Galvanic corrosion, Monel or other material?

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G12

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Hi everyone,

I have a question for those of you with knowlege of Metallurgy or Chemistry.

I want to attach a halyard clutch to my mast using M5 machine screws. I do not want to use stainless steel for this owing to the corrosion issues arising.

My question is - What material would be best? I am currently looking at Monel or Monel K500. There is a place in Yorkshire that would appear to manufacture things like that but before I ask them I would like to research on exactly what would be best to have made.

All input greatly received as always.
 
G12 said:
Hi everyone,

I have a question for those of you with knowlege of Metallurgy or Chemistry.

I want to attach a halyard clutch to my mast using M5 machine screws. I do not want to use stainless steel for this owing to the corrosion issues arising.

My question is - What material would be best? I am currently looking at Monel or Monel K500. There is a place in Yorkshire that would appear to manufacture things like that but before I ask them I would like to research on exactly what would be best to have made.

All input greatly received as always.
Click to expand...

You can use stainless screws providing you coat everything with Duralac.
 
Thanks, but Duralac doesn't work well enough on threads for my liking. I want to be able to remove the clutch when the time comes without having to re-tap the mast and beat the living daylights out of it with an impact driver.
Duralac is good however under winch pads etc where it is able to form a proper barrier between the materials, used it for years for that.
 
30boat said:
You can use stainless screws providing you coat everything with Duralac.
Click to expand...

To avoid galvanic corrosion altogether use aluminium bolts! :D

Monel has excellent corrosion resistance itself but its position in the galvanic series in seawater is very little different from the 300 series stainless steels. The problem is not the bolt corrosion, but the aluminium of the mast. So 30boat is quite correct, use a good sealant and inhibitor such as Duralac.
 
OK so howcome all the monel rivets work well on my mast without corroding? (That is how I arrived at my original choice of material).

I would like to use aluminium screws but was worried that it'd be unable to take the load.

As for the Duralac issue, I have always found it made next to no difference on threaded areas. As I said before, it does exactly what it says on the tin where it is able to form a decent barrier though. It also goes hard, again, something I wish to avoid on a threaded area. I do intend to use a threadlock compound on it but one I will be able to pick the strength of.

Thanks for the input so far, learning is what this thread is about.

So, Monel bad, Aluminium good. Are we now looking at an aluminium alloy of some description then or is there something else lurking out there that might be good? What about titanium? I have just had a quick google and found it is somewhere close to Aluminium in the reactivity series. Anyone know what is it like when exposed to sheer stresses?

EDIT: More extensive google reveals I was looking at the wrong data anyway. http://www.tifab.com/pdf/Galvanic Series.pdf seems to be the right kind of data. So, based on previous PDF, in the words of Doc Emmet Brown "If my calculations are correct......." then if I were to use titanium then I would be setting up a tasty little cell for galvanic corrosion or am I being a donkey again?
 
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G12 said:
OK so howcome all the monel rivets work well on my mast without corroding? (That is how I arrived at my original choice of material).

I would like to use aluminium screws but was worried that it'd be unable to take the load.

As for the Duralac issue, I have always found it made next to no difference on threaded areas. As I said before, it does exactly what it says on the tin where it is able to form a decent barrier though. It also goes hard, again, something I wish to avoid on a threaded area. I do intend to use a threadlock compound on it but one I will be able to pick the strength of.

Thanks for the input so far, learning is what this thread is about.

So, Monel bad, Aluminium good. Are we now looking at an aluminium alloy of some description then or is there something else lurking out there that might be good? What about titanium? I have just had a quick google and found it is somewhere close to Aluminium in the reactivity series. Anyone know what is it like when exposed to sheer stresses?
Click to expand...

Read my post again. There is absolutely nothing wrong with Monel, nor stainless steel. The problem with galvanic corrosion is that the more anodic metal, in this case aluminium, corrodes preferentially trying to protect the more noble metal, the bolts. In theory a big anode protecting a small cathode is a good thing, so the whole mast would slowly corrode and leave the bolts perfect. In reality the aluminium corrodes first around the bolts, making them difficult to remove without bringing lots of aluminium hydroxide with them.

If you object to Duralac, I don't understand why, then use a simple sealant like Sikaflex to exclude the water.
 
vyv_cox said:
Read my post again. There is absolutely nothing wrong with Monel, nor stainless steel. The problem with galvanic corrosion is that the more anodic metal, in this case aluminium, corrodes preferentially trying to protect the more noble metal, the bolts. In theory a big anode protecting a small cathode is a good thing, so the whole mast would slowly corrode and leave the bolts perfect. In reality the aluminium corrodes first around the bolts, making them difficult to remove without bringing lots of aluminium hydroxide with them.

If you object to Duralac, I don't understand why, then use a simple sealant like Sikaflex to exclude the water.
Click to expand...

Hi Vyv,

Yes, I could pop down to the local chandlers and buy some SS machine screws and screw the clutch to the mast using duralac and be all finished in next to no time as well as being quite secure.
The thing is, I've had plenty of experience of removing stainless machine screws from corroded aluminium even where Duralac has been used (I'm quite sure you must have seen this a time or two yourself). As I've already mentioned, the Duralac works fine except on the threaded portion where it is unable to form a barrier. Usually once the screw is removed it ends up with the aluminium having to be redrilled and retapped a size bigger. This is something I wish to avoid on my mast as well as the fact that it's a pain in the backside trying to removed seized items.
I don't need this done in a jiffy and I'm not worried if the cost per bolt is high as I only need two so I thought I'd investigate a better solution.
 
rotrax said:
Hi, I am pretty sure that an application of Zinc Chromate paste would inhibit corrosion and allow the screws to be removed/replaced without trouble.
Click to expand...

Not sure if or where you would find zinc chromate paste. The Duralac paste mentioned is barium chromate though.
 
G12 said:
Hi Vyv,

The thing is, I've had plenty of experience of removing stainless machine screws from corroded aluminium ........

......I don't need this done in a jiffy and I'm not worried if the cost per bolt is high as I only need two so I thought I'd investigate a better solution.
Click to expand...

Exactly. It's the aluminium that corrodes, so changing the material of the bolt isn't going to make much difference.

Have a look at Rivnuts. I drilled stainless pop rivets from my boom, which was badly corroded around them. I replaced them with stainless Rivnuts bedded in Sikaflex. The fittings screw into the stainless and the aluminium surrounding the nuts doesn't seem to be corroding a lot. It isn't too critical an attachment if it does, as they will never be taken out.
 
Tef-Gel is an alternative to Duralac which is often suggested/ recommended.

See HERE

Not happy with Duralac then try Tef-Gel. It's favoured by some
 
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Vyv beat me to it..
My first thought was to use stainless rivnuts... I think the 5mm rivnut will need a 7mm hole drilling. Mount with the duralac paste then you will be screwing stainless into stainless which should be easy to remove when the time comes.

If you have not got the tool to set them with your rivet gun then just take a 5mm threaded stud and grind the threads off for all but about a cm of it's length. It will then screw into the rivnut and the shaved end fit the rivet gun.
 
vyv_cox said:
Exactly. It's the aluminium that corrodes, so changing the material of the bolt isn't going to make much difference.

Have a look at Rivnuts. I drilled stainless pop rivets from my boom, which was badly corroded around them. I replaced them with stainless Rivnuts bedded in Sikaflex. The fittings screw into the stainless and the aluminium surrounding the nuts doesn't seem to be corroding a lot. It isn't too critical an attachment if it does, as they will never be taken out.
Click to expand...

Hi Vyv,

My understanding is that if I were to put a fastner into the mast that behaved in a similar way to the aluminium then it wouldn't corrode because there would be no cell set up. Maybe I am not correct with this?
I have used rivnuts many times before but never on a boat. I had not thought of them for this job. I will investigate what's available as they're very easy to fit and also easy to replace if required. Have you had any problems with yours becoming loose? (the rivnut itself, not whatever you fastened to it). The ones I've had to replace at work in the past were mainly down to the fact that they had worked loose and were spinning in their holes.

Vic, I might get some of that gel, it looks like good stuff to have onboard anyway! Thanks for the link.
 
I've had good results with Loctite as well.When I assembled my furling gear 7 years ago I had no Duralac so used Loctite.I removed several small screws last summer and there was no corrosion at all.
 
I think the Op is missing a possible trick-

Use an alloy winch base type 'pad', riveted to the mast, and simply bolt the new fitting through that using s/steel bolts and accessible nuts.
Oversize the holes if you like but, either way it will always disassemble in the future.
 
Blueboatman said:
I think the Op is missing a possible trick-

Use an alloy winch base type 'pad', riveted to the mast, and simply bolt the new fitting through that using s/steel bolts and accessible nuts.
Oversize the holes if you like but, either way it will always disassemble in the future.
Click to expand...

I like this idea too, will see what design I can come up with. I doubt anyone makes an off the shelf one for a single clutch.

Just ordered some Tef-Gel from eBay..... Just under a tenner delivered.
 
G12 said:
Hi Vyv,

My understanding is that if I were to put a fastner into the mast that behaved in a similar way to the aluminium then it wouldn't corrode because there would be no cell set up. Maybe I am not correct with this?
Click to expand...

Yes, that's true. The problem of course is that aluminium is weak and fasteners in it would almost certainly not be up to it. Aluminium rivets are OK if there are enough of them but I don't think that's what you want.
 
I would second the vote for Tef-Gel. Superior performance to Duralec in my experience. If you can isolate with nylon hat washers etc that may be even better, but it does alll depend on getting access to the reverse side!
 
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