Occasionally, we’re asked about the possibility of building a boat of stainless steel; the simple answer is: Don’t! This material has no place below the waterline on most boats including those built from non-metallic materials. The problem is shielding corrosion caused by oxygen starvation, which, in turn, will promote crevice corrosion. The important factor is the amount of oxygen in contact with the surface of the steel; one part of the steel must not be starved of oxygen while another part has it available. This phenomenon is known as the oxygen differential, and it will set up an electrochemical cell that will lead to rapid deterioration of the metal.

   Stainless steel is ideal, however, for deck fittings, chainplates, and stanchions. Stainless is also required as a liner in areas where dock and anchor lines would soon wear off the paint. On items such as stanchions, always paint 2 inches (50 mm) onto the stainless area to prevent galvanic action between any defects in the painted mild steel and the uncoated stainless fitting.

   The types of stainless steel most commonly used in boatbuilding fall into the 300 series, namely 302, 304, and 316. The 316 grade is considered the best for marine use and should be used wherever ultimate strength and freedom from corrosion are required.

When the quoted number is followed by the letter “L,” it indicates a low carbon content; this feature allows welds with good corrosion resistance by avoiding loss of chromium at the grain boundaries. The free-machining grades, type 303 or 303e, should never be used in seawater because they corrode. These specialized steels contain sulphate particles that facilitate the machining operation; however, the particles create a surface with numerous built-in alloys to particle galvanic cells. (See Chapter 12, Corrosion Prevention.)