Blacking refers to the much thicker layers which can be formed in high purity steam systems the longer they are in operation. These matt black layers are generally firmly adhering layers of magnetite: Fe3O4. At the beginning these are slightly transparent and can grow to a chalky surface structure. The high temperature of steam and the iron oxidation due to defects of chromium oxide layers influence respectively the formation of blacking. Different  impurities are also a source for the formation of layers, see chapter “cleaning /passivation”. In advanced stages the degradation of  surface layers may be observed and it could be possible that particles become detached.

Removal of blacking layers
Blacking layers are very stable and very difficult to dissolve: they can only be removed with a very well adapted treatment. Usually there are  the high-grade steam generators themselves which are most affected the longer they have been in operation for. But here also the conclusion is valid, namely that well-built systems, which have been well cleaned and passivated from the beginning, are much better and demonstrate a far better resistance to the formation of layers. If these systems are also cleaned regularly from the start, then they will remain metallically clean.