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Rivet corrosion questions
I've had good experience making marine hardware out of 6061 and doubt that priming (unless the material is *very* thoroughly prepped first with cleaning and conversion coat) makes much difference to longevity. The material just doesn't corrode much, even in salt air. What concerns me though (house and airport less than a mile from the ocean) is corrosion of the steel stem inside each of these 14,000 Avex rivets. I know Chris Heintz based his studies of this rivet system on there being no stem present so I'm not concerned about the loss of the steel, but of the loss to the aluminum after some time outside.
The stems of the Avex rivets are made of zinc "coated" low carbon steel. I've heard of older aircraft using these rivets with rust clearly visible inside the rivet (since the broken stem of the rivet is just plain steel, with no plating.) Some have suggested filling the rivet with zinc chromate (paint or epoxy) to seal it against moisture and provide some protection for the steel. Anyone experience with such methods?
Replies to This Discussion
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Permalink Reply by Chris Aysen on
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David - I don't think you can extrapolate the experience you had with 6061 in the marine industry, usually a thicker product, to that of an aviation product; much thinner. Where to pieces of metal, ferrous or nonferrous, meet the introduction of moisture in that joint accelerates corrosion, of course various metals decay at far different rates. But in the case of aluminum moisture at the joints is NOT good, add salt and you accelerate the corrosion process. Material thickness also plays a part in the decaying process in relation to length of time. I live in South Louisiana and we have had aluminum float planes loose tail sections within a few years of operation because of corrosion. I don't know what alloy of 6061 you used in the marine industry but 6061 T6 seems to hold up well in moist conditions but I would recommend some kind of corrosion protection at all joints no matter what climate conditions are where the plane will be kept. I agree with you about the stem question. Being so close to salt air for carbon steel this is not good as you know. I don't think there is any "silver bullet" answer to this one. But I have heard of builders dunking there rivets in corrosion protection right before they pulled them.The layer of primer and paint should also help. Best of Luck.
Chris
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Permalink Reply by David Josephson on
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Actually not all that different -- our parts were 6061 sheet metal, typically 0.032 - 0.065. Yes, thicker than most airplanes use but not that much. The aluminum antenna parts I used in an installation in the Gulf coast seemed to hold up pretty well too. My understanding is that the aluminum itself, and joints where aluminum contacts aluminum, are not the problem. What would the corrosion mechanism be with overlapping pieces of the same alloy? I have never seen a production airplane with corrosion protection only at the joints, have you? The problem only arises when there is some different metal nearby, just a few millivolts of potential difference will start galvanic corrosion.
6061 is the alloy, corrosion should be no different whether it's T0 or T6 or T651 or T6511 extrusion. These are just different tempers (heat treatment and stretching cycles) and chemically identical. Or so I've been told, if someone has different information I'd be interested to know.
I've seen the rivet dunk process too and am not sure what it accomplishes. It's the joint between the steel and the aluminum, inside the rivet, that concerns me more than any aluminum-to-aluminum joint.
Finally I'm not at all sure about the average homebuilder (that includes me) being able to be very effective with a "layer of primer and paint." All the failures I've experienced with corrosion in aircraft have been underneath primer and paint, and have spread undetected because the paint wasn't firmly attached to the aluminum -- it can't be, there's a complicated layer of aluminum oxide, which is porous -- a slight flaw in the paint allows moisture to seep in between and start to work. I worked in a helicopter shop for a few years. If we cleaned a part well, etched, washed, alodined, washed, washed again and then applied zinc chromate, we got good results. But anything less seemed to be worse than nothing at all.
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Well I found a couple of corrosion spots my last annual a month ago; right where the side skin meets the top longeron. Both are T6. What's the cause? I could only guess. But it would be evident that it's not only electrolysis between two different metals.
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That's good information. Do you think there was moisture trapped there, or some contaminant that combined with water? Would priming at the mating surfaces have prevented it?
