Derouging of stainless steel
What is Rouge?
Rouge is a common problem in pharmaceutical facilities, most often found in high-purity water and clean-steam systems fabricated in austenitic stainless steel.
Rouging is a catch-all expression to describe the discolouration that forms on the stainless steel surfaces; ranging from a pale yellow/orange, through to red/brown and onto a dark violet/black. It is composed predominantly of iron oxides and/or hydroxides.
In the absence of any industry standards, rouge is often categorised into three types:
Type 1 rouge is usually a lightly-adhered orange “dust”, generated at an external source – such as the cavitation of pump surfaces – which then migrates through the rest of the system. This is the easiest form of rouge to remove. It can sometimes be simply wiped off, but chemical cleaning and re-passivation will give better results.
Type 2 rouge is formed in-situ on the stainless steel surface, where the passive layer has broken down. It is usually tightly-adhered and will need chemical cleaning to remove. Type 2 rouge is typically darker and denser than Type 1 and is a deep red/orange colour.
Type 3 rouge is a dark violet/black colour due to the high content of magnetite (an iron oxide). It is commonly found in hot steam systems and can be very resistant to standard derouging chemicals. More aggressive chemical solutions or mechanical polishing may be required in order to remove it.
Where does rouge come from?
There are various theories on how rouge manifests itself, including; the passive dissolution of the chromium-oxide layer by the low-ion water, metal oxide particles released via pump cavitation and the more obvious sources of iron contamination from defective welding, heat-tints and poor cleanliness controls during fabrication/installation. Any or all of these can result in the formation of rouge.
There are also several other observations that should be taken into consideration:
- The formation of rouge occurs faster – and with more severity – with the increasing temperature of the water
- Cold water systems are not immune. Particularly ones that are treated with ozone
- Equipment that generates turbulence (such as pumps or spray-balls) are much more likely to develop rouging than pipework
- Rouge migrates – as evidenced by the orange discolouration often found on PTFE gaskets throughout an affected system
- If left unattended, rouge will progressively intensify, corroding the underlying stainless surface. Cleaning (derouging) systems that have been contaminated for a long period of time often reveals pitting and a deterioration of the surface profile
- Systems with rouge are more vulnerable to developing bio-films (the crystalline nature of iron hydroxides provides a site for bacteria to take up residence)
- Effective passivation carried out pre-commissioning reduces the rate at which rouge will develop
- Routine, scheduled re-passivation will reduce the rate and severity of rouge development
The derouging process
An effective derouging process will remove the iron oxide deposits without damaging the underlying surface. Whilst highly-corrosive products (such as chemical solutions containing hydrofluoric acid) can be effective at removing rouge, they will also inevitably etch the stainless steel, ruining polished/machined surfaces and exacerbating corrosion sites. As such, they should be avoided whenever possible.
Following the derouging, a thorough passivation treatment should be carried out to create a high chromium-iron ratio on the stainless surface. This will optimise the passive layer and the equipment’s corrosion-resistant qualities.
INOX Passivation Ltd are able to derouge your process pipework, pumps, vessels, WFI stills, lyophilisers, autoclaves or any other equipment.
We take each project on a case-by-case basis as it is rarely a situation where one solution fits all. As such, we offer a range of products for the work, ranging from the familiar phosphoric acid-based blends, chelating organic acid options, to modern pH neutral chemicals that are friendly to your equipment and the environment.
Before job commencement, we will attend your site and produce cGMP compliant documentation for the process, including a detailed step-by-step protocol, marked-up P&ID’s to identify the system boundaries and flushing loops and define the testing/verification methodology at each relevant stage.
Our equipment has only ever been used in pharmaceutical applications and full traceability for each pump, hose and fitting is available upon request (detailing the chemical products they have handled, contact times and dates of use).
Upon completion, all waste products will be disposed of in accordance with local legislation.
If you need assistance with rouge, passivation or any other stainless steel related issue, please click on the link below.