https://cooking.stackexchange.com/a/5863/6168
If you put any acid in cast-iron, you are harming your seasoning, and leeching iron into your food. This will affect the taste of your sauces, I find pan sauces taste metallic when made in cast iron.
Indian dishes often require tomatoes in most of the dishes. I am not intending to cook any tomato sauces in the Kadhai. Will I never be able to cook any Indian dishes requiring tomatoes in a well seasoned iron Kadhai (assuming I'll re-season again)?
The PH scale ranges from 0 - 14 with the lower numbers being more acidic than the higher number. Canned tomatoes are generally around 3.5-4.5 on the acidic scale, in other words just above the half way point below neutral 7.
As long as your tomato based sauce is not left in your seasoned Kadai (as Kristina rightly points out) for a long timeframe and your kadhai is washed out immediately after use no damage to the seasoned coating should occur.
Just remember to wash out your Kadai (with warm water and a cloth) immediately after use to preserve the seasoned coating.
I've made tomato-based dishes in my seasoned cast iron pans including spaghetti sauce and chili with no perceivable off-taste or damage to the pans' seasoning as long as I removed the food soon after cooking since prolonged exposure, from my experience, will affect the seasoning of the pan, if not also the flavor of the food.
According to this site,
For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it is formed rather than depositing on the metal surface to form a film.
Once ferrous oxid (i.e. iron oxid) becomes soluble, your body is able to metabolise it. Your body is able to regulate iron levels to some extent, so while it doesn't seem likely that this, on its own, would lead to iron poisoning, anyone taking iron supplements might want to avoid cooking canned tomatoes in cast-iron. Fresh tomatoes have a pH between 4.3 and 4.9 according to this online source and should be alright.
Note also, from the site in my first link, iron corrosion increases drastically below a pH of around 3.8. If spiceyokooko is correct, and the pH value range for canned tomato is between 3.5 and 4.5, I think you would do well to measure the pH value of your tomatoes before using cast-iron. There are various inexpensive pH test kits and instruments available. It's quite likely that your pharmacy stocks alkacid paper or similar.
I also recommend you read this excellent answer about cooking wine and vinegar in cast-iron.
