CD Structural Fasteners - Alert

[Dick Sisson]

I posted this below in the post about Typhoon Rudder Situation, but I think it bears posting separately. In the course of pulling off my easily accessible wood to varnish it, I pulled off the coaming boards which serve as supports for the winch pedestals.

This is a 1970 Typhoon, #167. The bronze (?) winch pedestals bolt to the coaming boards, with 2 bolts, and to the deck with 1 bolt. The 2 bolts thru the wood were fine, but both single bolts into the deck, that is, on both sides of the boat, were corroded to the extent of significant section loss.

They have the appearance of being markedly "necked down" in the portion of the bolt that was in the deck core. One is necked down to the point of being scary, just like a bolt looks after it has been tensile tested to destruction. The other is not as extreme, the threads are still mostly visble on this one. Eyeball measurement of these threads leads me to be fairly sure this is a case of corrosion loss as opposed to elongation from being over stressed.

I guess I will replace them with stainless steel bolts. Is it thought there is any problem with stainless steel fasteners used with a bronze fitting?

Is it plausible that this situation is being caused by "stray currents" from other boats in the marina?

Now I am nervous about the other structural fasteners, like for the fore and back stay fittings, and, perish the thought, the ones down inside there for the chainplates.

Much obliged for all comments, opinions and advice.



dickee03@rivnet.net

[Catherine Monaghan]

Dick,

Those fasteners should have been electrically bonded to the boat's grounding system. If they weren't, you'll need to do it to your new nuts/bolts. If you don't, they will fall victim to galvanic corrosion (if that was the cause) as well.

catherine_monaghan@merck.com
CD32 Realization
Raritan Bay

I posted this below in the post about Typhoon Rudder Situation, but I think it bears posting separately. In the course of pulling off my easily accessible wood to varnish it, I pulled off the coaming boards which serve as supports for the winch pedestals.

This is a 1970 Typhoon, #167. The bronze (?) winch pedestals bolt to the coaming boards, with 2 bolts, and to the deck with 1 bolt. The 2 bolts thru the wood were fine, but both single bolts into the deck, that is, on both sides of the boat, were corroded to the extent of significant section loss.

They have the appearance of being markedly "necked down" in the portion of the bolt that was in the deck core. One is necked down to the point of being scary, just like a bolt looks after it has been tensile tested to destruction. The other is not as extreme, the threads are still mostly visble on this one. Eyeball measurement of these threads leads me to be fairly sure this is a case of corrosion loss as opposed to elongation from being over stressed.

I guess I will replace them with stainless steel bolts. Is it thought there is any problem with stainless steel fasteners used with a bronze fitting?

Is it plausible that this situation is being caused by "stray currents" from other boats in the marina?

Now I am nervous about the other structural fasteners, like for the fore and back stay fittings, and, perish the thought, the ones down inside there for the chainplates.

Much obliged for all comments, opinions and advice.

catherine_monaghan@merck.com

[Russell]

[Olli Wendelin]

Dick,

Metal corrosion is primarily electrogalvanic. A "battery" is created with + and - poles. Below the waterline this is greatly increased by the saltwater electrolyte and possible stray currents from electrical sources.

Bonding of metal components above the waterline is primarily for lightning protection and elimination of sparking aroung fueling connections. Also salt water and spray on the deck can provide increased corrosion by introducing an electrolyte.

The situation you have described sounds like crevice corrosion. The ends of the bolt are in air, a good source of oxygen. The middle of the bolt is in an oxygen deficient area (the bolt hole). This creates a potential between the two areas, current flow, and corrosion. Add a little salt water in the hole and it works faster. Add the soaked deck core which will keep the area wet and it just gets worse.

Your winch base it bronze. Use bronze replacement bolts. Bronze corrodes slower than stainless steel. If you use stainless bolts with a bronze fitting, the stainless will act as a sacrificial anode. Just like the zinc on your prop shaft. The bolts will corrode faster.

Olli Wendelin
BLUE MOON
Charleston, SC



wendelin@spawar.navy.mil

[Jerry]

Your winch base it bronze. Use bronze replacement bolts. Bronze corrodes slower than stainless steel. If you use stainless bolts with a bronze fitting, the stainless will act as a sacrificial anode. Just like the zinc on your prop shaft. The bolts will corrode faster.

FWIW, the original bolts on my CD30 bronze winch base are stainless and are still OK after 15 years.

[john churchill]

my understanding is that crevice corrosion is primarily a stainless phenomenon, which relies on an oxidized surface layer. bronze does not. i do not have a good alternative explanation, however.

[Olli Wendelin]

All metals can be subject to crevice corrosion, it is just a matter of degree.



wendelin@spawar.navy.mil

[Dick Sisson]

They appear to be bronze, as does the pedestal.



dickee03@rivnet.net

[John R.]

Your winch base it bronze. Use bronze replacement bolts. Bronze corrodes slower than stainless steel. If you use stainless bolts with a bronze fitting, the stainless will act as a sacrificial anode. Just like the zinc on your prop shaft. The bolts will corrode faster.

FWIW, the original bolts on my CD30 bronze winch base are stainless and are still OK after 15 years.

Ditto on our '83 CD30. In fact, I've rebedded much of the deck hardware and all bronze fittings are fastened with stainless as are all bronze fittings on a '37 classic daysailer I have. All stainless bolts are in superb condition on both boats and many others I have worked on over the years. I've only witnessed stainless fastener corossion on fittings that are improperly bedded or are suffering from compromised bedding integrity due to age. I was inspecting chainplate bolts on a 34' ketch that had very minor weepage and found 3/8" diameter bolts had about 1/3 of their diameter corroded away. The bedding had broken down under the pan heads of the bolts and allowed seepage. Water seepage into the bolt hole area is the problem not the minimal contact areas of the bronze fitting and the stainless bolt head. Moisture in the substrate material itself will create acids that will attack stainless, bronze or other alloys. That can be witnessed in wooden boat hulls frequently, in particular near metal floors and keel bolts. To repeat what others have noted, stainless corrodes when its surface passivity is compromised. That is caused by lack of oxygen and the presence of moisture. If the bolt is well bedded preventing moisture intrusion you will not have any problems.