Corrosion with nylon and aluminium

[PeterKroll]

I pulled a couple of very old nyon oar clips off my tinnie today. They were held in place by two stainless screws. I had to break the clips to get them off, and the screws twisted their heads off rather than come undone.

So the whole area behind the clips seemed to have what looked like the standard whitish stainless/aluminium corrosion. I was a little surprised because, while I expected corrosion around the screws, I didn't expect it behind the nylon. So I have some questions.

Does nylon react with aluminium over time or is it likely simply to be the residue of nylon breaking down?

[tunaticer]

That is salt buildup that causes the corrosion, a very strong blend of aluminium oxides and sodium chloride. A simple hose down does not remove salt from such places and every time more salt spray goes there it is absorbed and strengthens the cocktail for more corrosion.

[Shark Poker]

Black nylon, especially fibre reinforced nylon of the injection moulded kind can indeed. As it has a reasonably high carbon content.
Although on applications above the waterline this is a very minor threat.

More likely it is just a build up of salt in between the fitting and the alum surface.

In order to create corrosion one needs to maintain the following principles:

ALUMINIUM + STAINLESS STEEL + SALT + TIME = CORROSION

All four parts are important.

After obtaining a vessel or trying to maintain one, all sst screws, nuts and bolts should be removed.
Replacing them with alum rivets is the best option, but if this is not suitable there are anti corrosive protectors like Lanolin, Duralac and Tef Gel that should be applied wherever sst fasteners are used.

This applies to an old tinny or a new modern plate boat.
Bubbling of the paint around certain fittings in any alum boat more than a few months old will show evidence of zero pre treatment.

Pauly

[PeterKroll]

Gents, that makes it a lot clearer. This also makes me a little anxious about what may be lurking under the floor. Fortunately, it's only 8 screws or so to pull it up, so I'll do that tomorrow. Fortunately, due to a previous thread on Ausfish, I do have a tube of Duralac for when I screw the floor back in.

It must be daunting with the bigger boats with all their nooks and crannies.

[PeterKroll]

One last thing.... All the ss screws that I attempted to undo broke off. Is there something I can put on them that might make unscrewing them easier? (I only have access to the screw heads. I guess it would have to be something that worked via osmosis).

[wags on the water]

Vice grips....

[thrillseeker1]

Ingress of salt water and a process called electrolysis, between S/S and Al is the problem. Had too many failures with Duralac, it sets hard and eventually cracks, my suggestion is Tef-Gel. I'm using it on reel screws and anything that locks up with corrosion around boats.
Sandblast or wire brush the affected area and use a product called Ali-Brite to get it really clean. Wash with detergent and dry. A thin coating on connecting surfaces will stop the problem for ever.
Frozen S/S screws can be removed by heating the surrounding Al, there is a different coefficiant of expansion between the two metals, the problem with that is the heat travels rather quickly through the Al to places you don't want heated so be aware.

[PeterKroll]

Thank you, thrill seeker. More good info.

[sporty1]

Hi guys
After having my motor serviced by my mobile mechanic recently, he suggested there is a school of thought that you can reduce the effects of electrolysis by running a small alligator clip from the hull (Chine a good area) to a earth (metal point in the garage or in my case the home earth is close by ) apparently salt can never be completely washed of the boat as it gets into everything, the hull,will develop a positive charge caused by this electrolysis ( use a multi meter to check this out)!and by reducing this current it is possible to slow the bubbling cause around dissimilar metals. Any others have opinions on this?
Murray

[thrillseeker1]

Hi guys
After having my motor serviced by my mobile mechanic recently, he suggested there is a school of thought that you can reduce the effects of electrolysis by running a small alligator clip from the hull (Chine a good area) to a earth (metal point in the garage or in my case the home earth is close by ) apparently salt can never be completely washed of the boat as it gets into everything, the hull,will develop a positive charge caused by this electrolysis ( use a multi meter to check this out)!and by reducing this current it is possible to slow the bubbling cause around dissimilar metals. Any others have opinions on this?
Murray

"Electrolysis is the passage of a direct (http://en.wikipedia.org/wiki/Direct_current) electric current (http://en.wikipedia.org/wiki/Electric_current) through an ionic (http://en.wikipedia.org/wiki/Ion) substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials.
The main components required to achieve electrolysis are :

An electrolyte (http://en.wikipedia.org/wiki/Electrolyte) : a substance (http://en.wikipedia.org/wiki/Chemical_substance) containing free ions (http://en.wikipedia.org/wiki/Ion) which are the carriers of electric current (http://en.wikipedia.org/wiki/Electric_current) in the electrolyte (http://en.wikipedia.org/wiki/Electrolyte). If the ions (http://en.wikipedia.org/wiki/Ion) are not mobile, as in a solid salt (http://en.wikipedia.org/wiki/Salt_(chemistry)) then electrolysis cannot occur. "
I don't believe that earthing the hull will have any positive affect at all.
The circuit and dc current is created between the disimilar metals by the liquid electrolyte(salt water). The less noble metal is then corroded(Al)
Sacrificial corrosion with zinc anodes is exactly the same, while ever the electrolyte is present, current will flow, letting it flow to an earth doesn't stop it.

