Anodised aluminum corroding out when in contact with stainless steel.

[berkshire]

I have a boat, which is one of several I maintain, that has an anodised aluminum jib car track corroding out at each fastening hole. The corrosion is cone shaped, widest at the bottom and barely visible at the top of the hole.
When I replace these tracks would I be better off with Monel bolts instead of stainless or will I run into different corrosion problems.
I am suspecting that part of the problem is incomplete anodising in the screw holes allowing corrosion cells to occur.
B.E.

 

[pauljohn]

Very common what you are experiencing. Problem is not so much stainless touching alum. but really salt water laying in contact with both of them when they are in contact with each other. I see it much less on fresh water boats.

Yes monel would definately be better but has less strength although should strong enough for sail/car track. Another option is to assemble fasteners with plenty of sealant such as 3M 5200 or a good silicone or a product such as Tef-Gel. I found that 5200 works best. Regular wash down with freshwater helps. You should assemble the monel fasteners with a sealant as well.

 

[berkshire]

pauljohn
Thank you for the response. I have had problems in the past trying to dis-assemble parts installed with 5200
What do you think of 4200?
It sounds to me as though the only real solution is to keep the two dis-similar metals apart. This can be difficult on a sail/car track.
B.E.

 

[pauljohn]

Hi Berkshire

Yes 4200 is good although not quite as aggresive as 5200. That is why I like it on sail/car tracks and other spar fittings that don't come off very often because it is so difficult to remove. There is a product that is supposed to remove it but I have had limited success with it. If you remove the fittings often then Tef-gel might be a better product.

http://www.tef-gel.com/

I have found it in the local chandleries - west marine, boat u.s. etc.

I think keeping stainless fasteners seperate from the Aluminum on masts.spars etc. would be almost impossible. Open to any ideas though.

 

[pauljohn]

When I was working in a rigging shop in the caribbean we tried many things to isolate, from primer too pieces of plastic that I would carefully cut up hoping that if we isolated some of the fitting, it would slow down the corrosion but it never worked very well.

As long as the stainless fastener is tapped into the alum, I can't see how it is possible. Like I said we then started to try and prevent contact of the salt water and that was more sucessfull.

We also used monel rivets when we felt strength was not compromised and that seemed to work well. Also rivets are a little easier to insulate as their contact with the aluminum is less than a threaded fastener.

 

[berkshire]

pauljohn (
after researching this it seems to me that trying to isolate the two materials is physically impossible, no matter how carefully you install the screws it is still possible to nick the anodic coating with the stainless screw.
I have found a possible solution I think, I am waiting to get more details from the manufacturer.
A fiberglass tape impregnated with corrosion inhibitors that is applied to the underside of the sail-track, this material is designed to stop corrosion and stainless in the joint interface.
B.E.

 

[berkshire]

somehow I lost a bit of this in the posting

I should have said " This material is designed to stop corrosion between aluminum and stainless in the joint interface,"
B.E.

 

[pauljohn]

Berkshire

"A fiberglass tape impregnated with corrosion inhibitors that is applied to the underside of the sail-track, this material is designed to stop corrosion and stainless in the joint interface."


Not quite sure how that is going to stop the contact between the threads of the fastener and spar or screw head and fitting, which are in my experience the biggest problems, not between spar and fitting. The corrosion that occurs between spar and fitting is fairly easily stopped by sealant.

That is why I mentioned the rivets it is much easier to seal the contact area between the back side of a rivet that contacts the inside of the spar and the front face of the fitting than it is to seal the edges of the stainless thread that contact the aluminum.

Maybe I am not understanding the concept of the fiberglass tape.

 

[berkshire]

It contains corrosion inhibitors that become active when dissimilar metal corrosion starts and neutralises it, at least that is what the manufacturer claims.
In this application, the aluminum jib car tracks are bolted to a fiberglass hull, using stainless bolts, the tape will be applied to the back side of the jib car tracks contacting the stainless screws and the aluminum.

The material is originally designed to go between stainless steel and aluminium sheets.
As you say contact between the threads of the fastener is just about impossible to prevent. Maybe this will stop the resulting corrosion.

 

[pauljohn]

A few other things to check. The stainless fasteners should be tapped into stainless backing plates under the deck or held by nuts. I have seen regular steel a few times moulded into the deck (ugh! lost a mast this way) Also make sure someone hasn't run wires onto the metal to bond all metal on the boat together. This will make corrosion rampant in some circumstances.

One thing I am not understanding is this. You will have to use sealant to seal the fitting and fasteners to stop water going below, how is that going to affect your fiberglass tape and it's 'corrosion protection'? I'm a little skeptical but if you decide to use it let us know how well it works.

 

[berkshire]

paul john
I am still waiting for the manufacturer to get back to me on his claims.
Until then I will not know.
This particular boat has no bonding wires, although I do have others that do. I have not noticed any difference one way or the other from this. The best results I have had, are on jib car tracks that are bare aluminum. I am presuming this is because the electrolysis is spread over a wider area.
B.E.

 

[stevenal]

I have used nylon shoulder washers to achieve complete electrical isolation.

http://www.seastrom-mfg.com/seastrom_manufacturing_catalog/ShoulderWashers.htm.

Don't know if raising the screws would interfere with the track.