From what I've read, even high quality stainless (ie 316) needs oxygen to be effective. I suspect in dhornsey's example, a lower grade stainless was used. Be interesting to know if those peices are slightly magnetic.
Also that shackle pin is under a fair amount of tension and movement (although slight) through wave action on the bob stay and sail pressures through the forestay. Pin could have developed a slight crack and the water did the rest.