Alright mfs I'm 3 months late to the argument but whatever.
Firstly, 304 stainless steel has inferior corrosion resistance to 316 stainless steel. For comparison, even in seawater (which is much harsher than an inland environment), 304SS needs 145 years for 1mm of penetration while 316SS needs 260 years. <
https://bssa.org.uk/bssa_articles/technical-library-pitting-corrosion-pre-numbers/>
Secondly, in this translation at least, she said "stainless metal" which could mean any metal, not just steel. Grade 5 titanium has even higher corrosion resistance than 316. If we're also comparing titanium under typical seawater, it straight up does not corrode. You'll start to see corrosion if your seawater is >500F. <
https://www.azom.com/article.aspx?ArticleID=1226>
Lastly, the source that says '304 is sometimes referred to as “marine grade”' is straight up wrong. Nobody says that. Even if you google "marine grade stainless steel", 316 pops up.