The metal that can corrode easier will loose materal, while the other will be more or less not harmed. Sounds like suppressors are not steel. IF they are made of aluminum, the suppressor threads loose. If it is made out of titanium, the barrel threads loose.
Combustion byproducts are carbon dioxide and water. Need water to rust, acids from carbon dioxide and others materials just act like an accelerant for corrosion. Alot of nitrogen in gun powder, so nitric acid is probably being made too.
In aluminum outboards, they put a zinc anode on for freshwater and a pure aluminum anode for saltwater. Saltwater conducts electricity alot better.