The marine environment is tough on metal hardware. Steel rusts. Aluminum pits. Even shiny chrome finishes aren't immune to the corrosive elements of salt and water. For many years, bronze was the preferred choice for deck hardware, and it's still the best option for underwater fittings such as through-hulls and sea strainers.
Above the waterline, stainless steel is the metal of choice. In addition to having a bright finish, it's strong, relatively light and corrosion-resistant. Several alloys are known as stainless steel. The lowest-quality alloy, known as 430, is not suitable for marine use. If in doubt, grab a kitchen magnet; 430 stainless is magnetic, while higher grades are not.
The two stainless alloys considered marine-grade are 304 and 316. The 304 stainless steel is typically stronger and more often used for hardware such as screws and bolts. It won't rust, but it will tarnish, and it may need occasional cleaning. The 316 stainless has greater corrosion resistance and will maintain a brighter finish. It's more often used for cleats, rails and other visible hardware.
Stainless Will Stain
Stainless steel may seem like a magical metal that is impervious to the elements, but in reality it will show small amounts of surface corrosion over time. All that’s usually needed to restore the luster is an easy cleaning with a mild metal polish or a lemon peel.
The secret to this metal’s lasting luster is a molecular-thin oxide surface coat that’s restored by contact with oxygen. There’s plenty of oxygen in the air, and even stainless submerged in water will scavenge dissolved oxygen — though the chlorine in salt water will eventually cause problems.
Stainless really begins to develop problems when it’s enveloped in materials that retain moisture but don’t allow the free exchange of oxygen. A prime example would be a stainless bolt running through a cored deck. If left unchecked for a period of years, this bolt would deteriorate through a process known as crevice corrosion.
Preventing Crevice Corrosion
The solution to reducing or eliminating crevice corrosion isn’t to substitute bronze bolts. The mixing of bronze and stainless steel — which are dissimilar metals — would create devastating galvanic corrosion. Aluminum and stainless are more compatible, but if possible, plastic or composite isolation washers should still be used between these two metals to prevent galvanic action.
When using stainless fastening hardware, make every effort to ensure that holes and cavities are sealed against water intrusion, and bed the attachment hardware in caulk when installing. Even when properly caulked and sealed, stainless-steel fittings that are embedded in the deck or installed in poorly aerated places should be removed from time to time and inspected for corrosion or cracking.