marks in 2024 bar stock - what are they?

[NickB]

I am currently cleaning up some 2024 bar stock which has been stored in my shed for a long time and am noticing marks on the material. The marks cannot be removed by sanding (hand sanding with emery cloth) but seem to be removed by using a 3m deburring wheel. However after using the deburring wheel and then lightly sanding again the marks re-appear. Just wondering if anyone could educate me as to what these marks are, is it corrosion or a defect in the manufacturing process? Are there any treatments to remove them and what impact on structural integrity will they have? The 4th photo is the original material with no sanding.

Thanks
Nick

[LSA052]

Intergranular corrosion?

[Bdflies]

That's corrosion.

Bill