A past issue of Ocean Navigator had two articles pertaining to stainless steel keel bolts. One article described the sinking of a yacht en route from Bermuda to the U.S. due to loss of its keel due to stainless steel keel bolts. The other article described how a keel should be installed using stainless steel (A151 316) keel bolts.
In April of last year, I presented a paper to the New England Section of The Society of Naval Architects and Marine Engineers on this issue. I quote as follows from this paper: "Stainless steels are widely used in shipbuilding and often not used correctly. There is the 300 series, which is basically 18% chrome and 8% nickel. This material is non-magnetic, can be formed and worked, and is not heat treatable. Its corrosion resistance depends upon maintenance of a film of chromium oxide on the surface of the material; consequently, it should not be used in an oxygen-free atmosphere.
"Correct applications of this material include countertops, yacht rigging, and air system piping. Wrong applications of this material include diesel and gasoline exhaust piping. These fuel exhausts contain sulfurous acid (which forms from the combination of sulfur dioxide and water: SOO = H). Sulfurous acid is unstable and wants to absorb oxygen to become highly corrosive sulfuric acid (H).
"This material has been used for keel bolts on yachts and these have failed due to lack of oxygen.
"The 18-8 material must be stabilized by the addition of columbium (type 316) or titanium (type 347) if it is to be welded. If unstabilized 18-8 (303 or 304) is welded, some chromium combines with carbon in the weld metal and there is not enough chromium in the weld area to maintain the protective oxide film . . ."
The probable mechanism of failure of stainless steel keel bolts is that the oxide film breaks down in places. This permits intergranular corrosion to start and ultimately to proceed to the point where the keel bolt fails. This type of failure is referred to as chloride stress corrosion.
Keel bolt material should be silicon bronze. If this is not strong enough, K monel keel bolts should be used. K monel is stronger than stainless, but it is more expensive.
Russell W. Brown is a naval architect and engineer and an adjunct professor at the Virginia Polytechnic Institute. He lives in Niantic, Conn.