As you can see the copper... is peeling off. Copper does not de-laminate from the cheap tin core like this post cap is doing.
These beautiful and inexpensive copper topped post caps you find at the big box stores are not what they seem. They are anodized coatings that eventually corrode off and leave you with the cheap tin top you paid for. There are authentic copper caps available from specialty retailers, but be careful. you can test them by scratching the finish in an unobtrusive place. Real copper will be the same color inside as it is on the outer surface.
You can expect to pay $35 or more for authentic copper caps. Copper caps will turn a green patina as they oxidize and they don't rot...unless you have 3 or 4 hundred years to wait.
When it comes to copper clad post caps...if it sounds too good to be true, it usually is.