galvanic corrosion boom/mast fittings
Dear All, Looking for best solutions to repair the galvanic corrosion shown in the photos below. Have had everything from a Kevlar collar, to cutting and soldering a new piece of aluminum, to riveting an aluminum collar suggested, but would appreciate all of your experiences in regard to this issue, and of course Lars your advise is always precious. Please remember that just taking it all down and replacing it is not one of the options in my little world...Thanks in advance to all. (and Lars have not forgotten the photos of the spreader, but the weather was just too bad before. will be back on board next week and will get this done stat. Cheers, Jayne & Aorangi 47/047

I am afraid but my advice is only one: undo the fastenings to check under them and anything similar to what you see in the first picture should be fixed the way you can see in the other pictures. With a good new insulating material (PTFE for example) between stainless steel and aluminum, and Barium Chromate compound on the screws.
I had it done few years ago and I do not regret it.
Our boats are worth it and 30 years are an awful long time for aluminum close to SS and exposed to weather.

Daniele, 411/004.

Dear Jayne,

had the same problem on Only You spreaders attachment, and after having dismantled the stainless steel plates, the mast had been cleaned, then a new alluminum plate below the stainless steel attachmen had been welded, look at the pics!

Fiar winds

Matteo (38/067 Only You)

Jayne

Basically the same as already proposed, some details added.

Remove the stainless fittings, and clean the area behind them thoroughly. Cover the back side of the fittings with thick electrician's insulating tape, bed the fittings down in Duralac or Sikaflex applied to the entire surface when putting them back, taking care that there are no voids between fitting and mast wall where water can get in and stand. Duralac also provides corrosion protection, Sikaflex only keeps water out. The edge of the bedding should be visible around the circumference of the fitting, and could be nicely radiused.

Screws need to be secured, particularly in the rig. The recommendation is to use Loctite thread glue, different strengths available. This will also prevent water from getting into the threads. Also put some Sikaflex under the screw heads to keep water out.

If threads in aluminium have been damaged Helicoil inserts can be used to restore them. It is advisable to put one of the products mentioned here into the Helicoil threads to keep water out.

Lars

For stainless screws in aluminium which need to be opened now and then there is a product called Tef-Gel, which prevents corrosion by keeping the water out, and works as a lubricant.

Thank you to everyone! seems the approach to resolving this is unanimous so that's that. Lars, was happy to hear you mention Tef-gel as have been using it but in place of Duralac. You mention both, so just wondering if there is a reason. Have both, but stopped using Duralac as was advised by a rigger that Tef-gel was better long term. Your thoughts? thanks again, and will post results after completion of repairs. Cheers, Jayne 47/047

Jayne

The difference is that Duralac has galvanic protection properties, i.e. works like a zinc for the metals in contact with it, while Tef-Gel keeps water away from the surfaces and provides lubrication. Both prevent corrosion.

Duralac is suitable for protecting aluminium in contact with stainless, also when the Duralac layer for practical reasons is not fully intact nor filling threads completely. The goal should be a complete fill.

Tef-Gel has to cover the whole surface to be protected, and there is usually a small brush provided for working the stuff into threads. The amount applied to threads has to fill the slots between internal and external threads.

For stainless against stainless threads Tef-Gel is preferable

Lars

Dear All,

   When I was fitting a new gooseneck to my "41" mast, I was recommended by an experienced rigger to always use pop rivets in preference to stainless screws when attaching fittings to an aluminium mast. Monel pop rivets apparently do not induce so much galvanic corrosion as stainless screws or rivets. They also do less damage to the mast as there is no thread to be cut and later to corrode away. The fitting can be removed by drilling off the rivet heads and punching the body of the rivet through. It can then be replaced with new rivets using the same holes.

 Durulac is barium chromate, and is a strong oxidising agent. This helps keep the essential oxide layer on the aluminium. It should be used liberally between the fitting and the mast. The pop rivets should be dipped in it before fitting. 

regards Cosmo Little

Hi Cosmo Little,
I agree with you that monel rivets are more friendly than stainless steel screws to aluminum but I believe that they are fit only if the major component of the stress is toward the contact surface, like in the case of the boom gooseneck.
Are they fit also when the major stress is tangential?
I ask because I am just going to replace a mast winch which is fastened by 5 stainless steel M6 countersunk screws and never considered changing the kind of fastening.