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I believe some contaminant of some sort was there prior to primer and paint. I believe the conventional belief is if the mating surfaces are PROPERLY coated by a corrosion preventive prior to joining the two surfaces the joint would be resistant to corrosion. To say that applying corrosion preventives is worse than having nothing would imply that these corrosion preventers contain a substance that actually causes corrosion. I'm not prepared to say that but I have never done experimentation that would prove or disprove the theory. I don't think the conventional wisedom of corrosion protection is some type of fallacy; I'm sure there is some science and experimentation supporting it. If your theory of no corrosion protection is the best approach I'm sure it would be great news to some and bad news to others. At the same time I'm not discounting your theory. How tap water (from washing the plane) reacts to the metals versus rain water or salt water and air for that matter....... It just seems to me that for the amount of people that use corrosion protection there is little corrosion occurance. Maybe if some builders would build their planes without corrosion protection we can then have specimens to compare.
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I'm not suggesting that no protection is better; I'm suggesting that improperly attached protection may be worse than none at all, and I want to learn more about that attachment. That first layer between the native aluminum oxide and whatever comes next is the key, I think. We do have many years of factory-built airplanes from DC-3s to Cessnas that were assembled without any corrosion protection. And as one might suspect, if you disassemble riveted assemblies there is often more pitting in places where sheets overlap -- due I can only guess to moisture remaining trapped there for longer than on exposed surfaces. In Mooneys, most of the troublesome corrosion spots between the spar cap and the wing skin, and at the trailing edge of control surfaces, are places where water and atmospheric contaminants can collect.
As for the conventional belief, I think we should be cautious about grouping "corrosion preventers." Primer and paint are moisture barriers but only as good as their bond to the surface beneath. Indeed self-etching primer, for instance, does promote corrosion to a point (which is desirable, so that the polymer system has something to grip.) My hunch is that chemically reactive surface preparation is even more important than we usually think. I am still learning about the various conversion coating processes (saddled as they are today with the restriction on using chromium compounds). My data point on this is that some brackets I made from 6061 20 years ago have a tiny bit of white deposit while some from the same batch that I alodined don't.
On some other homebuilt forums "primer wars" have apparently gone on for years. I don't claim to know what's best but would like to understand the chemistry a little better.
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Yes, I'm thinking something like that, ACF-50 or some other penetrating stuff that would stay stuck to the stem. But it would be nice to seal it in, though, so it wouldn't weep out during painting, sounds like a bad recipe otherwise.
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Another approach, wonder if anyone has considered this, is to just drive the stems out of the pulled rivets. As mentioned above, CH refers in several places to shear strength of the Avex rivets with the stem removed. I wonder how much bother this would be.
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Permalink Reply by Bob Pustell on
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Before doing that, I would find out if driving the stem out would uncrush/deform/weaken the shop head end of the rivit. Since the stem pulls the end of the rivit stem to form teh shop head, and since it is embedded in the resulting head, it could be that driving it out would loosen the clamping force or the grip of the finished rivit.
Maybe it is a problem, maybe it is not, but I would want to know before doing it. If it does not weaken the rivit, it would certainly fix your worry about galvanic corrosion. It would also create 10,000 little holes into the interior of your aircraft structure. Maybe that would let in moisture and contamination, maybe it would just ventilate things and keep it dryer than it would otherwise be. Dunno.
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Yep. I am not excited about this approach.
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Permalink Reply by Jacob on
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There is always the option of using solid rivets? You can drop back a size because they're stronger.
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That's probably the best compromise, and I will do that wherever practical, they are lighter too.
Scott Ehni estimates 16 pounds difference between pulled and driven rivets in his turbine 701, but I think that might be a little high. He did come up with a clever procedure to make it more consistent. He rigged a timer and solenoid valve in the air line at the rivet gun, connected to a footswitch. As he describes it, the riveter puts the gun in place and holds the trigger, while the bucker has the switch. When the bucking bar is in place, the bucker presses the switch and the gun runs for the prescribed interval. Then the bucker says either "next," "again" or "remove" to indicate a good shop head, one that needs more hits or one that needs to be marked for later drilling out and replacement.
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