Stripped Threads on Mast

[foolishsailor]

Nice clarification Gord, as usual

The question is really is it possible to passify 316 when it is threaded into aluminium - I dont know?

Is 316 stainless passivated if an appropriate barrier coat like tefgel or loctite is applied?

I would love to know this as an owner of an aluminium hulled boat as well, in addition to the use of 316 vs its much more expensive and less available distant cousin monel as a tool for mast work.

For example; all my rivets are monel rivets and they hurt the pocketbook and are not readily available in more distant ports...even in ireland although after living here for 8 years I tend to call it a "distant port"

[Celestialsailor]

Originally Posted by s/v Jedi

o.

p.s. for strength in thin-walled materials, use a course thread.

Is that your opinion or from a technical paper/report ?

[GordMay]

Originally Posted by foolishsailor

1. The question is really is it possible to passify 316 when it is threaded into aluminium - I dont know?

2. Is 316 stainless passivated if an appropriate barrier coat like tefgel or loctite is applied?
...

1. No - passivation occurs prior to inserting (while oxygen is available) the S/S fastener into the Al. substrate*.
2. No - a dielectric barrier (like TuffGel) isolates the two dissimilar metals, preventing galvanic interaction.

* Passivating Stainless Steel requires cleansing & descaling (/w Nitric or Oxalic Acid), then immersion in a heated bath of phosphates or salts which forms a protective oxide film.
In fact, once the metal is clean, the oxygen in the atmosphere will reform the protective chromium oxides instantly. The steel will nearly as passivated as if it was dipped in acid.
Passivation may also be accomplished by Electropolishing; an electrochemical process that is a super passivator of stainless steel and results in a more passive surface than the other methods mentioned above.
If/once the oxide layer begins to break down (or is damaged), in the absence of oxygen, the stainless steel becomes active, and its corrosion resistance is greatly reduced.

[foolishsailor]

So does the process of threading a passivated 316 bolt into a mast "abrade or damage" the micro passivated skin enough that basically negates passivation?

Effectively is any "passivated" 316 just regular ol 316?

[steamgoat]

Just be sure,if you re tap a hole or tap a new one, to use tapping fluid, something like 'tap magic' that will give you a much nicer,cleaner, thread.

[GordMay]

Originally Posted by foolishsailor

So does the process of threading a passivated 316 bolt into a mast "abrade or damage" the micro passivated skin enough that basically negates passivation?

Effectively is any "passivated" 316 just regular ol 316?

That's a very good question. I don't know, to what extent, the passive coating is damaged in threading in.
I expect, that's where the dielectric thread lock/sealant comes into play.

[roverhi]

My mast is 43 years old. Have pulled it and R&R'd all the fittings. Got almost all the fasteners out using a combination of battery powered impact screw driver, using jumper cables to set up an arc on the fastener and penetrating oil. Had a few heads twist off SS machine screws which was undoubtedly due to crevice corrosion in the SS. The only fasteners I couldn't get out were those that passed through or were threaded into thick aluminum like a winch pad or a cleat. Threads in the aluminum are still reusable. Doubt that any of these fasteners were installed with any kind of an insulating material like lanolin or LocTite. Fittings that I installed 5-6 years ago with TefGel came out real easy. The older installed fasteners were, shall we say, a challenge but they all came out if the head stayed on.

SS that passed through, not threaded into, thick aluminum were impossible. Learned a lesson right quick with my Wind Pilot Pacific Plus self steering vane. Installed bolts for the rudder coupling without using lanolin. After only two years of service, they were nearly impossible to remove because of corrosion build up in the thick aluminum. Took me three days of the jumper cable trick and penetrating to get them out and they still fought me all the way. Good thing they were hefty 10mm cap screws.

I find these galvanic tables to be a little misleading. Bronze is closer to aluminum than 304 in the above chart. Have actually seen a Bronze winch that was bolted directly to a mast without insulating material neatly corrode a big hole in the mast in less than 10 years. Never had that kind of an issue with SS.

SS and aluminum work fine together, just insulate with TefGel on threads and Lanolin on shafts of fasteners. Use electrical tape or some other insulator when putting flat plate dissimilar metals together. Don't use self tapping screws, drill and tap for machine screws.

GordMay, neat chart but how do I read it. Looks like 316 active is the closest to aluminum. Maybe I missed it but what constitutes passive or active in SS???

FWIW, most of the fasteners are not 300 series but 18-8 SS. You can get 316 fasteners but have to look long and hard for them. Usually order from McMaster Carr if I need enough fasteners in 316 to justify the freight and hassle. 18-8 doesn't seem to be as corrosion resistant as the 300 series. Okay for above the waterline but not below.

Always use a lubricant like Never Seize or LocTite when screwing SS together. Have had Norseman terminals seize up solid in a quarter turn. Seems to be entirely random that it happens. Have installed 40 plus Norsemans before the first one seized. Felt a little resistance and the next thing I knew had seized metal. Had to get big Crescent wrenches with long cheater bars and help from friends to get it apart.

[s/v Jedi]

Originally Posted by Celestialsailor

Is that your opinion or from a technical paper/report ?

It'll be in plenty engineering handbooks as all tapped holes in aluminium are course thread (like engine blocks that get high load). Into soft materials like aluminium, fine thread strips out easier.

[estarzinger]

Originally Posted by Celestialsailor

Is that your opinion or from a technical paper/report ?

This is common knowledge among almost anyone who has mechanical/machining knowledge. . . . But here is a link to one technical paper/site that agrees with Jedi's comment http://www.eisc.com/support/coarsefine.htm. There are dozens of others on the web.

