OXSID SCALING
The oxide scale formed comprised a three-layer structure, similar to that formed on pure iron and low-carbon steel. For the high carbon steel, however, the scale was essentially a two-layered because of the low proportion of hematite (Fe2 O3) formed.
Scaling occurs when water has high levels of minerals like calcium carbonate, which can build-up on surfaces. Slight scaling can be considered beneficial in that the inside surfaces of metal pipes become coated with harmless minerals that act as a barrier to corrosion.