I did not say titanium was not prone to crevice corrosion. In fact crevice corrosion is the corrosion that these alloys will fail through. I did say say that for the marine environment, corrosion is not typically the main concern for titanium alloys (crevice corrosion or any other form of corrosion). And yes, the common Ti alloys are superior to the commonly used stainless steel alloys with respect to corrosion in the marine environment. There are many other stainless steels not used often in the marine environment that are much better than the commonly used austenitics depending upon where they are being used.
Corrosion is not the single mechanism for failure. If it were, then the Ti alloys would certainly be hands down superior. In reality, Ti6Al4V has other failure mechanisms that are not commonly found in the austenitic stainless steels.
The use of Ti6Al4V - the real only commercially available Ti based structural alloy, there are others, but they are obscure - has two aspects that the commonly used stainless steels do not share and that is notch sensitivity and the inability to withstand contact motion.
Additionally, the marine industry has been using the common austenitic stainless steels - most notably 316 (and it's close brother 316L) in less than desirable applications. These have always been known to not be rated for submersed application and yet we use them all the time that way. So of course we are going to find examples where they have failed and blame it on the alloy. Well, if an alloy is used in a way it is not supposed to be used, it will fail.
The description you give above is not an inside-out mechanism as depicted in the video I mentioned. It is an outside-in mechanism. It is an insidious mechanism and difficult to see although to the trained eye not so much. In your above to images, I'm not sure what the left hand image is, but the right hand image looks like a keel bolt with a likely SCC crack showing.
What the aerospace industry does, that we do not do in the marine environment, is frequent inspections. So in your keel bolt example above I'm sure that the nut was never taken off that bolt to inspect the bolt prior to there being a visible concern and then it was found what you've photographed - likely many years after initial service. I wonder if that same corrosion would have occurred if there had been an annual or even a bi or tri annual inspection where the nut was removed and the bolt was inspected. That process in itself likely would have eliminated that failure. The aerospace industry identifies known critical components and points of failure and then establishes an inspection method and frequency where those points are inspected at defined usage intervals. We don't do that with our sailboats. Nor do I propose we do,but I do support that alloy selection be done with greater scrutiny.
dj
p.s. While the Ti6Al4V is stronger than the 316, there are other common stainless steels stronger than the Ti6Al4V. Some a lot stronger.
. I was told at the time that it was the only source available. Could have been a $$ issue too. While Allied Titanium says "We manufacture in America" that does not mean their raw materiel doesn't come from somewhere else like Russia or China and you could be the victim of supply chain issues (even before the Ukranian war)
