Investigations recorded in literature sources show, among other aspects, that
- 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 have an influence on the formation of rouge. Thus the amount of residual oxygen and the CO2 concentration, forming carbonic acid can play a role
- 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
One theory on its origin is that aggressive water breaks down iron by oxidation at very small weak points of the passive layer and is accordingly released and “bleeds” out. 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.