[GordMay]

Originally Posted by s/v Jedi

It'll be in plenty engineering handbooks as all tapped holes in aluminium are course thread (like engine blocks that get high load). Into soft materials like aluminium, fine thread strips out easier.

I'll need a specific reference; because i get (easily) confused..

See ➥ http://www.cruisersforum.com/forums/...ads-49871.html

And ➥ https://dcc.ligo.org/public/0028/T97...T970244-00.pdf

Coarse Threads:
Coarse threads are commonly used where rapid assembly or disassembly is required.
If corrosion or damage from handling or use is likely.
They allow for easier starting with less cross threading.
In relatively low strength materials such as cast iron, aluminum, magnesium, brass, bronze, and plastic, coarse threads provide more resistance to stripping than fine or extra fine threads.
If subjected to heat, they are less likely to seize than fine threads.

Fine Threads:
Fine threads are commonly used for nuts and bolts in high strength applications.
While applications vary, in general, fine threads are approximately 10% stronger than coarse threads.
They have less of a tendency to loosen under vibration because the smaller lead or thread helix angle provides better wedging action when the assembly is tightened.
Fine threads are also used for fine adjustment and thin walled applications due to the shallower thread height.
Fine threads are generally easier to tap. Since the thread height is shallower, the chip load per tooth and chip volume are lower, resulting in less tapping torque and breakage, particularly in difficult to machine materials.
Less chip volume also means that more lubrication will reach the cutting teeth resulting in longer tap life.
Fine threads require larger tap drill than for coarse threads, which improves the performance of the drill and tap

[estarzinger]

Gord,

When building Hawk, just for fun one day, we actually did a thread test. 15 machine screws in coarse thread and 15 in fine thread. The mean torque for breaking the threads was 135 (in-lbs) for fine, and 155 for coarse (with standard deviations of 10 and 9). That's 1/4" machine screws in 5083 H113 aluminum plate, but I doubt the results would be much different in 6061.

Frankly I think we are all dancing on the head of a pin here and most of the solutions suggested will work for the OP, but that stainless inserts will give him the best chance of being able to remove the bolts 10 years from now if that is at all important to him.

[s/v Jedi]

That CLOD Gord is truly master of the hyperlinks

I am not a mechanical engineer (electronics, later IT) but in those days you all started with 3-4 hours a week in the machine shop in school for the first year. I remember every detail of this because the teacher was of the kind that loved demonstrations that always ended in disaster In the case of tapping and threads, the whole load-cell shattered before the threads stripped so I never got the proof of it

[transmitterdan]

Originally Posted by estarzinger

^^
As to the aluminum inserts . . . I was looking at datasheets and many of them are 5052 aluminum (which has copper in it) rather than the 6061 the mast is probably made of. I have never used 5052 inserts but I have used 5052 machine screws and the copper in them does appear to cause some incompatibility problem with the 6061 aluminum, and they are quite weak (it is very easy to twist off their heads).

Copper is closer (less corrosive) to aluminum than either stainless or monel. I don't think the 5052 is incompatible with 6061 so far as my experience using tens of thousands of fasteners goes. I have seen many stainless inserts spinning uselessly in their aluminum holes with no way to remove the screw from a blind panel such as a mast.

If the screw is designed properly for the application the correct size rivet nut will be stronger than necessary.

The biggest disadvantage of the rivet nut is the special countersink required to properly prepare the hole. If you don't use the right countersink per the manufacturer's instruction then they do not work correctly. You also have to use the correct rivet for each particular wall thickness. They are not one size fits all.

I agree with Evans that we have 1,000 angels dancing on the head of a pin in this thread.

[transmitterdan]

Originally Posted by s/v Jedi

Is that your opinion or from a technical paper/report ? The Monel rivets in our masts do not cause any corrosion at all (19 years old) while every stainless steel fastener has major corrosion around it. This leads me to believe that Monel is much better in aluminium than stainless steel. It must also be the reason that mast manufacturers use Monel.

I think others have quoted from sources on this but it is easy to Google the charts. From the charts 304 stainless is closer to Al than Monel. So it should be the case that Monel is worse than 304 stainless for galvanic corrosion of Aluminum. I was not aware than any mast manufacturers installed Monel hardware. My experience is that Monel and sea water will make a galvanic corrosion problem with many metals including steel and aluminum. Perhaps the Monel hardware is plated with something that makes it more compatible with Al? In the old days Cadmium plating over steel was used for this purpose but it is seldom used today.

The wiki page on Monel has a link to a story that seems humorous now but at the time I'm sure it wasn't.

[s/v Jedi]

Originally Posted by transmitterdan

I think others have quoted from sources on this but it is easy to Google the charts. From the charts 304 stainless is closer to Al than Monel. So it should be the case that Monel is worse than 304 stainless for galvanic corrosion of Aluminum. I was not aware than any mast manufacturers installed Monel hardware. My experience is that Monel and sea water will make a galvanic corrosion problem with many metals including steel and aluminum. Perhaps the Monel hardware is plated with something that makes it more compatible with Al? In the old days Cadmium plating over steel was used for this purpose but it is seldom used today.

The wiki page on Monel has a link to a story that seems humorous now but at the time I'm sure it wasn't.

What I find with these tables is text like this:

"Please Note: Galvanic corrosion is a complex problem with many variables which are difficult to predict. The information above is provided for guidance only - and is only a short summary of the issues involved. "

For example, copper is really bad with aluminium; we have had examples of a copper cent laying on an aluminium tank, falling through it a short while later. Monel, which is copper with nickel, is the best metal fastener on aluminium. It's not a coating.