As far as I can recall monel is OK against naphthenic acid- and HCl- promoted corrosion. Chrome-containing stainless steels usually used on toppers should be adequate protection against sulfidic corrosion above 260oC but aren't better than C-S on the naphthenic acidity issue.
I have heard that 316 SS, and better even, 317 SS (with at least 3.5% Moly.), are used to stand naphthenic acid corrosion, which appear to be intensified by turbulence. Naphthenic acidity may corrode at temperatures above the 230oC level depending on the TAN value of the particular cut. Protection against this type of corrosion is frequently obtained by blending crudes to reduce the crude TAN values to around 0.5 mg KOH/g.