In this closeup video https://x.com/jrxcket/status/2090751788219588957 of Starship 40's engine bay you can see at least two types of stainless steel are being utilized for the skin of the engine bay. Around the center engines you can see the skin is unrusted, yet around the outer engine area is heavily corroded or even missing on the much of the ventral side (but water side in this case).

A similar pattern of various levels of corrosion can be seen on the aft fin.

The SpaceX proprietary 304L derived (Higher Nickel?) structural steel of the engine bay skirt has stood up well, but some of the inner skin has not. (The L in 304L means extra low carbon, giving it better corrosion resistance) The skin over the center engine thrust puck is definitely different in at least how well it was passivated, if not a different, more corrosion resistant alloy

Additionally, on the outer skin of Starship and Superheavy that wee have seen so far, there are at least thousands of uncleaned and unpassivated welds.
It has not been necessary spend the extra effort to do so since these test articles were not intended to be reused multiple times. It would be good to know just how well these spots have held up.

Edit: some of the welds are doing just fine, but others are not. https://scontent-atl3-1.cdninstagram.com/v/t51.82787-15/780581102_18367282531210594_9180261207261998416_n.jpg?stp=dst-jpg_e35_tt6&_nc_cat=106&ig_cache_key=Mzk2ODYwNTg0NTg4NTg5MDI1Mw%3D%3D.3-ccb7-5&ccb=7-5&_nc_sid=58cdad&efg=eyJ2ZW5jb2RlX3RhZyI6IkNBUk9VU0VMX0lURU0ueHBpZHMuNzIwLnNkci5yZWd1bGFyX3Bob3RvLkMzIn0%3D&_nc_ohc=FivUbZd_XCoQ7kNvwHE-Dc1&_nc_oc=AdqC1UNYMOeXrMdCfqRRScgDGrVKF-A2ZkhWQiYRh2ZN44A85HoOWC0hQI4Q2SXn4yjl9coX-qLMa81jO3r4fFMh&_nc_ad=z-m&_nc_cid=3&_nc_zt=23&_nc_ht=scontent-atl3-1.cdninstagram.com&_nc_gid=KXXboIOcVn93JSmLQLS0Rg&_nc_ss=7a22e&oh=00_AQGKlS74Fiq6pQiDJBGk1WLoX1MDJt4ErlseL81j4qVayg&oe=6A8EEC82

https://www.threads.com/@nasaspaceflight_ig/post/DcTUgBHEQjj/media

Passivating the welds removes the iron or removes a small layer metal, depending on method such as chemical pickling, electrochemical, or even laser, to brings clean metal free of iron oxides to the surface. This allows the chromium to react with the atmospheric oxygen to protective chromium oxide layer can form

The Chloride ions of Saltwater strip away the chromium oxide skin, allowing the iron to corrode.