| Author | Message | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: |
Looking for advice/recommendations on two things re: re-skinning. 1) how to isolate galvanized steel sheet metal from aluminum panels (where they overlap by about 2 inches) to prevent galvanic corrosion? I was thinking of using thin, closed-cell foam tape + butyl rubber caulking. Any thoughts? 2) do people commonly use solid aluminum buck rivets in this type application (layer of aluminum panel overlapping a sheet of galvanized, all fastened to mild steel frame members)? Anything I need to do to help minimize corrosion? Thanks in advance! John | ||
| Jayjay (65.134.221.160) Rating: N/A |
Phosphoric acid etch, chromic acid conversion coating, zinc chromate primer... in that order. (for both metals) Aluminum rivets dipped in zinc chromate (the green aircraft type, not the yellow used on building steel)and "bucked" while still wet, should do the trick. Cheers...JJ | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: N/A |
Thanks Jay Jay. Sounds like a recipe. Where do I get the ingredients? john | ||
| Ross Carlisle (Ross) (207.88.97.11)
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Im not 100% on this so I'll pose this as a question. I was always under the assumtion that in order for corrosion to occur, the material had to be exposed to a corrosive medium. Water, for instance, will make steel rust and salt will make just about anything corrode. Take a piece of steel or aluminum and place them in the Arizona desert and in a years time they would probably both show no signs of corrosion. Take the same materials and drop them in the ocean and they will begin to disappear rapidly...Especially the aluminum. Now...the question is...Will galvanic reactions between two dissimilar metals actually promote corrosion even in the absense of a corrosive medium. For instance, if you were to rivet a piece of steel to a piece of aluminum with a stainless rivet and place that out in the desert...Would it corrode just because the metals are dissimilar? I don't think it would. There is no question that it would corrode in salt water considering salt water is a highly corrosive medium. In fact, salt water acts as a catalyst to galvanic corrosion. I think it has something to with the conductivity of salt water. Just ask anyone with a salt water boat how long aluminum lasts without sacrificial annodes (zinc). If in fact dissimilar metals will not corrode in the absence of a corrosive medium, then preventing galvanic corrosion becomes a simple matter of sealing the joints to prevent the corrosive medium from entering. I know that the galvanic reaction between dissimilar metals expedites corrosion, but is it the real cause? What do you all think? Ross | ||
| Ross Carlisle (Ross) (207.88.97.11)
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John... I would not use aluminum rivets to rivet steel to aluminum. Aluminum is more suseptable to galvanic corrosion than steel. If you did have any galvanic corrosion issues, your rivets would act as sacrificial annodes and would disappear (or atleast weaken) first. I would use steel rivets to fasten aluminum to steel. Ross | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: N/A |
Ross, Thanks for the input. Thought I'd pass along what seems to be a consensus opinion that using aluminum rivets to rivet steel and aluminum is acceptable practice. I've heard that from several folks on the boards, and the rivet supplier. FWIW, the original aluminum skins are buck riveted to a steel frame using aluminum rivets. I've seen little evidence of any corrosion of those rivets in the absence of general corrosion of an area (i.e., if the frame is corroded, then obviously the adjoining rivets are; but where the frame/skin is sound, the rivets are fine). I'm assuming that there shouldn't be anything different about the aluminum/steel contact in the case of using steel skins...? John | ||
| Dale Fleener (64.66.215.236)
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John; According to the notes I took at the last Bus Conversion Convention, this is not so. At the seminar held there we were told that aluminum rivits in steel was a no-no, but the other way around (stainless rivits in aluminum)was ok. My $0.02 worth. Dale | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: N/A |
Dale, Thanks. OK, now I'm worried. Hmmm... already ordered the aluminum rivets, and was planning to do the work next week. Time to rethink this a bit. Hoping to hear from others re: what they've used to solid buck rivet steel sheets to steel frames. I was under the impression that one really doesn't want to try to use steel buck rivets in a "backyard conversion" type setup (requires heating the rivets and using much greater force than used on aluminum rivets). So what have y'all done? I guess something like monobolts is an option... Thanks, John | ||
| Ross Carlisle (Ross) (207.88.97.11)
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I used steel 3/16" pop rivets with umbrella caps. I just dipped them in zinc chromate before pulling them. I use buck rivets all the time in building airplanes. I didn't want to go there on the bus. Would have been a month long job for atleast 2 people. Most, if not all of the forces imposed on the skin fasteners are shear forces. I'm not sure what the shear strength is on a 3/16 steel rivet, but one of the kit plane manufacturers who use 1/8" steel pop rivets in thier airframes extensively tested the 1/8" rivets to over 900 pounds. Multiply that times the munber of rivets. Thats quite a bit of strength and not a ton of money. Also...The window area of the bus you are reskinning was never designed to have any structural integrity anyway. I would not replace structural rivets with pop rivets, but with the new skins, all the rivets need to do is hold the skins on. Ross | ||
