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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).
It looks to me like they are gotten out of a trxformer that caught fire.
You could look up whether high heat can affect permeability of ferrite...but i doubt it does.
I bet they are ok....you coudl also put a coil round them and test their inductance and satuarion current is correct.
I have seen this on Ferrites myself but I assumed it was a mark left by the baking process when it was manufactured. I have never seen it change any magnetic characteristics.
The core above is unsed EPCOS EE42/21/20 N87 cores, bought in 10/2019.
I went to buy small EE cores , but also bought some ee42,36 etd 39 etc.
I didnt have any bag with me so I took a packing box they gave me , shown below.
I didnt not move the cores out of the box, took few for some projects. I put this box in a wall cupboard and still
it is there. recently when I opened the box I saw this discoloration on few cores. small EE cores are okay but 4 of the 6 EE42 and couple of ETD39 have this issue.
My room has some humidity issues. Last year , last quarter after heavy rains most of the boxes/cartons (stored in open air ) in my room got fungus/mould. This box was inside the cupboard and shows no sign of anything like that. so this discoloration got anything to do with humidity?????????
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