So I am quite new to this cast iron thing. After few uses I tried to clean it with kosher salt. Now some parts of it look like it's all flaky, as if the layer will come off if I scratch it with my fingernail. The part is grayish in color. Is it the seasoning that has come off, or have I damaged my pan? How do you really know the difference? I mean, how easy it is to damage a cast iron pan in a way it is no longer useful, and what will that damage look like?
If it's flaking, it wasn't seasoned right to begin with. However it really only matters on the cooking surface. You want seasoning to build up, but it shouldn't flake.
See if it really is coming off, try with you fingernail. If it's coming off then get all of it off that you can by chipping it away. Scrape it off. If it doesn't come off then leave it alone, and season as normal another new layer.
If you aren't happy with it, you can always strip it and start over but that's usually something you end up wishing you hadn't done. You do this by putting your cast iron in the oven and running it through a cleaning cycle. Be warned, don't do this unless you already know what to do and how to season your newly bare cast iron.
Old cast iron seasoning is the best, you don't want to start over. I have some with a 1/4 inch of seasoning (I am exaggerating) on the outside of the pan and it looks crusty, but the inside seasoning and finish is baby skin smooth and slicker than any Teflon coating. I keep it that way by scrubbing lightly with a green scrubby to "tip" the bumps down before putting back in the hot stove to cook another thin layer of oil.
You can't ruin cast iron except by breaking it. Don't worry, it's not ruined, it just needs some care. Some of my best cast iron was a lump of rust I rescued from the junk store.
I have a decades old Indian "tava." It's an 8", round, concave, cast iron surface for cooking flat breads on a stove top. The outer 1" of the tava (the 1" around the edges, furthest from the center or the flame, routinely flakes for two reasons: 1. Because the outer edge typically doesn't get hot enough, as it's furthest from the flame. 2. Because the outer edge typically doesn't get cooked upon. (A typical flat bread is less than 7" diameter). So, oils that I add to the flat bread only minimally, if at all, reach the outer edges of the tava.
Bottom line, flaking is normal under these conditions. About once a year I have to scrape the flaked material off. After doing this I very lightly coat the entire cooking surface with canola oil and place the tava in a 450-degree oven to season it. Regardless, the outer edge of the tava will start to flake again because of how I use the pan. I suppose I could overcome this imbalance in the cooking surface by routinely seasoning the outer edge, but that's a lot of work. The main thing I care about is that the inner 7" is smooth as glass and performs beautifully - even after 50+ years (My mother used this tava and now I use it).
I had that problem at first — I believe it was because I had been too aggressive in my initial seasoning process, by using too much oil.
I decided to start over by leaving the pans in a super hot oven (550°F) for a few hours — you could also use the self-cleaning cycle if your oven has one. After cooling, all the old seasoning can be scrubbed off with soap and water and you'll be down to bare cast iron. It's important to immediately season the pan again or it will start to rust.
People have differing philosophies about what kind of oil to use. I used plain old canola oil, with very good results. The most important thing I learned was to only use a very thin coat of oil — wipe it on, and then off again with a paper towel. If there's a "layer of oil" on the pan, that's what will form that soft coating that flakes off.
It'll take a few cycles of seasoning before the pan is ready to use, but the thin coatings will result in a much harder seasoned surface that will remain beautifully non-stick, the more you continue to cook with it.
The flakes are burnt oil. They occur because oil residue has built up in the pores of the pan and when it gets too hot they flake off. Companies who make such pans often deliberately place such oil deposits on the pan to retard corrosion. Also, many cooks will bake on oil to regenerate this oil layer, called "seasoning," as kind of a non-stick surface.
You can recondition the pan by scouring it with Scotch Brite and renewing the seasoning oil.
Personally, I don't mess with seasoning at all because it is a hassle and it can absorb bad odors and bacteria. Corrosion is caused by acids; so long as you keep the pH of the pan basic it will not rust. This can be done by rubbing it with baking soda.