Tinned Copper or Aluminum Wiring

Question on aluminum wiring. At first glance I called this aluminum. The wires are for the furnace and are on a 30 amp breaker. But when I looked closer the wires have been nicked and look like copper underneath. I have heard of tinned copper but when I google it I get tinned copper is cloth covered. Any ideas why I see the copper color underneath?

Single strand aluminum would be defect, not multi strands.

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You nailed it Tim THANKS!

That’s tinned copper, the RW or RHW insulation is the confirmation.

Thanks Robert, that was my question.