Yakaholic, your experience is very helpful. The galvanic cell involving the xbar SS Soc Caps/aluminum brace/water (saltwater when sailing on the ocean) is really serious because it is difficult to wash clean and probably does not dry rapidly. It is a particularly serious type of corrosion--actually has a name, crevice corrosion.
Well, its worse than you think
Beneath the rear aka brace on either side is not 2 but 3
threaded brass insert "holes" in the hull! You only use the outer 2 - I don't know what the center brass hole is for - unless for a sidekick mount (Adventure kayak mode?) Anyway, what this means is that there is a brass hole or "cup" to hold salt water under the rear aka x-brace creating a permanent battery hidden away under it.
Once salt water gets into this hole there is no way to wash it out. Washing it with a hose is just not likely to reach this hidden cavity.
This salt water "cell" of brass+steel+aluminum = an amazing mess under the xbrace.
I will have to pull the annodized xbraces to see for sure - but they have none of the telltale "crevice corrosion". Based on my experience and observation the annodized metal should have been kept.
PS - I'm not sure is there are 3 holes per side on the forward x-bar until I pull it.
PPS - It may be that my 2007 Island hull is unique with 3 brass holes per side on the rear x-bar mount, but that seems unlikely.