Hastelloy and Inconel Bolts for High Temperature & Low Acidic Environments

Q:We are currently using mild steel with a black oxide coating and the bolts are breaking down because they are stretching under tension because of the heat, and corrode in various places from our acids.The acids we use here are Hydrochloric (HCl) and Sulfuric (H2SO4), both are heavily diluted with water at this stage (the pH of the liquid at this stage runs between 5 and 6). Temperatures are at about 1200F. Any suggestions on a more reliable bolt?
A:
For a high temperature like 1200F and the fact that you are dealing with mild acids which can become concentrated and significantly more corrosive as you drive off some of the water. I would suggest considering an Inconel 625. This will give you the best of high temperature stability and very good acid resistance. It will cover you over a broad range of conditions and will be a significant improvement over carbon steel.
Another option is a Hastelloy C276 bolt. This is more geared for the acids but will certainly be OK at those temperatures as it’s a nickel based alloy.
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