Mine in unused/boxed have no sign of imperfections.

How long been used on layout?

Does anyone knows what is the material used to cast Marklin locomotive and wagons wheels?
I still dont know why did this coat/finish came out.

Tks

I looks like the paint came off. I'v no such modern loco's with painted wheels.

It looks to me like the plating of the wheels is coming off, my guess is it's probably nickle plating, which looks to have the newer black anodising treatment as well. I don't think anodising would flake off like that, so it's probably the plating. Somebody like Jacques with more knowledge of earlier Loks may give us an idea.
However, the only fix really, is to replace the wheels with new ones, I notice one of the drivers has a chip out of the flange also. You could renickle and paint them but I think that brand new ones would give the best and I believe less costly result.
However you will need a professional to do this as the driving wheels will need to be "Quartered" as they are being press fitted to the axles. The tools can be bought on the Internet, but not worth it for one Lok.
List members, jvuye in France, or drwhittle in New Zealand are two that have the tools and knowledge to remove and fit new wheels for you, that I know of. They both have the knowledge to access to the new parts from Marklin if they don't have the correct ones available allready
Good Luck

edit; it also ocurred to me that Marklin of course have a repair service too, and I have heard tales of out of warranty jobs being done for free if they think they have some culpability or are just in a good mood that day. I suspect their repair costs would be higher than the other private guys I have mentioned here.

Eduardo,Dave, Glen,kariosls37 and hemau, thank you all for the feedback.

The strange thing is the fact that this only affects one of the wheels.
Maybe i should leave it like it is, after all it's only one wheel.

Thank you all for your kindness and patience!

Regards

Hi Fred
Ring the marklin Service Department and discuss it with them
they will have seen it before for sure, and as someone above
has said they could give you some advice, or they may offer
to sort it for you,and give you an estimate to change the wheels
and I think also the M track suggestion could well be the joints
doing the damage, as the new c track are far better connection
and Tell them Toottoot referred you as they know us well.
regards
bryan old toot

Frankly, to me that looks like mechanical damage and not a fault in the finishing process. These brighter spots have very ragged edges which would be inconsistent with f.i. a bit of grease left on the surfaces after cleansing. Also on the first and 2nd photo there is smear of red wheelpaint visible on the axle stub. My guess would be that it might be caused by some kind of wheel press or similar tool, i.e. that wheel has been replaced or at least been pressed back on. Well, there is another cent, just one this time I guess.

Cheers, Robert