Is this corrosion or a void in the metal?

[rich4422]

Hello, I am looking for some opinions on marks I have seen, under the microscope, in a steel air rifle barrel.

This black mark is approx 1mm by 0.5mm.

I believe it's a void in the steel, exposed by machining and NOT corrosion.

What do you think?

 

[EdStainless]

A cross section would likely tell you more.
It could be either, or even corrosion initiated at an inclusion in the steel.
Air rifles barrels have a tough life. Every time you fire them the ID surface gets damp (from the cooling of the air as it expands). It is difficult to keep them pristine.
This is why competition barrels are either 410SS (for some corrosion resistance) or they are plated.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[rich4422]

Thanks Ed!

I really wanted cutting the barrel in half! LOL!

What I did notice, when machining a light skim off the face, initially in the hope of clearing the pits off, just exposed more...

That photo was of a really big one, but there are loads of tiny pits, even in the bore.

The rest of the rifle is pristene.

This is a phone pic looking down a borescope: