Duralac, aluminium, and stainless.

[sarabande]

I am now going for ali backing plates (see our Wooden Boat thread http://www.ybw.com/forums/showthread.php?396024-making-plywood-backing-plates-waterproof)


I can't see how Duralac acts as a corrosion inhibitor. With an ali backing plate, when you tighten down the nuts on the big washers, doesn't it all squeeze out from between washer and plate, allowing intimate contact between ali and steel, and thus allowing a potential for dissimilar metals corrosion ?

These backing plates are a once and only excursion into difficult recesses, so I really want to get them right.


Re-assurance and expert knowledge sought, please !

 

[vyv_cox]

There is a reasonably good explanation of its composition and method of working at http://cassellmarine.mobile034.com/p/337808/duralac---anti-corrosive-joining-compound.html

However, having read your thread on the Wooden boat forum, I see that nobody has mentioned my favourite for this type of job, Tufnol. Considerably stronger than plywood and possibly aluminium, impermeable and not corrodable, easy to work with. I mounted my Yacht Legs hull fittings on Tufnol pads nearly 20 years ago, undisturbed since, still in perfect condition. Not expensive, googling brings up many suppliers.

 

[oldsaltoz]

Spot on as usual Vyv,

I have also seen washers made of some sort of plastic, the shape of a top hat to avoid contact on the bolt head side of the fastenings.

Not sure if are available off the shelf, but would be a simple task to turn a few out on a lath

Good luck and fair winds.

 

[oldbilbo]

It's my understanding, from observing mil aviation practice over some years, that only a thin application with a brush is needed - and not a thick 'bed' which one might apply when using Sikaflex, CT1, or thickened epoxy 'bog'.

Certainly Tufnol is an excellent material for this task, together with some other enginering plastic sheet materials one may have lying around, and so also is preformed GRP sheet of 6mm - 12mm should there be some to hand, or recycled from a GRP moulders' scrapheap.

 

[rogerthebodger]

Most types of plastic can be used. Nylon washers and the top hat sections as Oldsaltoz are available from RS. Plastic cutting boards or my current usage is sheet ABS but I have also used HDPE.

I used lots of Duralac when fixing aluminium GEBO windows with stainless screws and have no sign of corrosion and on my mast used stainless plates isolated from the al mast with plastic shim stock where I needed to ensure the easy removal if item like all mast head fittings (nav lights, wind transducer, VHF antenna) but where I did not use Duralac stainless fittings corroded in with a year and had to be drilled out.

IMHO Duralac has some chemical way of preventing corrosion not just excluding moisture.

 

[sarabande]

ah, Tufnol, of blessed memory...


Just looked on ebay. Is "Phenolic" (touted as a Tufnol substitute) a genuine alternative ? It seems relatively cheap....

 

[rogerthebodger]

Tufnol is a trade name, Phenolic resin is one of the components.

 

[sarabande]

sorry, I forgot the link about phenolic ...

http://www.ebay.co.uk/sch/i.html?_t...+sheets&_nkw=tufnol+sheets&_sacat=0&_from=R40

 

[Heckler]

I am now going for ali backing plates (see our Wooden Boat thread http://www.ybw.com/forums/showthread.php?396024-making-plywood-backing-plates-waterproof)


I can't see how Duralac acts as a corrosion inhibitor. With an ali backing plate, when you tighten down the nuts on the big washers, doesn't it all squeeze out from between washer and plate, allowing intimate contact between ali and steel, and thus allowing a potential for dissimilar metals corrosion ?

These backing plates are a once and only excursion into difficult recesses, so I really want to get them right.


Re-assurance and expert knowledge sought, please !

Ive always used Durulac BUT having just overhauled my mast and having had to remove spreader SS pins which were fitted with The stuff, I am having second thoughts. The oil that it is liquified with appears to be linseed oil. It hardens or oxidises and forms a hard almost glue like substance! It took a two legged puller, a propane torch and a 2lb ball pein hammer to shift the barstewards! I put them back with a slathering of Marine grease!
Stu

 

[doug748]

It would be fine. A good plan. On this size of boat any of the proposed schedules of work would do the business.

However, just to complete your misery.

I would follow your first idea and epoxy in ply backing pads which are compatible with the original structure, would enhance quiddity and spread the loads in a kind manner. You could then cut and drill stainless strap/plate to mate with the through bolts and complete the job.

 

[J Neeves]

For backing plates: We have made up sheets of 4 - 10mm fibre glass. Simply laid up on a piece of glass (to give one flat surface). The thin sheets are in case we need to fix a hole in a rounded surface - the thicker ones for backing plates. We made them up in one lot and cut them as needed.

We have glassed in marine ply. We have also used, real belt and braces, glassed in marine ply (over the original reinforcing that was insufficient as it looked stressed) and then 4mm aluminium plates. We used Sika between the aluminium and glassed in marine ply and where the bolts exited (or entered) the hull We used Duralac for the nuts and washers where they passed through the aluminium. Like yours the access is limited, its for the kicker wires that hold down the prodder, right up in the bottom of each bow. No sign of any issues after 10 years, or so (I check annually)

Is there an alternative to Duralac?

Jonathan

 

[alahol2]

Is there an alternative to Duralac?

There's green Duralac, but I've never yet seen any on sale. http://www.sanshin.co.uk/Pdfs/DuralacGreen.pdf

 

[Greenheart]

I bought Duralac for isolating the replacement rivets on my spreaders. It wasn't cheap, so I hope it stays good for years in the tube once opened - I'll only need a tiny amount. I'd also wondered how even a substantial blob remains in place between the rivet and the spreader when the riveter applies pressure...some contact of the metals seems inevitable.

I heard it's very toxic stuff.

 

[J Neeves]

Its based on Barium Chloride (historically it came from China - I used to know the importer for the UK) another application is as a rat poison.

Jonathan

 

[VicS]

Its based on Barium Chloride (historically it came from China - I used to know the importer for the UK) another application is as a rat poison.

Jonathan

Duralac contains barium chromate

Barium compounds are generally toxic but barium chromate has a very low solubility and is not as toxic as other more soluble salts. Only classed as harmful ITYWF

Surprised to learn that it is used as a rat poison!

 

[vyv_cox]

Duralac contains barium chromate

I know we have had this conversation before, Vic. All the aircraft applications I find for duralac say it is zinc chromate. I know that when I built a mast for an Enterprise something like 40 years ago the duralac supplied was definitely zinc chromate. So when did it change from zinc to barium?

 

[rogerthebodger]

Data sheet from the makers

http://www.llewellyn-ryland.co.uk/downloads/duralac.pdf

 

[J Neeves]

I was working on memory cells from the early 80's - obviously got my cells crossed

Jonathan

 

[VicS]

I know we have had this conversation before, Vic. All the aircraft applications I find for duralac say it is zinc chromate. I know that when I built a mast for an Enterprise something like 40 years ago the duralac supplied was definitely zinc chromate. So when did it change from zinc to barium?

I dont know, when or why.

Lower toxicity may be the reason. Lower solubility may be the reason OTOH its very low solubility may make it less effective as a corrosion inhibitor.

I wish Id known 15 years or so ago 'cos I faffed about making some zinc chromate and at about the same time disposed of a jar of barium chromate!
Vic's homemade anti-corrosion paste is zinc chromate and linseed oil!

 

[J Neeves]

So its no good looking to you for a, cheap, replacement for Duralac

I've just looked up my Merck Index, its dated 1968, Barium Chloride does not sound very nice but Barium Chromate has a use:as a pigment almost entirely in anticorrosive jointing pastes (zinc chromate is used as a pigment - no mention of anti corrosive pastes).

Jonathan