Scorched steel makes it easier to oxidize ..... and you live right near the ocean in a salt air environment.
Even here in the high and dry of the intermountain desert I find a thin layer of fine rust on the heat caps of some of my Solar Engines and PM Research Stirling Cycle Engines!
My PMR's all will get some surface rust if I don't oil the end caps after running them, but they are carbon steel.
The Bohm end cap is stainless, I've seen a slight rust stain even on 316 stainless in marine settings, but that's a fair bit of rust on that end cap.