What Does Rouging Mean?
Rouging is corrosion that occurs in metals like stainless steel. It occurs due to stainless steel surface iron contamination. It can be brought about by welding on materials other than stainless steel for temporary means, such as for support columns. When these materials are welded off, they leave behind an area low in chromium.
Rouging mostly occurs in hot water systems with high purity and may take the appearance of a thin black or red slimy or powdery deposit. The exact mechanism that causes rouging is not completely understood. However, it is related to passive layer destabilization.
Corrosionpedia Explains Rouging
There are three types of roughing:
- Class 1 - The surface of the stainless steel and the ratio of iron and chromium on the surface underneath the deposits typically remain unchanged.
- Class 2 - Iron particles originate on inappropriate passivated or unpassivated surface of stainless steels. With its formation, the ratio of iron and chromium is altered.
- Class 3 - Iron oxides build up on high-temperature surfaces. In this, the ratio of iron and chromium is usually modified.
Essentially, "rouge" is composed of corrosion products of hydrogen and iron oxides in different states of oxidation. The color range is varied due to the various oxides of ferric ion and the corrosion products of hydrogen. The exact explanation for the rouging phenomenon is not yet clearly understood, but it is possibly the outcome of momentary destabilization within the passive surface.
One theory connects rouging to the interaction of passive layers that are rich in chromium with water. Momentary breaches in the passive surface leads to a localized type of corrosion to the steel beneath, prior to the reformation of the passive layer. Once the layer has reformed, short attacks have halted and the situation reverts to that leading to the destabilization of the passive layer.
There are many ways to prevent rouge formation, including:
- Choose steel suitable for high-purity water conditions like stainless steel 304 and 316.
- Perform electro-polishing to reduce the incidence of rouging.
- Avoid any iron source than can contaminate stainless steel system.