Rouging and Derouging: What Rouge Is, Its Classes, and When It Is Removed
Classes, evaluation, removal, and relationship to passivation of rouge formed on stainless steel in purified water, WFI, and pure steam lines.
Rouge, Derouging, and Passivation Are Not the Same Process
- Rouge: a colored deposit consisting primarily of iron oxides and hydroxides on stainless steel surfaces carrying high-purity water or pure steam. ASME BPE divides it into three classes.
- Derouging: chemical or electrochemical dissolution and removal of rouge, followed by cleaning and passivation.
- Passivation: removal of free iron and strengthening of the passive chromium oxide layer. It does not replace derouging, and derouging does not replace passivation.
- Decision: the presence of rouge is not a reason for treatment on its own; its potential impact on the product, the process, and long-term operation of the system is evaluated through analysis.
For Mikron's passivation and derouging service, see our passivation and derouging page.
What Is Rouging, and Where and Why Does It Form?
Rouging is described as the formation of a thin red, reddish-brown, orange, light blue, or black layer on a stainless steel surface exposed for extended periods to very pure hot water or steam. It is an oxidation process: metal ions dissolving in microscopic voids on the surface cause micro-scale potential differences, hydrolysis, and oxidation. The colored material is iron oxide, iron carbonate, or iron hydroxide, or a mixture of these.
Rouge is observed most frequently at insufficiently polished spots: burrs, scratches, sharp edges, material transition zones, and poor welds. ASME BPE defines rouge as a phenomenon that can develop naturally in high-purity stainless steel systems (including water or pure steam) and attributes the degree of formation to four factors: the stainless steel material used, how the system was fabricated (welding, surface finish, passivation), the process conditions to which the system is exposed (water purity, chemicals, temperature, pressure, mechanical stress, flow velocity, dissolved gases such as oxygen and carbon dioxide), and how the system is maintained.
Hot storage is cited as a particular risk in this regard: pharmaceutical engineering literature notes that hot storage and distribution maintained continuously above 70 °C is the most effective method for preventing microbiological growth, but brings the risk of rouging with it. Rouge is not an immediate hazard on its own. When the deposit grows, however, rust can form, surface smoothness degrades, and biofilm risk increases.
Rouge Classes: ASME BPE Definition
The class describes both how the rouge formed and how difficult it is to remove.
| Class | Character | Appearance | Composition and formation | Removal |
|---|---|---|---|---|
| Class I | Predominantly particulate. Migrates downstream and may settle on process-contact surfaces. Wipes off with a cloth and is visible on the cloth. Underlying surface composition is unaltered. | Orange to reddish-orange | Generally hematite (red ferric iron oxide); low carbon and other oxide levels | Relatively easy; weakly bound to the surface |
| Class II | Localized, active form of corrosion. May result from attack on the surface by chloride or other halogens. | Wide spectrum of colors: orange, red, blue, purple, gray, black | Mostly hematite; some chromium and nickel oxides and a small amount of carbon | More difficult than Class I; may require additional time |
| Class III | Surface oxidation formed at high temperatures such as in pure steam systems. Originates as a stable layer, rarely particulate in nature. | Gold, blue, and various shades of black as the layer thickens | Magnetite; may include chromium, nickel, or silica substitution; generally contains significant carbon | Much more difficult than I and II; chemistries affect surface finish to some extent |
System temperature and temperature gradient are among the factors that determine the type of oxide formed (Class I hematite vs. Class III magnetite) and the ease of removal. The success of passivation and derouging also depends largely on the operating temperature of the system.
Factors Affecting Rouge Formation
ASME BPE Nonmandatory Appendix D groups the factors by their influence. Those with strong influence are as follows.
During fabrication
- Alloy selection: low-carbon type 316L provides better corrosion resistance than higher-carbon type 316.
- Surface finish: electropolishing reduces the area of the alloy exposed to oxidizing fluids and halogens as well as micro-pitting initiation sites. Irregularities in mechanical polishing, by contrast, increase surface area.
- Welding and argon purge: improper welding creates chromium-depleted heat-affected zones and discontinuities; inadequate purge leaves heat-tint oxides that passivation cannot reverse.
- Passivation: the depth and chromium-to-iron ratio of the passive layer determine the propensity of the alloy to corrode.
During operation
- Corrosive fluid: bleach, halogens, and chloride initiate corrosion cells at breaks in the passive layer.
- High velocity and shear: erodes the passive layer at pump impellers, spray balls, and tee fittings.
- Temperature and its gradient: determines the type of oxide and ease of removal.
- Dissolved gases: oxygen and carbon dioxide content are considered influential even within conductivity and TOC limits; studies show that reducing oxygen in nitrogen-blanketed WFI tanks has an adverse effect on the passive layer.
