I have used nitric acid solutions for cleaning 304 & 316 SS HXers, usually 25-45 vol. % since this was readily available (Type VI Passivation solution per QQ-P-35C). This has the advantage of not attacking the metal. Also, after removal of the Fe2O3 (acid comes out clean), passivate with clean nitric solution and then rinse the HXer.
HCl will work quite well, but you should use an inhibitor. As Fe2O3 dissolves in HCl, the solution becomes more corrosive towards the metal.