It is very difficult to give a specific MOC recommendation without knowing many additional details about your process. The composition, concentrations, and temperatures are all important from a corrosion resistance perspective. Even low levels of some chemical species can significantly change the corrosivity. As you have found out, HF is very corrosive to many alloys including austenitic stainless steels. You also mention that chromic acid is present. Taking a quick look at Schweitzer's Corrosion Resistance Tables, a Ni-Cr-Mo alloy like C276 looks promising as a starting point. It is resistant to both HF and chromic acid depending on the concentrations and temperatures involved. Once you have some potential materials of construction selected, you might consider running laboratory corrosion studies to help you make a final choice. I am not a big fan of coatings for this kind of application given the elevated temperature and the potential for damaging the coating and exposing the metal substrate. If you decide to pursue this route, in addition to the general compatibility of your process with the coating, you need to be concerned with permeation by HF, etc. Permeation could lead to attack of the base metal and/or peeling of the coating from the substrate.
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