Anyone make 12" keel rollers that do NOT have steel sleeves inside?

Stainless needs oxygen to oxidize the protective coating that makes it "rust" resistant. This is why you don't use stainless fitting below the water line.

http://www.amteccorrosion.co.uk/papers/corrosionguide.html

Crevice Corrosion
Intense localised corrosion, ranging from small pits to extensive attack over the whole surface, can occur within narrow crevices formed by the geometry of a structure, for example: riveted plates or threaded joints. Crevice corrosion is characterised by a geometrical configuration in which the cathode reactant, oxygen, can readily gain access to the metal surface outside the crevice and less access to the crevice. The metal within the crevice is therefore anodic to the surrounding steel and suffers preferential corrosion.



Figure 1-6.


The crevice can be caused by any material & crevice corrosion is commonly found under insulation gaskets.When mud, poorly adherent coating, sand and other debris cover a passive surface it undergoes a similar type of attack to that occurring in crevices. Wherever loose debris collect, there will be a depletion of oxygen in a crevice. Consequently, the attack is localised there.​