I have a set of Calphalon cookware that is anodized aluminum. The set is at least 15 years old. On the stockpot and a saucepan, the anodized coating is gone. The area where the coating is gone in inside the pot up to a level where cooking liquid typically goes.
Does cooking acidic foods in these pots removed the anodized coating? What else could do it? Oddly, I don’t have this problem in the other pieces in the set that get much more use.
Acidic and basic solutions typically found in foods typically doesn’t impact it much.
As it is porous and isn’t nonstick often scouring pads will be used in those areas which will remove it over time.
Most anodized aluminum cookware is also not dishwasher safe, if you used one the alkaline phosphates in most dishwasher detergents dissolves the ‘passive’ aluminum surface as sodium aluminate. This reacts with the sodium phosphate to produce sodium aluminum phosphate which often dries into white spots.
As stockpot and a saucepans are typically the vessels that typically require the most cleaning due to stubborn stains I would assume it is the 1st in your case or a mix of both as you are unlikely to cook food with a large amount of alkaline phosphates.
I have done this a few times.
It’s annoying–typically happening only on the bottom, since the inside has a Teflon coating.
The worn areas are not the result of scrubbing. The coarsest material I used is a ScotchBrite pad. The wear patterns are very even, and resemble what you see as high-water marks on canyon lakes. My sauté pan has taken a beating in terms of scrubbing more than any other piece and show no sign of this.
I was wondering about acidity because once a year I make a cranberry mold by cooking whole cranberries in orange juice. I have never put any of them in the dishwasher; the manufacturer’s instructions specifically say not to do this. You are correct that I use alkaline phosphates very sparingly in my cooking 
It could simply be wear then, alkaline phosphates are typically the most common issue.
Gone? Or just changed colour? Sometimes the colour is less stable than the oxide.
Given the pattern you describe, it sounds like it is in fact the food interacting with the pan. Since anodized alum doesn’t interact, perhaps the anodized layer was removed ( a prior owner/house guest stuck it in the dishwasher) or maybe it was never present (unlikely but not impossible) or the anodizing was at too high a temperature, making it susceptible to wearing off. Another possibility is that there was some layer over the anodized surface with which the food interacted.
Complaining to Calphalon might help, notwithstanding the age of the cookware. They are pretty good at replacing stuff if the customer is insistent enough
Note that pan makers often make two levels of anodized pans. The good stuff and the poor stuff. The poor ones lose their coating fairly quickly. The cost savings aren’t worth it, IMHO.
Yes, I learned this the hard way. I just assumed that anything made by Calphalon had high-quality non-stick surfaces. But the set my husband bought for me about ten years ago is no longer non-stick.
Pitting on the bottom, as well as loss from the sides. We have a pot like that. I had assumed that it resulted from sitting in the sink for weeks (months?) full of dirty dish water. There was a time when my wife and I maintained separate homes, miles apart. One of us (perhaps both) was not very good at making sure the dishes in the sink were washed on a timely basis.
I don’t know if there is anything you can do about it. Replacement is probably the best choice. If you want to replace it, I suggest a high-quality heavy gauge stainless pot. Yes, their expensive, but it’s not like the Calphalon is cheap. The stainless will resist almost anything (except chlorides, do not brine in a stainless pot) and will last. The other choice is to just get a cheap one and replace it regularly. Being a stock pot, it generally doesn’t need to be non-stick.
I am not really looking for what to do with the pot so much to understand the process that causes this for the sake of science, and also so I don’t keep doing it.
You mean as in sodium chloride? How can they make a stock pot that you can’t put salt in?
Maybe it’s because you cook with gas? 
I have the same cookware, and it has the same perfectly-straight unanodized line on the inside at the typical water line. There’s no way it’s from scrubbing, as there’s no way I scrub the pot that straight. The pot is typically used for pasta, which we typically salt the water. Maybe the salt has an effect?
One interesting thing I noticed is that if I boil some aluminum foil, the unanodized part gets dark again. It lasts for a little while before wearing off. I’m not sure if it’s anodizing it or if it’s something else. The reason I boiled aluminum foil is I was making some eggs and didn’t want them to sit on the hot bottom and bounce around from the steam bubbles. I set the eggs on some crumpled up foil and noticed the darkening of the shiny part of the pot afterwards. (ETA, maybe it was the combination of eggs and aluminum foil. I never boiled just aluminum foil to see what happens.)
