Passivation vs Derouging: What Each Does and When Each Is Indicated

Passivation and derouging are two different chemical treatments that are often ordered as if they were one. Passivation removes free iron from a clean stainless surface and lets the chromium-rich oxide reform. Derouging removes iron oxide deposits that have already formed on top of that surface. The passivation vs derouging question is therefore not which one is better; it is which condition the system is in. A new or repaired system that has never rouged needs passivation. A system with rouge needs derouging, and then passivation, because the derouging chemistry leaves the surface active. Ordering passivation for a rouged loop wastes the campaign, and ordering derouging without the passivation step sets up the next one.

Two chemistries for two conditions

Passivation under ASTM A967 uses nitric acid or citric acid to dissolve free iron and iron-rich material from the outer layer of the alloy, leaving a surface enriched in chromium that oxidizes into a stable passive film in air or water. It does not remove scale, heat tint or rouge; it works on a surface that is already clean and bare. Derouging uses chemistry chosen to dissolve iron oxides and hydroxides: chelating and reducing blends built on citric, oxalic, gluconic or phosphoric acid for the common classes, and stronger mixtures, sometimes with hydrofluoric acid, for hard magnetite. Those chemistries attack the iron oxide, but they also strip or disrupt the passive film beneath it. A derouged surface is clean and unprotected, which is the condition passivation is designed for.

Passivation vs derouging side by side

Rouge Class I, II and III and which treatment each calls for

Class I rouge is particulate iron oxide that arrived from somewhere else, typically a pump impeller, a heat exchanger or a carbon steel component upstream, and settled on the surface. It is orange to red, loosely adherent, and wipes off on a white cloth. The treatment is to find and fix the source; a mild derouging or a thorough clean followed by passivation of the affected surface deals with the deposit. Class II rouge forms in place on the surface itself, on 316L in hot water for injection at typical loop temperatures, as the passive film breaks down locally and oxidizes. It is adherent, red-brown through purple to blue-black, and does not wipe off. It needs derouging with a chelating or reducing chemistry, followed by passivation, and it will return at a rate set by the system’s operating conditions. Class III rouge is black magnetite that forms on surfaces exposed to clean steam and high temperature. It is hard, glossy and tightly bonded, and it needs the strongest chemistry or mechanical removal, followed by passivation, and it is often left in place on clean steam surfaces where it is stable and not shedding. The class decides the chemistry; the location and the shedding behavior decide whether to act at all.

Why derouging is always followed by passivation

The derouging solution does not know where the oxide stops and the alloy begins. Chelating and reducing acids that dissolve iron oxide also reduce the chromium oxide of the passive film and pull iron from the outermost metal, leaving a surface that is chemically clean and electrochemically active. Left in that state under hot water, it re-oxidizes uncontrolled and rouges faster than it did before. Passivation immediately after the derouging rinse dissolves the freshly exposed iron, enriches the chromium and lets a coherent passive film reform under controlled conditions. That is why every competent derouging procedure ends in a passivation step, and why a derouging campaign that reports only “rouge removed” is incomplete. The derouging service and the passivation are one scope with one record.

Citric vs nitric passivation after derouging

Both chemistries are accepted under ASTM A967 and both restore the passive film on a derouged surface. Citric acid chelates the iron, works at moderate temperature, produces no nitrogen oxide fumes, and its spent solution is easier to neutralize and discharge. It is the usual choice for in-place circulation through a WFI loop, where the chemistry has to be pumped through the plant’s own piping past gaskets, instruments and drains. Nitric acid is a stronger oxidizer and forms the passive film faster, but it brings fume control, corrosive waste and a risk of attacking sensitized weld zones on a surface that has just been chemically stripped. Nitric remains the choice where an owner specification requires it, where citric has been shown not to reach the acceptance test, or on alloys and geometries where a job shop bath is practical. The citric vs nitric guide sets out the treatment designations. After derouging, the surface is bare either way; the decision is about safety, waste and what the specification says, not about which one works.

The order of work on a WFI loop

A derouging and repassivation campaign on a hot WFI distribution loop runs in a fixed order, and skipping a step is how loops come back rouged within months.