[Fed]

Peter's little project is suffering from galvanic corrosion not electrolysis.
http://en.wikipedia.org/wiki/Galvanic_corrosion
http://en.wikipedia.org/wiki/Electrolysis

Either way Murray tell him he's dreaming.

[sporty1]

Oh well I'll put that one away with the snake oil!

[thrillseeker1]

Peter's little project is suffering from galvanic corrosion not electrolysis.
http://en.wikipedia.org/wiki/Galvanic_corrosion
http://en.wikipedia.org/wiki/Electrolysis

Either way Murray tell him he's dreaming.

You are correct, my mistake for calling it electrolysis, I do know the difference. The remedy still applies.

[dnej]

Tef-Gel is essentially PTTE, or Teflon. That being the case why not just use Teflon plumbing tape on screws?
David

[Fed]

All the goop in the world won't help when you're screwing stainless screws into alloy, at some point the metals will touch & turn the hull into an anode in the surrounding area.

[PeterKroll]

That's the logic that's been running through my head. At some point, the screw thread has to bite into the metal.

[thrillseeker1]

All the goop in the world won't help when you're screwing stainless screws into alloy, at some point the metals will touch & turn the hull into an anode in the surrounding area.
"An electrolyte (http://en.wikipedia.org/wiki/Electrolyte) : a substance (http://en.wikipedia.org/wiki/Chemical_substance) containing free ions (http://en.wikipedia.org/wiki/Ion) which are the carriers of electric current (http://en.wikipedia.org/wiki/Electric_current) in the electrolyte (http://en.wikipedia.org/wiki/Electrolyte). If the ions (http://en.wikipedia.org/wiki/Ion) are not mobile, as in a solid salt (http://en.wikipedia.org/wiki/Salt_(chemistry)) then electrolysis cannot occur. "
The corrosion can't occur without liquid electrolyte even if the disimilar metals are touching. The Tef-Gel seals the salt water out. I suppose that thread tape could work but much harder to apply than the paste.

[Fed]

Point taken thrillseeker but the problem is how do you put the goop on to the inaccessible pointy end of the screw after it's been installed?

If Peter could get to it then it would be a whole different ball game with nyloc nuts & nylon washers and goop.

I'd be looking at pop rivets & goop everywhere as extra insurance Peter.

I'm assuming your fixing to a small box section on a tinny?

[PeterKroll]

Yes, Fed. Just screwing nylon clips to the struts. If I did it again,I might use rivets.

[SunnyCoastMark]

Yes, Fed. Just screwing nylon clips to the struts. If I did it again,I might use rivets.

Hey Peter,
If you use rivets - make sure you use ally rivets with an Aluminium stem - most have a steel stem and they will rust and also cause corrosion.

You would have to go to a specialist NUt & Bolt supplier to buy those rivets as they are not readily available at you average hardware store. Whitworths may have them, so you could have a look on their website.

Duralac works OK - but you have to recoat every couple of years in areas exposed to salt water. And yeah - as Wags said - if you have broken any screws - use vice grips. Pretty good idea though to give the screws a hit with a hammer on the end of a screwdriver before trying to remove them. - This will loosen some of them up (not all) - so there will be less broken ones to deal with.

Mark.

[thrillseeker1]

I've never tried this but I wonder,,,
If a solution of Ali-Brite was allowed to soak into the screw hole between screw and Al, by capillary action, would the acid dissolve the hardened corrosion and release the screw?
Worth a try.

[Shark Poker]

I've never tried this but I wonder,,,
If a solution of Ali-Brite was allowed to soak into the screw hole between screw and Al, by capillary action, would the acid dissolve the hardened corrosion and release the screw?
Worth a try.

Whether that would drift past or sink in would be the question.

And that is a metaphor for this subject indeed.

The issue is not exactly which alum rivet, tef gel vs. duralac, 304 vs. 316,
IMO, the big problem to overcome is that most people have no idea. So Any improvement is a good one.

[PeterKroll]

That's why this forum is so valuable.

[thrillseeker1]

Point taken thrillseeker but the problem is how do you put the goop on to the inaccessible pointy end of the screw after it's been installed?


Interesting question. Galvanic corrosion can't occur with (humidity)fresh clean water(no free ions). If salty air can't access the inaccessible part of the screw there won't be a problem. If it can, chances are you will be able to access it as well, to cover it.http://www.ausfish.com.au/vforum/images/icons/icon14.png