I have a chemical sewer that will be seeing Cl gas and weak HCl. What material is best to use? I am looking to replace a chemical sewer's manhole, and I have yet to find a company that can provide what we need.
You should be able to find polyethylene fittings, if not fiberglass/epoxy, in manholes and covers. Sealing same might be an issue. Try talking to a local sewer/septic tank supply house, one that deals in plastic tanks.
I would evaluate a high silicon cast iron (Duriron) pipe and fittings commonly used for underground chemical sewer pipe and gratings. Look at ASTM A518 and A 861.
Adding to the post by metengr recommending Duriron Pipe which will probably do the job and if it doesn't you might want to consider Durachlor Pipe. Both these material are made by Flowserve.