- CIP and SIP practice: SIP following inadequate CIP can accelerate corrosion mechanisms; mild acid cycles (for example, 2% to 20% phosphoric acid) have preserved and renewed the passive layer; ozone sanitization has been observed to retard alloy corrosion.
- Stagnant zones, system age, maintenance: new systems generate a disproportionate amount of Class I rouge compared to established systems.
The role of surface finish is debated in the literature. One view holds that highly polished stainless steel acts as a barrier against both rouge and biofilm. An engineering document on WFI production, meanwhile, reports that mechanical polishing, electropolishing, and passivation are applied for corrosion resistance, are not considered necessary to control microbial growth, and that unpolished surfaces exposed to pure steam that have only been pickled or pickled and passivated have been found more resistant to discoloration. Surface finish is therefore selected at the start of a project according to the product and operating conditions.
When Is Derouging Required?
Evaluation, rather than a fixed schedule, is required.
ASME BPE requires evaluating the presence of rouge according to its potential to affect the product, the process, and the long-term operation of the system. This evaluation is two-pronged: analysis of mobile constituents in the process fluid and analysis of the surface itself.
| What is analyzed | Method (ASME BPE Appendix D) | What it shows |
|---|---|---|
| Process fluid | Trace metal analysis by ICP/MS | Trace metal concentration in water; suitable for trend monitoring |
| Process fluid | Standard particle counting by light scattering | Particle count by size range; a baseline must be established for each system |
| Process fluid | Particle analysis by SEM/EDX | Size, composition, and topography of particles collected on a filter |
| Process fluid | FTIR | Potential organic film or elastomer-derived contamination |
| Surface | Microscopy (PLM, SEM) | Surface morphology; composition with EDX |
| Surface | Auger (SAM/AES) and XPS/ESCA | Depth and elemental composition of the surface and passive layer; which oxides are present. Requires a coupon and is destructive |
| Surface | Ellipsometry, electrochemical impedance | Surface state and real-time corrosion detection; field qualification is ongoing |
The standard also explicitly notes two situations: unnoticed corrosion formed in the installation or storage environment (contact with carbon steel, scratching, chemical contamination, stagnant condensation) may require derouging prior to passivation; established magnetite layers in pure steam systems may likewise require derouging prior to passivation. Failing to notice corrosion reduces the effectiveness of a normal passivation.
Derouging Methods: Guideline Ranges from the Standard
The values below are taken from the table in ASME BPE-2019 Nonmandatory Appendix D; they are not Mikron's application recipe, but a guideline summary from the standard. Concentration is percent by weight, and time and temperature are directly related to concentration.
| Class | Chemistry | Concentration | Time and temperature | Note |
|---|---|---|---|---|
| Class I | Phosphoric acid | 5% – 25% | 2 – 12 hours, 40 – 80 °C | Removes iron oxide without etching |
| Citric acid and supplemental organic acids | 3% – 10% citric | 2 – 12 hours, 40 – 80 °C | Without etching | |
| Phosphoric acid blends (with citric or nitric) | 5% – 25% phosphoric | 2 – 12 hours, 40 – 80 °C | Used under various conditions | |
| Sodium hydrosulfite (dithionite) | Up to 10% | 2 – 12 hours, 40 – 80 °C | Fast; sulfur fumes may form | |
| Electrochemical cleaning | 25% – 85% phosphoric | Approximately 1 minute/ft² | Accessible areas only, mostly vessels | |
| Class II | Phosphoric acid; citric acid and organic acids; phosphoric blends | 5% – 25% phosphoric; 5% – 10% citric | 2 – 24 hours, 40 – 80 °C | Similar chemistries to Class I, longer duration |
| Oxalic acid | 2% – 10% | 2 – 24 hours, 40 – 80 °C | May etch electropolished surfaces | |
| Sodium hydrosulfite | Up to 10% | 2 – 12 hours, 40 – 80 °C | Sulfur fumes may form | |
| Electrochemical cleaning | 25% – 85% phosphoric | Approximately 1 minute/ft² | Accessible areas | |
| Class III | Phosphoric acid blends | 5% – 25% phosphoric | 8 – 48 hours and above, 60 – 80 °C | Effective only on very light deposits |
| Oxalic acid | 10% – 20% | 8 – 48 hours and above, 60 – 80 °C | May etch the surface | |
| Citric acid and organic acids | 5% – 10% citric | 8 – 48 hours and above, 60 – 80 °C | May etch the surface | |
| Citric acid, organic acids, and fluorides | 5% – 10% citric | 8 – 48 hours and above, 60 – 80 °C | Etches the surface | |
| Nitric acid and HF or ammonium bifluoride | 15% – 40% nitric, 1% – 5% HF or NH4HF | 1 – 24 hours, ambient temperature – 40 °C | Etches the surface | |
| Electrochemical cleaning | 25% – 85% phosphoric | Approximately 1 minute/ft² | Accessible areas |
- Every derouging operation must be followed by cleaning and passivation; rinsing with deionized water is performed immediately after each chemical treatment.
