Dear Masoud: I had searched through my reference books to find anything about the effect of NH4Cl on steel but it was not fruitfull. So I tell you my own judgement which may be flawed but better than nothing! Ammonium chloride has got chloride which is very much damaging to the steel structures esp when its hot. I don't know if you had any observation of pitting on cast iron before but it seems to be a problem. But fortunately it is not acidic which would have increased the damage in that case. As far as I know cast iron is more susceptible to corrosion than mild steel because of possibility of formation of galvanic corrosion due to the presence of Grapfite so if you change it to carbonn steel it will be fine from corrosion perspective but not heat transfer in which the cast iron has the upper hand. Best Wishes Ali

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