| John Biundo (Jbiundo) (64.175.37.23)
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I found some "Randolph" brand Zinc Chromate Primer in Green. Not sure why the color matters, or what other specs to look for on the paint (I can find regular yellow locally, but have to mail order the green stuff). Does it matter? Is there a brand name I should look for? Or is any old zinch chromate primer OK for wet setting the rivets? Thanks, john | ||
| Ross Carlisle (Ross) (207.88.96.223)
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John... NAPA sells it but they do not openly admit that it contains zinc chromate. Look in thier store brand for a self etching primer. They sell two versions. One is a "Self Etching Primer" and the other is a "Chromate Free Self Etching Primer". The first contains zinc chromate and is green in color. I've used it on aluminum and steel with excellent results. They also sell Zinc Chromate in a spray can by Martin-Senour. I have not asked, but they may also carry the same stuff in quarts and gallons. Ross | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: N/A |
Ross, Thanks. I'll check out NAPA today. So... this may be a dumb question, but: to "wet seal" buck rivets, do I simply dip 'em in the primer, insert and buck? Anything else I have to worry about? Dumb question #2: should we wear respirators (i.e., does the zinc chromate atomize when you do this)? Thanks for the help! John | ||
| Ross Carlisle (Ross) (207.88.96.89)
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#1...Yes...Just dip and buck. The stuff dries fast, so dont waste any time bucking. Its gonna be a messy job. With pop rivets, you can dip the rivet without getting your hands in the paint. With the buck rivets, you'll probably have to just drop them in a pail of paint and pick them out one at a time. #2...No need to wear a respirator unless the solvent smeel bothers you. The paint will not atomize. Ross | ||
| John Biundo (Jbiundo) (64.175.37.23)
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Ross, thanks again. Local NAPA has it only in gallons. Checking into mail ordering a quart of it from Aircraft Spruce. Appreciate all the help. john | ||
| John Biundo (Jbiundo) (64.175.37.23)
Rating: N/A |
Ross, Struck out again -- Napa doesn't have it after all. Not sure whether Aircraft Spruce can ship it to me -- they had to check their freight dept. and haven't gotten an answer yet (hazardous material issues). Any other ideas for sources? I'm getting the picture that this stuff is relatively unavailable, or perhaps even banned from sale in California due to it being a hazardous material. If I can't get the zinc chromate, any suggestions on an alternative for wet sealing? Thanks, John | ||
| Ross Carlisle (Ross) (216.107.195.105)
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John... Did they know what you were talking about and just did not have access to it? Or did they not have a clue what you were talking about? If they didn't have a clue....The stuff is in thier "Econo-Body" line and the gallon part # is 15210. Ross | ||
| John Biundo (Jbiundo) (64.175.37.23)
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Hey Ross, That's it! They DO, in fact, have that product. I got a big runaround on the phone, but I jotted down the part # of the paint that came closest to your description. I went searching for it on the vendor's web site and came up blank. From the info I got from the sales guy, and the lack of info on the web site, I guessed it was the wrong product. Now another question for you: this apparently comes as a 2 part solution -- the primer and a "reducer". Sales guy said he thought I needed to use both, but didn't seem to know much about it. What do you say? Thanks for the diligent follow-up! Cheers, John | ||
| Phil (204.89.170.3) Rating: N/A |
If it is self etching primer you must use the second part. This is an activator/acid etch. | ||
| Ross Carlisle (Ross) (216.107.197.171)
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John... You have to use the self etching reducer. Not sure of the # on that. I took all the paint down to the bus today. The part # for the reducer is on the paint can along with mixing instructions. Ross | ||
| Tom Caffrey (Pvcces) (64.114.233.102)
Rating: N/A |
Ross, to try to answer your first question: the corrosive medium is necessary and the right name for it is "elecrolyte". When dissimilar metals are connected together and an electrolyte is present and in contact with both metals, they act as a battery. The current that flows between the metals causes metal to be removed from one of the metals and possibly deposited on the other metal. The reason that salt is so corrosive is that it makes water such a good conductor; even in small quantities, it can cause a good current flow between the metals. Water is a poor conductor of electricity if it is pure. Most water has enough contaminate to act as an electrolyte. If dissimilar metals are to be joined, sealing them so that not the slightest moisture can reach them can work very well to prevent corrosion. The devil is in the details. On saltwater wood boats, it is common to find aluminum bulwarks fastened to the wood by galvanized lag bolts. Most other fastenings to aluminum use stainess steel. We have found that galvanized bolts should be replaced before the galvanizing is gone because the steel of the unprotected bolt will burn the wood for an inch or so around the bolt. Zinc, aluminum and stainless can be very similar in an electrolyte while brass, copper and iron are very different from them. The greater the difference, the greater the corrosion. I hope this helps! Tom Caffrey PD4106-2576 |
