Hello Jim,
There are some 400 series stainless steels that don't corrode in salt water. The best of the more common 300s is 316. Even that will corrode when oxygen deprived like under washers or where tight against something like under the keel. Sometimes stress corrosion happens when a fastening or plate is under load, screws or bolts crumble or the fitting will develop cracks where corrosion begins. Only low carbon 316 should be welded, normal 316 will rust immediately at the weld.
Is that cup welded on the skeg in your pic? Whether something stainless will corrode is unpredictable, sometimes it won't, even tho it should. Water running by a rudder shoe provides oxygen which may help the fitting survive longer. Out of the water s.s. creates a thin passive protective film in the presense of oxygen.
Stainless has its own problems under water by itself, unless an electrical contact with an annode is constant it will always corrode. In the presense of bronze (copper doesn't like to change its voltage, so when a current happens to it, it is the likely one to corrode or cause the corrosion depending on what it's coupling with) the galvanic corrosion is again unpredictable.
On 338, which had a brass shoe with a s.s. rudder shaft in it, it was the shoe that experienced the worst of the pitting. One would think the copper alloy more noble, except the brasses called bronzes have a load of zinc and lead alloyed in them. If an alloy is a little bit off, and the electric current is just so, I guess the alloy itself can corrode. The s.s rudder shaft in this case showed no obvious deterioration. It was not zinced - the shoe was.
Two very different alloys: s.s and bronze have absolutely no business together under water. That is asking for trouble.
Maybe to say they will create some bad business together. Since you can't isolate rudder shaft and shoe you have to zinc the separate pieces. But either the zincs wouldn't work or they'ld dissolve so fast the shoe or the rudder would quickly become unprotected. Even similar alloys (two bronzes or or two stainless) are different enough to begin exchanging little particles.
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