Ferrite is one of the iron oxides, it might be converted to
another of them by surface chemical attack / oxidation.
Black oxide to red oxide etc. But I think this is more like
a "decoration" of material imperfections from molding or
shaping process, either physical or chemical (what does
uneven cleaning of a rough sawn surface usually do?).
If it -is- corrosion, do you care? It's not like there's any
electrical characteristic of interest, aside from magnetic
attributes. Right? Maybe some interest in adhesion of
any glue used in final assembly, that ought to have an
in-house clean (unless you're getting these pieces in a
vacuum bag w/ a sourcing spec on surface cleanliness
and keeping the material prostine once you get it,
which the pictures indicate otherwise).