Please forgive the way this copied.
The entire document can be downloaded from the following web site


http://www.ssina.com/download_a_file/corrosionfinal.pdf


Because stainless steel contains at least 10.5% chromium, the oxidation of the iron is changed to produce a complex oxide that resists further oxidation and forms a passive layer on the surface. This is a very thin layer (microns in thickness) but very tenacious and will reform if it is removed by scratching or machining. The addition of nickel to the structure (8% minimum in 304 and 10% minimum in 316) broadens the range of passivity established by the chromium. The further addition of molybdenum (2% minimum in 316) further expands the passivity range and improves corrosion resistance, notable in acetic, sulfuric, and sulfurous acids and in neutral chloride solutions including sea water. If stainless steel is properly selected and maintained it should not suffer any corrosion. Stainless steel will, however, corrode under certain conditions. It is not the same type of corrosion as experienced by carbon steel. There is no wholesale “rusting” of the surface and subsequent reduction of thickness. If stainless steel corrodes, the most likely form of corrosion is “pitting.” Pitting occurs when the environment overwhelms the stainless steel’s passive film and it cannot heal the interruption. It usually occurs in very tiny dark brown pits on the surface (hence the name pitting), and does not interfere with the mechanical properties of the stainless steel.

From another source:
In the marine environment, because of it slightly higher strength and wear resistance than type 316 it is also used for nuts, bolts, screws, and other fasteners. It is also used for springs, cogs, and other components where both wear and corrosion resistance is needed.
Type 316 is the main stainless used in the marine environment, with the exception of fasteners and other items where strength and wear resistance are needed, then Type 304 (18-8) is typically used

From a third source - just the conclusion of 1000 hr enviromental testing is copied here

CONCLUSIONS
All the observed corrosion on the stainless steel samples was surface corrosion
with no deep pitted corrosion. The electropolished samples showed no or very
little surface corrosion. The non-electropolished samples of 304 or 316 showed
about the same amount of surface corrosion whether they were passivated or
not. Throughout this test, the 304 and 316 stainless steel alloys showed very little
differences in the amount of corrosion, although several industry sources
recommend using only 316 or 316L stainless steel in marine environment
applications.

No opinions offered here. You'll have to decided what's best for you. I was comfortable with 304 for my chainplates

The expanded chainplates do seem a good idea. But the underlyinig trouble of rotted core should be investigated.
Here is the Port Bulkhead area we recored on Conundrum with a composite build up structure to replace the Dead Plywood Bulkhead. It sounded hollow and measured wet with the moisture meter after the deck was already repaired around the chainplate. Starboard side actually metered pretty dry.
Feathered the edge of the laminate and painted the whole bulkhead with Awlgrip matching the color. Better than new!

Hi Don,
We did a pretty through job of checking into the extent of wood damage on the boat. The main cause was due to lack of maintenance on the prior owners part to keep a fresh silicone seal between the chainplate and the deck. Therefore, that area of the deck went soft, and moisture got into the bulkhead. Funny how it always to be the port side tho. The combination of soaking the wood with Get Rot and epoxy without tearing the bulkhead apart seems to be successful after a year of pretty windy sailing.