• Assess: borescope and photograph reference points, take wipe tests, classify the rouge, and check gaskets, instruments and valve seats for chemical compatibility.
• Isolate: blank the loop from the generator and points of use, drain, and confirm the circulation path, temperature source and waste handling.
• Degrease where needed: an alkaline clean and rinse so the derouging chemistry reaches the oxide rather than a film.
• Derouge: circulate the chemistry at its working temperature, sample for iron in solution on a schedule, and continue until the iron reading plateaus and the reference points clear visually.
• Rinse: to an endpoint conductivity approaching the feed water, so no chelant residue remains.
• Passivate: circulate citric or nitric solution at the concentration, temperature and time of the chosen A967 treatment.
• Rinse again: to the same endpoint, logged against the clock.
• Verify: ferroxyl or copper sulfate at defined locations, water-break check, and a chromium to iron ratio on a coupon where the specification requires it.
• Return to service: sanitize or steam the loop, sample under the water program, and release on results.
• Document: one report covering assessment, derouging log, rinse endpoints, passivation parameters, verification results and the release.

How to decide which one the system needs

Three indicators answer the derouging vs passivation question. Iron: a wipe test with a white cloth that picks up red or brown is rouge; a clean cloth on a surface that still fails a ferroxyl test is free iron, which is a passivation problem. Color: red-orange that wipes off is Class I, adherent red-brown to blue-black on hot water surfaces is Class II, hard glossy black on steam surfaces is Class III; a bare grey surface with no deposit needs no derouging. Surface tests: the ferroxyl and copper sulfate tests detect free iron and say nothing about oxide deposits; a surface can pass both and still be rouged. Combine the three. Deposit present, and Class II or III or a recurring Class I, means derouging and then passivation. No deposit, but a failed free-iron test, a fresh weld or repair, a new system, or a surface that was mechanically cleaned, means passivation alone. A light, stable, non-shedding film on a clean steam surface may mean monitoring and nothing else, decided on the trend rather than the appearance.

What to require from a derouging and passivation provider

Require that the provider classify the rouge before proposing chemistry, in writing, with photographs. Require the derouging endpoint to be defined by iron in solution and visual clearance, not by elapsed time alone. Require the passivation step to be named as an A967 treatment with concentration, temperature and time, and the verification test and locations stated before the job. Ask how gaskets, instruments, valve seats and heat exchanger surfaces are protected or removed, and who is responsible if they are not. Ask how spent solution is neutralized and where it goes. Require one report covering both steps with the rinse endpoints logged, and ask to see a report from a previous campaign before awarding the work. A provider who offers derouging without passivation, or passivation for a system that has not been assessed, has answered the question for you.

What Paul Industries does

Paul Industries performs derouging and passivation as one scope on WFI loops, purified water systems, clean steam lines, tanks and process piping, and passivation alone on new and repaired systems and on batches of OEM stainless parts. We assess and classify the rouge first, select the chemistry from the class, define the iron endpoint before circulation starts, passivate to an ASTM A967 treatment immediately after the derouging rinse, verify at agreed locations and issue a single report covering both steps. Where the assessment shows a system needs passivation and not derouging, or monitoring and neither, we say so. Our passivation services page covers the in-place and batch work, and the high-purity water systems page covers the loops where most of this work happens.

Standards referenced: ASTM A967 · ASTM A380 · ASME BPE · 21 CFR 211 · USP 1231 · ISPE Baseline Guide

Frequently asked questions

Passivation vs derouging: which one comes first on a rouged system?

Derouging comes first, because passivation acid cannot remove an oxide deposit and only treats bare metal. The order is derouge, rinse, passivate, rinse, verify. Running passivation on a rouged surface passivates the rouge, not the steel, and the deposit continues to shed. Running derouging alone leaves an active surface that re-rouges quickly. The two steps are one campaign with one report.

Does passivation alone remove Class I rouge?

No. Class I rouge is a loose deposit of iron oxide particles that migrated from an upstream source, and passivation acid is not formulated to dissolve oxide. Class I is usually removed by cleaning or a mild derouging, and the affected surface is then passivated. The lasting fix is finding the source, often a pump impeller, heat exchanger or carbon steel component, because the deposit returns until that is corrected.

Can a system be derouged and passivated in one chemical step?

Some proprietary blends claim to do both, and on light Class I deposits a citric-based chemistry can remove the film and passivate the surface in one circulation. On Class II and III rouge the derouging chemistry has to be reducing and the passivation chemistry oxidizing or chelating, which are different conditions, so they are run in sequence with a rinse between. Treat a one-step claim as something to verify with a test.

How can Class II rouge be told from Class III without laboratory analysis?

By location, color and hardness. Class II forms on hot water surfaces, is red-brown to purple or blue-black, and is adherent but can be marked with a wipe or a fingernail. Class III forms on clean steam and high-temperature surfaces, is glossy black, and is hard and unmarkable. A magnet will attract the magnetite of Class III on a scraped sample. Laboratory analysis confirms; the field call decides the chemistry.

Does derouging change the choice between citric and nitric for the passivation step?

