I have a non stick Tramontina frying pan. Today, in an effort to remove the brown stains on the reverse (metallic) side, I put the pan in boiling water with baking soda for 30 min. After cleaning it with a sponge, I noticed that the outer bottom metallic layer seems to have come off quite a bit (see attached picture). I was surprised as I did not think that baking soda could do this--moreover the nonstick surface seems to be undamaged. So my questions are: What happened here? And what will be the effect on cooking?
Baking soda is a mild base (the opposite of being acidic). In the right concentration & environment, it can be a corrosive.
What happened is that in addition to taking the burned bits off your pan, it also took the shiny finish coat off the outside of the pan. What you're seeing is the unpolished aluminium, with a coating of aluminum oxide.
Since only the exterior of the pan is affected, you've not damaged the functionality of the pan, just the looks.