Rouging

The phenomena of rouging, which is named after the rouge used by women, mostly occurs in hot ultra-pure water systems and clean steam systems. It is assumed that waters which are extremely lacking in ions have a “sucking effect” upon the ions in the alloy because they want to improve their ion poverty state. Furthermore the CO2-content can change the acidity of the water in these systems due to the corresponding carbonic acid equilibrium.

At the end there are system parameters, which constitutes the aggressiveness of these pure waters. Even ozone treatments promote the formation of thin rouging layers. However even in storage containers for CIP solutions (NaOH or KOH solutions) discolouring in the passive layer has been observed. If, for example, the rouging in pure steam systems is not removed from time to time, then blacking could occur. With increasing time pollution with small particles out of these layers can not be excluded.

Systems which are affected by rouging usually show discolouring. This can range from a reddish brown to violet to light blue nuances. These discolourations are usually thin layers and may be either strongly adhesive, easily wiped off or may even have a chalky characteristic.

Relating to  definition of corrosion:”every physical-chemical interaction between a metal and its environment, which leads to a change in the characteristics of the metal and which can lead to considerable diminution of the function of the metal, the environment or the technical system, is described as „corrosion””, the rouging can be also classified as type of material corrosion.

Because the phenomenon of rouging is not yet clarified in detail, theories drawn from our own experiences and observations as well as literature sources are given here (see Literature)!

• rouging formation increases with the increasing temperature of the water systems
• pumps are far more susceptible than pipelines

• water treatment with ozone promotes rouge formation
• installations with defects (bad welding, using low alloy steels) can influence  the whole system
• on one and the same component rouging can occur in separate places with totally different adherence characteristics
  
• in the cases of nearly all thicker layers it can be wiped off to a certain amount

• he thicker the layer the greater the effort required to remove it
• phosphoric acid-based solutions have only a limited dissolving capacity for rouging layers
• systems which have been cleaned and passivated from the beginning are less susceptible to rouging
• systems which are regularly cleaned are generally less susceptible to the  formation of new rouging layers

• it deals with various ferric oxides and hydroxides, which apparently form on the surfaces or accumulate there
• it deals with corrosion processes, which are caused by hot aggressive waters
• the actually gas compositions, which are present in the systems, can ha ve an influence on the formation of rouge. Thus the the amount of residual oxygen and the CO2 concentration, forming carbonic acid can play a role
• the  surface condition influence the formation of rouge. The smoother, pure and more homogenous, the better the future performance is.
• the differences in susceptibility to rouging of alloyed materials of different quality is much smaller in cases where there has been good preliminary chemical cleaning

It is possible that the aggressive water with low ions content, could locate very small imperfections in the passive layer and decompose iron by process of oxidation thereby dissolving it out and “bleeding” it. These insoluble iron oxides  can easily deposit on other parts of the metal surface causing a weakness in the passive layer through adsorption. In pump casings and propellers e.g. the rouging is specially observed, because the iron oxides and hydroxides will be centrifugated there.

Removing rouging is a difficult task due to its low solubility. As already mentioned, pure solutions with H3PO4 can only remove rouging in its preliminary stages. The removal of advanced rouging without harming the electro-polishing, can only be achieved by using special solutions and requires qualified experience.