including "assemblies, equipment, and installed systems."[4]
What happens to clean steel that isn't protected?
It can start to rust almost immediately. An industry article on hydrotesting notes that "water alone will result in flash corrosion on the internal surfaces of a pipeline," and that "the longer the hydrotesting or construction process, the greater the risk."[5]
Over-flushing can make it worse. A UK building services engineer writing in Modern Building Services warns that carbon steel pipework "may well show the effects of 'flash rusting'" and that "closed water systems should be cleaned once (and only once), dosed with inhibitors and biocides."[6]
What does OCP say about the gap between flush and fill?
The Open Compute Project's PG25 guidelines are direct:[7]
"Flushing with uninhibited water is not preferred due to an increased risk of corrosion for certain metallurgy."
"Install the heat transfer fluid as soon as possible following the final flush, preferably within 24 hours."
"If the system will not be filled for 24 hours or greater, purge with nitrogen and seal the system."
OCP doesn't set a single passivation procedure. It points teams to its Modular TCS commissioning documentation, but it does say to document the work.[7]
What should be documented for passivation?
OCP's list is a good starting point: "Record the date, passivation method used, chemical concentrations, exposure time and test results."[7] Add:
Materials of construction for each loop
Cleaning chemistry and post-clean flush results
Time between final flush and fill, and how the system was protected in between
Any field welding or grinding after passivation, and whether those areas were re-passivated
Where Prime Equipment Solutions fits
Prime Equipment Solutions handles the work on both sides of passivation: engineered mechanical flushing before cleaning, and post-flush circulation, RO filtration and fill afterward, so the system goes from clean to filled without sitting unprotected.
Keep your system protected from flush to fill. Request a Quote
Frequently asked questions
What is passivation in hydronic piping?
A chemical treatment applied after pre-commission cleaning that promotes a protective oxide layer on metal surfaces, helping new piping resist corrosion.
Is passivation the same as flushing?
No. Flushing and cleaning remove debris, scale and oil. Passivation protects the clean metal surface left afterward.
What standards cover stainless steel passivation?
ASTM A967 covers chemical passivation treatments for stainless steel parts. ASTM A380 covers cleaning, descaling and passivation of stainless steel parts, equipment and installed systems.
Why does passivation matter for liquid cooling?
ASHRAE TC 9.9 warns that a single unpassivated component can jeopardize the system, and that damaged stainless steel can release free iron that fouls cold-plate microchannels.
How soon should a system be filled after the final flush?
The Open Compute Project recommends installing the heat transfer fluid as soon as possible, preferably within 24 hours. If that's not possible, purge with nitrogen and seal the system.
Sources
Solenis, "The Importance of Cleaning and Passivation in Industrial Water Systems," Dec 26, 2025.
ASTM International, A967/A967M-25, "Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts."
StorageReview, "ASHRAE TC 9.9 Flags Four Ways Direct-to-Chip Cooling Loops Go Wrong," Aug 30, 2026.
ASTM International, A380/A380M-25, "Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems."
The Australian Pipeliner, "Reducing the corrosion risk from hydrotesting new pipelines," Oct 2020.
Steven Booth, "Steel yourself," Modern Building Services, Feb 7, 2019.
Open Compute Project, "Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks," Rev 2.0, V1.0.0, Sep 2026.
