Rouging of stainless steel is sometimes found in high purity hot water systems, usually appearing as a thin red or black powdery or ‘slimy’ deposit. The mechanism that causes rouging does not appear to be fully understood, but is connected to destabilization of the passive layer. Measures that improve corrosion resistance can help prevent rouging. Passivation and smoothing of surface finishes by electropolishing have been used to reduce the risk of rouge formation.
What is rouging?
‘Rouge’ is belived to be comprised of be iron oxides and hydroxide corrosion products in various oxidation states. The range of colour observed is probably explained by the different ferric ion (Fe3+) oxide and hydroxide corrosion product. The exact cause of rouging does not seem to have been clearly establishe, but is the result of temporary destabilization in the passive layer.
One theory attributes this to ionic interactions between the water and the chromium rich passive layer. Temporary breaches in the passive layer resulting in localized corrosion to the underlying steel, before the passive layer has time to reform. Once reformed the short bust of attack is stopped, until conditions revert to those causing the passive layer de-stabilization.
Observations that rouging is affected by:
water oxygen content and pH
presence of any iron contamination in systems
surface finish (roughness)
non-metallic inclusion levels in the steel
welding defects, debris, heat tint or improperly cleaned areas
could all support this view as these are known to have an influence on the corrosion resisting performance of stainless steels.
The relative water solubilities of ferrous (Fe2+) and Ferric (Fe3+) ions are influenced by pH and result in the range of rouge colours formed from the ‘intermittent’ attack mechanism. Fe2+ ions are readily soluble, but when oxidized to Fe3+ ions are insoluble, forming oxides (Fe2O3 and Fe3O4) and hydroxides (Fe2O3.H2O) like the rusting process of carbon steels. These oxidized products have different colours. The near neutral pH of high purity water promotes the formation of insoluble ferric oxidation products and hence rouge formation.
The rouge corrosion products can be deposited away from the actual corrosion site, or as an even film giving the impression that corrosion is general. It therefore often difficult to identify why and where the attack has occurred.
Avoiding rouge formation
Resistance to rouge formation depends on several factors. These include:
In ambient temperature water, 304 type is normally considered suitable but for high purity, ‘hot’ water conditions where rouging is a hazard, 316 type has been found to be beneficial and is normally selected in pharmaceutical plant systems.
The extra nickel in 316 helps improve passive layer stability, while the additional molybdenum improves micro-pitting resistance, both of which appear to be beneficial in resisting rouge formation. For the same reasons, more highly alloyed stainless steel types could be expected to resist rouging better than 316.
Smooth surface finishes are important in the avoidance of rouging. Electropolishing is widely used to reduce the risk of rouge formation. This may also help improve the stability of the passive layer, so giving a double benefit.
Electropolishing of stainless steel
Surface imperfections from non-metallic inclusions can also be sites where the passive layer can become disrupted and become corrosion initiation sites. Steel made using good modern steelmaking methods should be better than older, perhaps ‘dirtier’ steels in resisting rouging. Remelted steels could also be considered, but their higher cost may preclude them. Sound, smoothly contoured weld joints, with any heat tint removed, should also help improve the resistance to rouge formation by minimising crevice effects and optimising the inherent corrosion resistance of the steel surfaces.
Avoid iron contamination
Any source of iron contamination or ‘carry-over’ corrosion product from other non-stainless steel parts of a system could promote rouge formation as the iron corrodes (rusts) relatively easily (compared to the stainless steel) in the process water.
This provides a source of iron ‘ions’ (Fe3+) that form the rouge products. Careful post fabrication clean up is most important.
Removal of rouge deposits
Often rouge formation cannot be avoided and has to be removed during routine plant shut downs. Besides being unacceptable to the plant operators as product contamination could result, build-ups of rouge products can lead to operational problems such as blockages in filters. More severe localized ‘shielding’ (pitting) corrosion could also result under rouge deposits. In some cases rouge deposits can be wiped off, but abrasion should be avoided as this will result in roughening of the surfaces and may reduce the resistance to future rouge formation. Acid treatments involving ‘moderate’ strength nitric, phosphoric, citric and oxalic acids have been used satisfactorily. The passivating action of some of these treatments could be beneficial, but ‘etching’ of surfaces could result in further problems as this could reduce rouge formation resistance, due to the slight roughening of the surfaces. Electropolishing could be beneficial to re-smooth and repassivate the surfaces.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)