pintle & gudgeon ss bolt failures

Posted By: Bob Pr. S2 6.7 #81, "The TARDIS", Topeka, KS
Date: 8/22/04 12:29p.m.

Report: outside forum guidelines

In the 7 years we've been a somewhat organized group, I'm aware of about 5-7 of our boats which have had rudder failures due to failures of the ss bolts attaching the pintles to aluminum cheeks of the rudderhead or the similar bolts which attach the gudgeons to the transom. A few years ago, through a fortunate meeting, I began e-mailing Dr. Harvey Hack, a metallurgical scientist in Northrop-Grumman's marine division, and he gave us a consult on failures of bolts attaching our pintles and gudgeons. All that was posted on this BB and I wish I could refer you to those posts. Unfortunately, when our BB was transferred from one company to another, about 6 months out of 7 years of posts were lost. Dr. Hack's comments were in that 6 months. As best I recall from Dr. Hack's explanation (which was far more elegant as well as surely more accurate) the contact between the aluminum (Al) and the stainless steel (ss) creates galvanic corrosion due to the induction of currents because of the dissimiliarity of metals in the "table of nobility." The farther apart the metals are in this table, the greater the galvanic current and the sacrifice (corrosion) of the less noble metal. Aluminum and ss are fairly far apart and the aluminum is the less noble. The presence of water -- especially salt water -- speeds the process. These galvanic currents will be conducted throughout any metals in contact with each other so the bolts attaching gudgeons to the transom can be affected as can those attaching the pintles to the Al cheekplates. The galvanic corrosion produces a distentegration/sacrifice of the aluminum (forming a white powder) and the effect of this generation of white powder is frequently to increase the "sealing off" of portions of the ss bolt from oxygen -- the ss bolt becomes oxygen starved. Whenever ss is starved of oxygen, from whatever cause, crevice corrosion can rapidly occur which turns the ss into something increasingly resembling crumbly Swiss cheese but without the flavor. When it's become sufficiently "holy", failure occurs. Dr. Hack said the effects are somewhat capricious and he knows of no way of predicting why, say, one pintle bolt might fail before another pintle bolt in contact with the Al would. (All the ss bolts holding the Al cheeks together are also subject to this but since there are so many, they're far less critical than those attaching pintles and gudgeons.) Boats that sail in saltwater will be at greatly increased risk for these failures -- (BUT we've had some similar failures occur on freshwater boats, too). Dr. Hack recommended either of two solutions: #1- replace critical bolts of ss with some made of titanium. Since exposure to oxygen is not as critical for titanium among other reasons, there will be much less possibility of crevice corrosion. (I've looked for sources of titanium bolts of the size needed but haven't found any. If someone can point me to a source, I'd appreciate it. Wouldn't that be something worthwhile for our class to stock and sell?) #2 - examine and/or replace critical ss bolts every 3-4 years -- but certainly more often the more saltwater contact there is. Since I don't have Dr. Hack's explanation to read and refresh my memory, I can't firmly say whether gudgeon bolt failure is due to the crevice corrosion since there's no Al in immediate contact with it. But I suspect it is because it's in the circuit and it would probably be easy for a portion of a gudgeon bolt to be deprived of oxygen. Any errors in the above are due to my memory functions and not to Dr. Hack's very generous explanations. Here are two of his articles covering much of this area: http://members.shaw.ca/diesel-duck/library/tech_notes/corrosion.htm http://www.diveweb.com/maritech/features/uw-su99.01.htm