Clean, Derouge, Passivate – A Tour of the Passivation Treatment Process

This video takes you inside the distribution piping of a typical water system to see the Cleaning, Derouging, and Passivation treatment process. This video will demonstrate the treatment process of a typical system and show how passivation can improve the corrosion resistance.

Contamination of any sort is the bane of those who operate systems where cleanliness and purity are absolutely critical. Rouge, therefore, is a literal nightmare for those with a high purity water component because it comes from within the system itself. No matter how carefully ingredients or components are screened or cleaned, certain conditions can cause the formation of ferric compounds on the inner surfaces of stainless-steel piping. Whether carried by the water from another part of the system, or forming on the stainless steel itself, these ferric compounds can grow over time to become an ongoing source of contamination to the overall system as well as the product. Regularly scheduled wiping to detect rouge only alerts the operator to the problem after it has started. Fortunately, there are steps that can be taken to prevent the formation of rouge in the first place.

Rouge: A Quick Recap

While rouge is a form of corrosion, there are three classes which help to better identify the nature of the problem:

Class I

– Free iron comes from a source other than the stainless steel itself and deposits on the surface. Carbon steel used elsewhere in the system is sometimes the source, often coming from pumps where the impeller is damaged due to cavitation or erosion.

Class II

– The ratio of chromium to iron in the surface of the stainless steel is not sufficient to form a strong enough passive layer. This means that there is enough free iron on or near the surface to interact with any moisture and oxidize, forming Fe2O3, or rust.

Class III

– Usually found in high-temperature steam systems where the formation of iron oxide scale alters the chromium/iron ratio of the passive film.

Regardless of source, if left untreated rouge increases the surface roughness of the stainless steel, reducing its ability to be cleaned and sanitized. This allows impurities in the water to get trapped on the steel’s surface, potentially setting it up for increased corrosion. As the rouge continues to grow, the ferric crystals can break off and be carried by the liquid and distributed elsewhere within the system, spreading the contamination. Additionally, if left untreated for too long, there is an increased likelihood of bacterial or microbial contamination.

Cleaning and Preventing Rouge in Three Steps

Step 1. Cleaning

Once rouge has been identified, the first step is to clean it. This isn’t to remove the rouge itself just yet, but to eliminate any organics or other contaminants that may be embedded in the rouge. To begin the cleaning process, Astro Pak technicians will connect their custom-designed pump skid to add chemistry to the system and circulate it throughout in order to dissolve and dislodge the embedded material.

3D Image of Rouge Crystals
Field and lab testing techniques are used to evaluate the efficacy of passivation treatment methods. Corrosion will result in pitting of the surface and appear as roughness or a dull haze. On the right, a 3D image of rouge crystals

If biological contaminants were detected prior to the service taking place, it is likely that microscopic organisms have taken up refuge within the rouge. This organic material, known as biofilm or bioburden grows and changes over time and can become increasingly harder to remove, becoming a slime-like layer that clings to even smooth surfaces. This will require further rounds of cleaning to remove this biomass. Astro Pak has created unique and distinctive formulas and processes designed for optimal effectiveness for removing both free-floating and surface-dwelling organics.

Following the chemical treatment, a high-purity water rinse is conducted which flushes the organics out of the system, paving the way for the second step to begin.

Step 2. Derouging

This step is also performed using the pump skid. This time a different set of specially formulated chemistry is used to dissolve the ferric crystals of the rouge itself. The specific chemicals needed will vary depending on the class of rouge present. Class I is the most easily treated while Class III can require a more aggressive approach because of how it was formed as well as its chemical makeup.

Again, this treatment is followed by another high-purity water rinse to literally flush the system of the contaminates. Once the water tests clear of the debris, the technicians can move onto the third step.

Step 3. Passivation

While Class I rouge is easily dislodged in Step 2, in the case of other types of rouge the ferric crystals were embedded through the chemically inert passive layer of the metal. Their removal leaves exposed areas on the steel’s surface where the problem could re-occur. Passivation restores that passive layer, by removing any exposed free iron on the surface of the stainless steel, thereby increasing ratio of chromium to iron atoms to a point where the chemical resistance of the metal is restored. The free iron is not only removed from the surface, but is chelated to render it incapable of binding with the surface of the system before being flushed out with the chemistry.

While stainless steel naturally forms a passive layer of chromium oxide (chemical formula: Cr2O3), it is only a few angstroms (0.0001µ) thick. Chemical passivation treatments enriches this layer to increase its effectiveness and durability. Astro Pak’s patented UltraPass® treatment regularly achieves chromium-to-iron ratios far in excess of the 1.5 to 1 that is accepted as industry standard. It is strongly recommended that even Class I rouge is followed up with a passivation treatment to ensure optimal protection from further instances of rouge.

After the final high purity water cleanse, the pump skid is disconnected and the system is ready to be returned to service. It is also important to note that, with few exceptions, the chemistry utilized by Astro Pak is safe for disposal in sewers without requiring special handling. As a result, the area around the system being serviced is largely unaffected with no need for hazmat suits or other extraordinary protocols. This further accelerates the process of returning the system to use with optimal resistance to future corrosion.

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Picture of Daryl Roll

Daryl Roll

Astro Pak Consultant, Daryl serves as the primary senior technical advisor for corrosion, surface chemistry and stainless steel Passivation. With over 40 years of experience in chemical processing, Daryl has been published in MICRO, UltraPure Water Journal and Chemical Engineering for his papers on passivation and rouge control. He is a participant on the ASME BPE Subcommittees for Surface Finish and Materials of Construction requirements and a leading contributor for the Rouge and Passivation Task Groups. Daryl holds a B.A. in Chemistry and Earth Science from the California State University of Fullerton and a Professional Engineer's license from the State of California.

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