Not on effectiveness; both restore the passive film on a derouged surface under ASTM A967. It changes the practical case. Derouging is done in place, by circulation through the plant's own loop, and citric acid is easier to pump, fume-free and simpler to neutralize in that setting. Nitric is chosen where a specification requires it or where the citric acceptance test has failed. The surface is bare either way.

What does the iron reading in a derouging solution tell you?

It is the endpoint. Iron in solution rises as the oxide dissolves and flattens when the deposit is gone. Sampling on a schedule and plotting the curve tells the crew when to stop, and the total iron removed, with the loop's surface area, indicates how heavy the rouge was. A reading that keeps rising after the reference points look clear means the chemistry has reached base metal and circulation should end.

Is derouging vs passivation a question for a new system that has never run?

No. A new system has no rouge; it has free iron from fabrication, heat tint at welds and possibly embedded particles from tooling. It needs cleaning, tint removal where required and passivation to ASTM A967 before commissioning. Derouging chemistry on a new system removes nothing useful and strips the surface for no reason. The first derouging decision arrives after the system has run hot for a period.

Can derouging damage the surface finish or the welds?

It can, if the chemistry is wrong for the class or left circulating past the endpoint. Chelating and reducing acids pull iron from the alloy once the oxide is gone, and hydrofluoric mixtures used on Class III will etch 316L and electropolished surfaces if uncontrolled. Sensitized weld zones are the most vulnerable. A defined iron endpoint, temperature control and a chemistry matched to the class keep the treatment on the deposit.

Why does a passivated 316L loop still rouge at WFI temperatures?

Because rouging on hot high-purity water is an operating-condition phenomenon, not a passivation failure. Hot deionized water with little oxygen and no ions slowly breaks down the passive film locally, and the iron that releases oxidizes on the surface. Passivation restores the film and slows the rate; it does not change the water chemistry or the temperature that drive the process. The realistic goal is a managed rate, not a permanent cure.

How is the endpoint of passivation checked differently from the endpoint of derouging?

Derouging ends when iron in solution plateaus and the reference points clear visually; the surface is then bare. Passivation runs for the time and temperature its ASTM A967 designation sets, and is then verified by an acceptance test on the surface: ferroxyl, copper sulfate, water break or a coupon Cr:Fe ratio. One endpoint is read from the solution, the other from the metal after a fixed treatment.

What happens if a system is derouged but the passivation step is skipped?

The derouged surface is left active. Under hot water it re-oxidizes without control and typically rouges faster than before the campaign, and free iron left by the derouging chemistry fails any A967 acceptance test. The system may also shed particles into the water in the weeks after return to service. A derouging record without a passivation record is a compliance gap as well as a technical one.

Should Class I rouge be derouged, or wiped off and its source fixed?

Both, in that order of importance. Class I rouge is a symptom of an upstream source shedding iron, and no treatment lasts until that source is corrected. Once it is, the deposit is removed by cleaning or a mild derouging and the surface is passivated. Skipping the source investigation and repeatedly derouging a Class I system is the most common way a plant turns a small problem into a recurring campaign.

Do gaskets and instruments have to come out before derouging or passivation?

Depends on the material and the chemistry. EPDM and PTFE gaskets tolerate citric passivation and most chelating derouging solutions; some elastomers and instrument sensors, conductivity cells, TOC analyzers and certain valve seats do not, and hydrofluoric mixtures attack more of them. The assessment step lists every wetted component in the circulation path and states which stay, which are removed and which are blanked off.

Does a clean steam system get derouged the same way as a WFI loop?

No. Clean steam surfaces carry Class III magnetite, which is hard, stable and often left in place unless it is shedding or the owner requires removal. When removal is required, the chemistry is stronger and the risk to the surface higher, so the campaign is more selective and the endpoint tighter. A WFI loop with Class II rouge is derouged with milder chelating chemistry by full circulation. The passivation step is the same for both.

What separates a derouging record from a passivation report?

The derouging record documents the assessment and classification, the chemistry and temperature, the iron-in-solution curve to its plateau, the visual clearance at reference points and the rinse endpoint. The passivation report documents the ASTM A967 treatment with concentration, temperature and time, the rinse endpoint, the acceptance test with locations and results, and the release. A single campaign report holds both, in sequence, with one signature block.

Related: Rouging & Derouging Services · Rouge in Stainless Steel: Class I, II & III and Derouging Methods · Passivation Services · What Is Passivation? · Passivation of Stainless Steel: Citric Acid vs. Nitric Acid · What Is WFI Water? Water for Injection Explained · Request a quote