Dear PassageMaker Magazine:
"I have rust on the screws holding the stainless steel railings to the deck of my boat. The boat is 4 months old and I assumed that stainless screws were used to hold the railings to the boat deck to prevent rusting. The factory has already replaced a hinge on the rode-box cover due to rusting. What seems to be the problem? Cheap screws?"
Thanks for your note and query. As a variation of the old axiom goes, there are screws and there are screws. Most ordinary stainless fasteners used in the recreational marine industry are made of an alloy known as 18-8, which means it contains 18% chromium and 8% nickel. This is a good , all around alloy for above the waterline marine use, however, it does have the propensity to stain brown, particularly in on deck applications where it frequently becomes wet with seawater. If it's used in an application where water is allowed to remain in contact with it, in the recess of a cleat fastener well for instance, the staining will be exacerbated. It, like many stainless steel alloys, is also susceptible to crevice corrosion (look for a profile of this problem in an upcoming What's Wrong With This Picture column) if it's used in a wet, oxygen depleted environment. This frequently occurs below and sometimes above the waterline (in wet core material or even if water becomes trapped in the hole in the deck through which a fastener passes). The staining that occurs in 304 stainless fasteners used on deck is typically a cosmetic rather than a structural issue. Severe staining, however, may be indicative of crevice corrosion or other structural issues that require further investigation.
Some builders and yards, including me, specify 316 stainless fasteners for on deck applications in order to avoid the unsightly staining. 316 is one of the most corrosion resistant marine stainless alloys thanks to the addition of 2-3% molybdenum. Although not impervious to staining, it's much less likely to occur in this alloy. Polishing any stainless alloy (on a polishing wheel) will harden its surface and remove micro cracks and pits, which will further reduce its tendency to stain or corrode. As you might imagine, 316 stainless steel fasteners are more expensive and not as common as their 18-8 brethren.They can, however, be ordered from dedicated fastener suppliers, a number of which are accessible through any internet search engine.