VANS air force discussion on Duralac

4 years ago

04-26-2016, 10:22 PM

Join Date: Nov 2012

Location: Launceston, Tasmania, Australia

Posts: 751

Dissimilar Metal Jointing

Hi All,

I put this in the primer category, as it seemed the most appropriate section for this post. If the moderators think otherwise, please feel free to relocate.

I live on an island in a corrosive environment and have primed my entire aeroplane with Ekopoxy. I'm at the point where I am joining things together and have had conflicting input regarding dissimilar metals. One local gentleman is telling me to use Duralac. He has built an RV-8, and has sailed the world and swears by it in corrosive environments. The other gentleman is Glastar builder and was a seaplane operator for many years, and won't touch Duralac because he says it hardens, cracks and allows moisture egress and subsequent corrosion. I failed to ask him what he is using as a substitute (if anything at all).

Doing some digging on the forum, I have found nothing definitive regarding the use of jointing compounds. A little more research on boating forums revealed a common dislike for Duralac due to the cracking. As a non hardening replacement if chromated paste is your thing, people seem to be using JC5A. If you're not a big fan of chromated anything, then people seem to be using a PTFE based compound called Tef-gel which essentially fills the voids and stops moisture egress. I find this one attractive, as I know I can source it locally.

Anyway, I was wondering what people are using for intermetallic corrosion prevention, and if anyone has any experience in aircraft use with Tef-gel?

Cheers,

Tom.

04-27-2016, 07:05 AM

Dean Pichon

Join Date: May 2006

Posts: 509

I have never used Durlac, but have only heard positive reports. I looked into using it for work, but we never actually tried it. (I work for a DoD contractor and we design and manufacture shipboard and aircraft systems.) In principle, when joining dissimilar materials we follow these guidelines:

1. Avoid dissimilar materials, if possible

2. If not possible to avoid, pick materials with minimal separation on the galvanic scale. (Here's a useful link: http://www.preservationscience.com/m...etals/PGC.html) A classic example of this is not to use stainless fasteners in contact with aluminum.

3. If necessary, isolate one material from the other. We install almost all fasteners "wet". The fastener is dipped in epoxy primer and installed while wet. We often coat fasteners with a brusbable version of polysulfide. We use a variety of specialty platings, but these are generally too expensive for general use.

For fasteners that are unlikely to be removed frequently, I would install wet or coat with polysulfide. For fasteners that are removed regularly, I would try Duralac or BoeShield. (I spray everything with BoeShield...)

Good luck

04-27-2016, 07:41 AM

Low Pass

Join Date: May 2006

Location: Houston

Posts: 1,969

Quote:

Originally Posted by tgmillso

Hi All,

I put this in the primer category, as it seemed the most appropriate section for this post. If the moderators think otherwise, please feel free to relocate.

Anyway, I was wondering what people are using for intermetallic corrosion prevention, and if anyone has any experience in aircraft use with Tef-gel?

Cheers,

Tom.

What about CorrosionX treatment?

__________________

Bryan

Houston

04-27-2016, 09:21 PM

tgmillso

Join Date: Nov 2012

Location: Launceston, Tasmania, Australia

Posts: 751

Quote:

Originally Posted by Low Pass

What about CorrosionX treatment?

If I owned an old, unprimed Cessna, I'd be all for the CorrosionX, however I'm fully priming my aircraft, so I'm not sure this is the right situation for it. I'm looking for a product specifically for the joining of dissimilar metals in the build, and I know that CorrosionX bleeds away over time (hence the 3 year re-application that generally occurs down here).

Anyway, I did a little more research into Tef-Gel and found out that the manufacturer was actually here in Australia (Queensland). I gave them a call and spoke with the inventor of the product. I asked if he was using the product in aircraft applications. His response was that he has a number of aircraft operators that have been using the product for up to 10 years, including both land based and seaplane operators, all the way up to SAAB 340 turboprops, but his focus had been on the maritime industry because the growth had been so rapid as the marine sector was moving away from chromate based products. He stated that the product was generally being used in highly corrosive parts of the aircraft, such as elevator hinge points in seaplanes, or for the lubrication of brake fittings (it can handle temperatures up to 536 deg F). Because the product is not petroleum based, it does not crack off or dry out over time, and thus leaves no path for moisture egress in the future and I think this is the real key to it. It is also electrically conductive, hence the reason I noticed B&C Specialty sells it (or a similar product) as lubricant/sealant for electrical connections, just be sure not to smear it across the insulating surface onto an unintentional ground. One thing to keep in mind is the impact on final torque values, as it will reduce the fastener drag as it is a lubricant.

