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I have found black areas in the the surface of some steel. Are these voids from the original manufacure or are they created by corrosion?

This one is approx 1mm x 0.5mm.

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Richard
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  • From those pictures it really isn't possible to tell. Voids seems unlikely, some machining issues might explain it, as would some surface corrosion. Without better metallography it is hard to say. – Jon Custer Feb 25 '19 at 14:44

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That looks like a slag inclusion, left over from the rolling operation in the mill. This is common in low-grade hot-rolled steels.

niels nielsen
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  • Interesting. Thanks. The object is an air rifle barrel. The supplier is saying it's corrosion, but it looks to me to be inside the steel and exposed by the machining. – Richard Feb 22 '19 at 23:17
  • the next thing to do would be to either cut it up & put it in a scanning electron microscope for a closer look, or electropolish it and then do a grain boundary etch to see if there's evidence for intergranular corrosion. Do you have money to spend on this? – niels nielsen Feb 22 '19 at 23:43
  • Hi Niels, alas not. I agree that sectioning it would show 100% that it's not corrosion caused by bad ownership but I don't really want to destroy the barrel... It's currently fitted with a bonded on carbon fibre sleve, so if that was removed and there were pits in the surface underneath, that would prove it's not corrosion too. – Richard Feb 23 '19 at 09:53
  • I see. how old is the barrel? – niels nielsen Feb 23 '19 at 16:45
  • I would guess it's about 8 years old. – Richard Feb 23 '19 at 19:13
  • Some more photos added. – Richard Feb 24 '19 at 09:06
  • looking more to me now like intergranular or pitting corrosion, from storage in the damp, but can't be sure. – niels nielsen Feb 24 '19 at 19:20