- Derouging waste may be classified as hazardous waste due to its metal content. After derouging, an oily or loose black residue resulting from carbon accumulation may remain on the surface and require specialized cleaning.
- Application methods include fluid circulation, gel application for welds and surfaces, and spraying for vessels and equipment.
- Chemistry and duration vary with rouge thickness, material, and surface finish. Selection is made through engineering evaluation.
Passivation After Derouging: Sequence and Limitations
Sequence
The ASME BPE passivation guideline describes the process in six steps: preliminary survey and preparation, flushing, cleaning, passivation, final rinse, and verification. On a surface where derouging has been performed, this chain is completed with the cleaning and passivation steps. The passivation provider qualifies the process with a written procedure and documents critical variables such as time, solution temperature, chemistry, endpoint determination, and final rinse water conductivity.
Limitations
- Post-weld passivation removes free iron; it does not remove heat tint. Removing heat tint requires an acid more aggressive than nitric or citric acid.
- Pickling does not replace passivation.
- Passivation cannot reverse the effect of improper argon purge.
- Even optimal passivation only makes the material as corrosion-resistant as possible for that environment; it does not eliminate the need for a more corrosion-resistant alloy where required.
For which tests are performed after passivation, which documents are required, and the acceptance criteria, see passivation verification tests; for design details that reduce rouge, see hygienic design and dead legs. Sanitization choices that affect rouge risk are compared in PW and WFI loop sanitization.
Documentation of rouge and passivation requirements during plant construction is addressed under system validation and qualification and engineering solutions as part of qualification and commissioning. For water systems, see our purified water (PW) and water for injection (WFI) pages.
Sources: ASME BPE-2019 (GR-8 definitions, SF-2.6, SF-2.8, Nonmandatory Appendices D and E); Boeke and Le Brun, Practical Pharmaceutics, Chapter 27; WFI production engineering documentation.
Frequently Asked Questions About Rouging and Derouging
Rouge classes, timing of derouging, surface impact, and relationship to passivation.
What is rouging?
Rouging is the colored deposit and discoloration that forms on the surface of stainless steel systems carrying high-purity water or pure steam. According to the ASME BPE definition, it consists primarily of iron oxides and hydroxides. It may appear red, reddish-brown, orange, light blue, or black, and typically initiates at spots that are not sufficiently smooth, such as burrs, scratches, sharp edges, material transitions, and poor welds.
Is rouge harmful?
It is noted that rouge does not create an immediate hazard on its own. The concern is that as the deposit spreads, surface smoothness degrades, rusting begins, and biofilm risk increases. ASME BPE likewise requires evaluating the presence of rouge based on its impact on the product, the process, and the long-term operation of the system; in other words, not all rouge carries the same urgency.
What is the difference between Class I, Class II, and Class III rouge?
ASME BPE defines three classes. Class I is predominantly particulate in nature, migrates downstream, and can be wiped off with a cloth; the underlying surface is unaltered. Class II is a localized, active form of corrosion that may be associated with halogen attack such as chloride. Class III is a magnetite-based, highly stable surface oxidation formed at high temperatures such as in pure steam systems. Difficulty of removal increases in the order I, II, III.
When is derouging required?
There is no fixed schedule. ASME BPE recommends evaluating the potential impact of rouge on the product, process, and long-term operation, using analyses of both the process fluid and the surface for this purpose. The standard also notes that derouging may be required prior to passivation in cases such as corrosion formed during installation or storage, or established magnetite in pure steam systems.
Is passivation required after derouging?
Yes. ASME BPE requires cleaning and passivation of the treated surface following derouging operations. Derouging dissolves the rouge; restoring and strengthening the passive chromium oxide layer is the role of passivation. The two processes do not replace one another.
Does derouging etch the surface?
It depends on the chemistry and the class of rouge. In the table of the standard, phosphoric acid and citric acid based mixtures are stated to operate without etching the surface for Class I and II, whereas oxalic acid may etch an electropolished surface, and fluoride-containing mixtures used for Class III definitely affect the surface. Chemistry selection is therefore made by evaluating the surface finish and rouge thickness.
Are rouge and rust the same thing?
No. Rouge is a deposit of iron oxide, carbonate, or hydroxide and may not harm the surface on its own. However, if the deposit progresses, it can eventually result in rust formation. For this reason, rouge is monitored early and managed when necessary.
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