Daniel, 411-004

Dear Daniel,

  I don't know what stresses pop rivets can take. I used 1/4" (6mm) on the gooseneck. These had to be pulled up using a special power tool, and certainly seemed very strong. On modern spars the cast alloy plates that carry the winches are certainly secured with pop rivets, however I think that the winch is then usually bolted to the plate.

regards Cosmo

Dear Daniel, Regarding mast winch: in July we installed a 46winch on the mast of Aorangi with Monel rivets and using Tef-gel on the bolts (which in the meantime had to be removed for repair work), and have to say that to date we have had no issues. We installed a mounting plate of course, but any "play" so far is nonexistent. It is the mainsail winch fyi.
PS: Thanks for you input!
Cheers,
Jayne
Aorangi 47/047

correction on last post: not bolts but stainless steel screws. sorry about that. J.

Jayne,
rivets for the mounting plate and countersunk s/s screw for mounting the winch to the plate. Is this what you meant?
In my case there is an aluminum mounting plate solidly soldered to the mast so my question regarded only the fixing of the winch.
Lewmar does not consider rivets in their instructions but I am willing to learn new things.
My winch is also for the main halyard: I am installing a self tailing of same brand and size but, not surprisingly, the places of the mounting screws are all different.

Daniel, 411/004

Dear all,

The pictures included in this thread are quite scary and make me wander if this is something we should all be worried about? Someone told me that this sort of damage is less likely to occur in the Northern parts of Europe, due to all the rain we get during the summer. The theory being that the fresh water washes the salt away. (If so, at least it's good for someting, but I'm not convinced, as the problem is not rust.) Is is required to remove the mast fittings in order to make a proper inspection? My mast looks to be ok, but of course, I cannot see under the fitting.

Grateful for comment.

Kind regards,

Terje.

Hi Daniel, you are absolutely right. we installed the mounting plate with the monel rivets and the winch with imbedded screws. While I am certainly not the right person to give suggestions on your situation, I feel that there must be a similar solution using the rivets for installation of the winch as well. makes a real difference when you have to do any maintenance. please post your results if possible, thanks.
Jayne Aorangi 47/047

Terje,
I do not think there is anything to be scared of but almost anything must be checked once in a while! Especially in a marine environment. Regarding galvanic corrosion, for example, usually a careful visual inspection is sufficient because most of the serious problems, even if well hidden, often display tiny perceivable signals: swelling, whitish powder, oozing, cracks in the seals, etc.
I would advise to check periodically all parts where different metals are in contact, replace the insulating sheets whenever necessary and apply duralac or tef-gel.

Jayne,
thank you for your kind post: I was indeed curious of your experience but your reply confirms that countersunk screws are the right choice. The winch base fastening holes are indeed bored for this kind of assembly and the heads of standard rivets would not fit right.

Daniel, 411/004

Dear All, I am thrilled to say that the repairs on the boom of Aorangi have gone very well, but not without surprises!!! The damage was far less than what appeared on the edges thank heavens. (see Terje there's hope!) and the new plate seems to be ready for the next 30 yrs. at least. A real work of art; and spending the entire day watching all of this happen made things an incredible discovery! for better and for worse...Upon removing the sheet plate for the main the screws were seized in a way that drilling them out was necessary. and in doing so two of the welded nuts inside the boom the hold the counterplate in stainless inside the boom fell off. so there were two empty holes that had no more threading. Here is where I would like Lars to intervene if possible... It was decided to recreate the support running aluminum welding into the holes and then rebuilding the threads. (the same was done on the boomvang screw holes in precedence (photo attached). Everything seems to be ok, but somehow having two screws out of 6 on one side (and in line of course) missing the welded nuts inside makes me nervous. Have thought of putting a spectra security tie on the shackle attached to this fixture and running it around the boom just in case, but if there is anyone who has input on this will be happy to listen and learn. thanks for all the help to date, it's been precious in getting this done well. Jayne & Aorangi 47/047