Thank you for your question.
The difference between 904 grade and 316 grade steel is the higher chromium content of the metal.
904 has approx 2x more chromium. The higher chromium content makes 904L harder and more resistant to corrosion.
This is why the 316 flue liner has a 15-year warranty and the 904 flue liner has a 30-year warranty.
Hope this helps?