Titanium is a reactive metal and its corrosion resistance is due to the protective titanium dioxide film that rapidly forms on its surface in most environments. As with any metal or alloy, titanium is not immune to all environments and should not be misapplied.  For example, titanium is unsatisfactory for strong sulfuric acid and corrodes in even dilute hydrochloric acid. You have not given any information regarding the environment (components, concentrations, temperatures, etc.) your titanium vessel is seeing but based on your observations it would appear there are better materials of construction for your application.

Regarding your question about sulfuric acid, this is a very complex subject as sulfuric acid corrosivity varies significantly depending on temperature, concentration, and even fluid velocity. To make matters worse, intermediate concentrations of sulfuric acid tend to be more corrosive than dilute or very concentrated solutions. Whether 316L stainless steel or 904L offers better corrosion resistance will likely depend on the specifics of the application. There are entire chapters of books and even entire books devoted to selection of materials of construction for sulfuric acid service. I suggest you consult one of these after you have fully analyzed the details of your process and understand the chemical environment.