My 2000 380DA was designed to continuously retain salt water in the bottom half of the exhaust system, causing it to rot out every 3-4 years, making me discard / replace expensive cast iron w/ new parts.
My solution was to redesign the exhaust system, from impossible to drain / impossible to flush, to now I flush w/ fresh water, flush w/ soapy water, flush w/ Salt Away, then drain bone dry after every usage.
My recent disassembly for inspection after 3 years of salt water usage revealed the interior passages to still look brand new. My speculation is that I am done replacing exhaust parts.
Yes, I would expect brackish water to be less damaging to cast iron.
My recommendation is to perform an annual disassembly for internal passage inspection and to replace parts when the SOLID wall thickness becomes insufficient.
