Helicoil into aluminum - corosion

[Paul L]

I need to put some helicoils for stripped bolt holes on an aluminum mast base fitting. This regularly gets soaked with saltwater. Any thoughts on how to avoid corrosion between the stainless helicoil and the AL plate that they will be in?

[sailormed]

Tefgel

[model 10]

I have seen green locktite used to prevent corrosion in helicoil threads. I'm a big fan of anaerobic sealant. Kind of like locktite but does not get hard.

[barnakiel]

Lanocote, Tefgel, Duralac.

b.

[skipmac]

If you want to remove it in the future, Tefgel. If you never anticipate taking it apart, use red or green Loctite.

By the way, I recently learned that Loctite recommends using their primer with what they refer to as inactive metals which includes SS, anodized Al, or pure Al. Not sure if that would include Al alloys common in marine usage like 6061.

[Hobie_ind]

Paul,
Is the application structural or just holding a cover on? If non-structural, or possibly just shear loads, have you considered using a Rivnut? I normally despise them from my days working on airplanes, but in this instance it would reduce the issue of galvanic corrosion with the mast as they're also aluminum.

You'd still have the issues of galvanic corrosion between it and the stainkess fastener, but with Tef-Gel it's reduced and they can be drilled out and replaced relatively easily and replaced if you do get some thread corrosion in a few years. Just a thought.

Mark

[a64pilot]

Rivnuts are horrible things, spawn of the devil they are.
Seriously what happens quick is corrosion between the SS fastener and the rivnut, then you know what you have right?

I have much experience with Loctite, both green and blue, just do not use red. Astonished me when Forespar recommended Red to me to mount my whisker pole track. Red is stud and bearing mount, meant to be permanent. On the bottle it warns that it may take excessive force and or heat to remove.
I have a two ounce tub of Tefgel that I plan on using based solely on others recommendation. Sure looks a lot like Teflon pipe dope, just way more money, but if it works it's worth it.


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[Paul L]

I usually use tefgel. I was just thinking that there wouldn't be much left on the helicoil to base material side after installing the helicoil.

[model 10]

Originally Posted by a64pilot

Sure looks a lot like Teflon pipe dope, just way more money, but if it's worth it's worth it.
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I'm telling you they are not the same, it's worth it. If you don't believe me check the teflon content of both and also check the difference between Forespar and Tefgel because you will find Forespar has more Teflon.

[FATS]

Helicoils weren't always intended just to repair damaged threads. In the aircraft industry they were used in aluminium to prevent damage should a bolt require to be withdrawn frequently.

Some aircraft find themselves in pretty hostile environments, corrosion-wise, much worse than our beloved boats. We engineers had to make sure minimal corrosion, coupled to demountability. Thus, whilst we used stainless steel helicoils against stainless steel bolts we avoided stainless against aluminium. The maxim being that potential between two metals must be as low as possible but never more than 0.5 volt. I do not recall the actual figures (I could look it all up, but you can have fun doing the!), stainless to aluminium is 0.9 volts and consequently a high potential and electrolytic action in a wet environment (basically you have a battery corroding away) by electro-plating the helicoil with cadmium, which would stay in place between the helicoil and the aluminium and have less than 0.5 volt potential to each. The cadmium would wear away between the bolt and the helicoil, but that didn't matter...they were the same metals/potential difference. While I am a fan of loctite and the rest, metal to metal contact is inevitable although electrolyte (sea water) is lessened. It might be better to avoid it, just in case one day you want to take that dogged little helicoil out. Merry Xmas!

[FATS]

I forgot to say that these days cadmium plating is considered too toxic to be commercially viable but I believe zinc plating does much the same job, potential-difference-wise.

[brianlara 3]

Been helicoiling seriously expensive alloy cylinder cylinder heads (Merc, bmw, ferrari etc) for 5 decades. Yes, the environmental electrolyte in automotive is different, little or no salt, temps well in excess of boiling must be tolerated and in the early days glycol didn't exist.
You'll have permanent success with Loctite or Duralac.
in any case, the masts aluminium will try to produce its own ( preventative) oxide.
If only ALL mast tappings were Helicoiled.
Appy the stuff liberally and forget it.
FWIT, in automotive... 3 metals are present...HT steel, alloy, s/steel....all in a terribly hot environment.
Worldwide standard practice for over half a century. Fact.
Sleep well.

[rostan1]

No one's mentioned copper free marine anti-seize. I've installed SS screws into an aluminum mast and was able to remove and replace the screw 5 years later without any problems. Boat was a 46' ocean going catamaran.

[reed1v]

In a pinch use silicone sealant. Good to have on any long distant voyage for sealing whatever. We re bedded an entire mast rack with it. Mildly adhesive. Clear tubes best all around use.

[colemj]

They make titanium, inconel, and other types of threaded inserts if you want to go that far.

Mark