The following is an excerpt from Brion Toss’s book “The Complete Rigger’s Apprentice – Tools and Techniques for Modern and Traditional Rigging” (International Marine; Camden, Maine 1998).  It's a good book, you should pick it up.  If you can't find it, let us know and we'll get you one.

"Corrosion is always a problem in a saltwater environment, particularly when you mix antagonistic materials such as aluminum and stainless or stainless and carbon fiber; these materials are on different points on the galvanic scale, and when joined by the conductive medium of salt water they set up an electrical potential.  The resulting activity corrodes whichever of the two materials is least noble (lower on the galvanic scale).  While not as serious with rigging as with permanently immersed items such as hull fastenings, galvanic corrosion can over time weaken aluminum and carbon fiber spars and clog such machinery as winches and blocks.  

"The first thing to do is to take note of places where dissimilar materials are in contact.  Then go about isolating them with some form of nonconductive bedding.  This can be as low-tech as parceling and serving to isolate galvanized steel wire from bronze thimbles, or it may involve 3M 101, Alumelast, or other polyurethane or plastic compounds to isolate aluminum or stainless fasteners.  When surveying an aluminum spar, pull a few fasteners and examine the screws and the holes they came from.  If you see a white powdery substance on either, you’ve got galvanic corrosion.  If the fasteners shear off when you try to pull them, you’ve got serious galvanic corrosion.  The most likely places for galvanic corrosion to occur are at the mast step, under sail tracts, and under winches.

"Stainless steel, the dominant material in rigging today, is susceptible to its own special form of decay: crevice corrosion, also known as oxygen starvation.  Stainless steel contains significant amounts of chromium.  When exposed to the atmosphere the surface oxidizes slightly and a thin film of chromium oxide forms, preventing any further oxidation.  If exposed to water, salt or fresh, without the presence of air, this film will not form and the metal will corrode.  If the water in question is salt water, the process is accelerated.

"You risk oxygen starvation anytime you cover stainless, as when applying spreader boots, shroud rollers, or service.  The trick is to exclude both water and air.  When serving, some anhydrous lanolin covered with proper parceling and service works fine.  Lanolin or mineral oil under shroud rollers is also good.  Just rinsing stainless with fresh water whenever you can will lessen the corrosive effects of salt water.

"When surveying for stainless corrosion, don’t be distracted by stains.  Contrary to what the name implies, the stuff does stain, mostly from bits of mild steel scraped off the extruding dies when the wire is formed.  But do look closely, preferably with a magnifying glass, for any sign of pitting in the metal – the surface will seem to have teeny-tiny craters in it.  Any significant pitting is cause for replacement.  When in doubt, proof-test, running the wire up to 50 percent of its rated strength on a testing machine.  If nothing breaks, it was a relatively cheap way to check.  If it breaks, you’ll feel very prudent for checking.  Proof-testing is a good idea after five to ten years in northern climates, or three to five in the tropics, even without evidence of pitting, just to make sure the wire hasn’t fatigued or suffered other damage. (Bo'sun Note: We usually recommend to customers that they seriously consider replacement of the rigging within these timeframes.  Proof-testing is not always possible or practical. Better to be safe than sorry!)  

"Just how frayed, rusty, or old, or whatever, must a piece be for it to be condemned?  There are plenty of people out there who can point to a battered but still-functioning wreck and tell you that it’s held up fine and they’d still trust it in a gale.  Gear will sometimes hold together far longer than anyone could reasonably expect.  But the point is to have, not a long-lived rig, but a safe long-lived rig.  Why incorporate a safety factor in a rig design, only to erode it away?  In view of the possible consequences of gear failure, it seems foolhardy to go out with anything but the strongest, best-conditioned rig that is compatible with performance and your purse.  Watch, understand, and respond as if it were an instinctive feeling of your inner self.  

"Sometimes metal just plain rusts.  Stainless steel rusts more slowly, but tropical climates will get to it in just a few years.  Galvanized steel left untended can dissolve in a matter of months.  Any survey of metal must be a survey for rust. ..."