Bright, shiny stainless steel fixtures on sail or power boats — from cleats and fishing rod holders, to mast shrouds and winches — can really make a vessel “pop” and serve their purpose well if maintained properly.
Of course, boat owners have a more vested interest in caring for onboard stainless steel equipment, but boat charterers, particularly on voyages of more than a few days in a saltwater environment, should also know how to maintain and care for stainless steel fixtures because the material requires constant, but, fortunately, simple care.
A short and non-scientific discussion of stainless steel metallurgy, presented below, along with some tips on caring for stainless steel, can help effectively maintain stainless steel equipment, fixtures, and fittings on board a vessel.
Stainless steel is an amalgam of metals that contains more than 10% chromium. Marine grade stainless steel (known by its industry number, Type 316) also contains amounts of molybdenum, nickel, and nitrogen. All four elements work together to make stainless steel the metal of choice for a variety of applications in harsh marine environments, especially saltwater. Stainless steel resists corrosion (called pitting), maintains its strength at high temperatures, looks good, and is easily maintained. Stainless steel differs from carbon steel by the amount of chromium content in the metal.
While stainless steel is corrosion resistant, it is not corrosion proof. When exposed to harsh air and moisture conditions, especially in tropical climates and around saltwater, stainless steel corrodes or rusts (also called oxidation), which creates iron oxide. The chromium in stainless steel inhibits this process by producing its own oxide, chromium oxide, a rough, corrosion-resisting microscopic film on the steel surface that resists further oxidation. In freshwater environments, stainless steel corrosion still occurs, but at a much slower rate.
The electrolysis phenomena is an electro-chemical process that occurs when two dissimilar metals come in contact with each other in a saltwater environment. For example, when aluminum and stainless steel are in contact, the chloride in saltwater or salty air creates an electrical charge that, if not controlled, causes corrosion to the involved metal lower on the galvanic scale (in scientific terms, the less noble metal).
Cathodic protection is the most common technique used to inhibit corrosion on dissimilar metals. In its simplest form, the technique involves attaching a sacrificial anode, a material with a more negative electrode potential, like zinc, aluminum, or magnesium, which takes the brunt of the formed electrical charge. The sacrificial anode eventually corrodes away and needs replacing. Protection can also be provided by connecting an electric power supply to oppose corrosive galvanic currents. Boat manufacturers go to great lengths to design and build vessels that minimize the use of dissimilar metals that contact each other, but there are areas on a boat where the contact is unavoidable, like lower units.