Marine stainless steel fasteners

Any damp areas where stainless steel fasteners are deprived of  oxygen, such as below-the-waterline wooden-boat plank fastenings (bronze, or galvanized steel, screws should always be used).

(Note that if a boat is moored in a protected harbor, or in a canal where there is little or no current, it is a good idea to move the boat at least once a week to ensure that there is a fresh supply of oxygen-laden water in contact with any underwater stainless steel hardware.)

Sailboats are vulnerable in a number of additional locations:

• Inside the lower terminals of swaged-on rigging fittings, which tend to collect water, and inside barrel-type turnbuckles. In both instances corrosion is invisible and the first sign of trouble is likely to be a rigging failure.

• Inside centerboard trunks-particularly around hinge pins-where the water is stagnant and oxygen levels are depleted.

• Wherever stainless steel fasteners pass through the hull, deck, or spreaders-if any moisture becomes trapped in the hole the moisture will eventually become deoxygenated, causing the stainless to turn active, often with no external evidence

(through-deck chainplates are particularly vulnerable since the flexing of the rig almost always eventually breaks down any caulking seal, allowing moisture to penetrate).

316 stainless steel fasteners

Besides using 316 stainless steel or some other specialized alloy, you can enhance corrosion resistance by polishing the surface of the metal (either electrochemically or mechanically) and by ensuring that if a fastener must be used in an area where moisture may penetrate, the fastener is properly bedded in a completely waterproof sealant (5200, for example, but not silicon, which is minutely porous).

Despite these measures, 316 stainless steel, despite its popularity, is not the best choice in many instances for applications in which it is commonly used.

(Note that even where pitting and crevice corrosion are not a problem, austenitic stainless steels in certain environments are subject to stress corrosion cracking and corrosion fatigue, both of which occur at the microscopic level, making problems almost impossible to detect

until a sudden failure occurs).

Finally, it should be noted that when many stainless steels are welded, the differential heating of the metal in the area of the weld causes the chromium to combine with carbon in the steel, removing the chromium from its passivating film-forming role.

These areas become anodic with respect to the rest of the stainless steel, leading to galvanic corrosion that goes under the name of weld decay, or intergranular corrosion (particularly noticeable around the welds on many stainless steel fuel and water tanks, which soon develop leaks).

On small welded pieces intergranular corrosion can be avoided by heating the entire piece, after welding, to around 2,000°F (600°C), and then rapidly cooling it. For larger structures where this is not practicable a low-carbon-content stainless steel should be used (identified by the letter "L' after its number, e.g., 304L or preferably 316L) for all welded structures.