Anyway, I'm going to speak with some of the A&P's at the aircraft maintenance facilities that are using it, and I'll post what I hear.

I'l also still open to any other input on this product or any other solutions that others may have.

Cheers,

Tom.

04-29-2016, 07:48 AM

tgmillso

Join Date: Nov 2012

Location: Launceston, Tasmania, Australia

Posts: 751

Fay Sealing and Seam Sealing Of Wing and Fuselage Skins

So I spoke with the seaplane operator/builder again and he had some further comments that may be of interest here. First, I asked if he wasn't using duralac, what was he using for dissimilar metal use, and his response was anhydrous lanolin. He says that he has pulled apart aircraft bolts that have been in operation for 40 years in seaplanes that were coated in lanolin without a problem. It appears that lanolin is used because it is 97% waxy esters, so it doesn't evaporate over time, and has hydrophobic properties, which I guess is why it's the coating of choice for sheep on a stormy night.

As I'm about to start riveting my wing together (tomorrow), I also asked him about fay sealing these surfaces, as I know he did it on his Glastar. He is adamant that an aircraft should at least have any lap joints fay sealed, and anywhere there is an intersection on the outside of the aircraft (e.g. at the butt joints on the skins at the main spar). He also dipped all rivets on every rib. Some may say overkill, but I guess this is what years of seaplane operation do to you.

Anyway, I also spoke with a local A&P here today, who is normally very supportive of corrosion protection measures, and he said that this fay sealing was overkill for a land based aircraft. Instead, if an aircraft was fully primed, he would only run a fine seam of sealant around the edges of the surfaces that are exposed to the elements after the riveting was complete but before the final paint was applied. He said that at the end of the day, you've got to have some faith in your primer, and that this seam of sealant when applied properly would stop that capillary action egress that causes much of the corrosion.

Given that I can't stand the smell of proseal, hate mixing it, and it is almost impossible to source here, I asked the seaplane builder/operator about using a Sikaflex product instead. He said that about 15-20 years ago that had a situation where they had no access to proseal when reskinning a section of a seaplane, so they went to the local marine store and purchased a tube of Sikaflex 291 or similar. He said that all these years later, the Sikaflex has served it's job remarkably well.

For a fay sealing an aircraft, the problem with Sikaflex 291 is that it only has a short working time (I've found well less than 30 minutes in the summer) which is not enough time to set the rivets in a skin. Thus, I called Sika today and spoke with a technical specialist and said I was looking for a product for use with aluminum with a two hour plus working time. His recommendation was to use Sikaflex Pro, which has two to four hour working time, and is a polyurethane based product that can be sanded or painted. He said that it took a few weeks to fully cure, but this is not an issue in our situation where we take months or years to complete the aircraft. As far as I'm concerned, this would be a far better product for fay sealing/ wet riveting than proseal for applications that are not fuel tanks. No mixing, minimal smell and 4 hour work time... sounds good to me. A local builder here recommended the use of white instead of the black, as for some reason he found he gets a longer working time with the white. I called my seaplane friend and the A&P about the use of Sikaflex Pro in this fay sealing application, and both were very supportive of its use. The A&P did warn me against the use however if I was just using it to run an external sealing seam/bead after riveting instead of fully fay sealing, as Sikaflex Pro is not UV stabilized. He said he had another compound he used for this application (which I failed to get a name of).

Anyway, I purchased a tube of Sikaflex Pro and will use it at this stage for dissimilar metal riveting (e.g. aileron pushrod rod ends) as I've not got to the end of my Tef-Gel research yet. I'm leaning toward just running a bead/seam of UV stabilized sealant around the wing skin perimeters once I'm done riveting. After two fuel tanks, I think I'm finished with wet riveting for now. From a weight perspective, I calculated I'll use approximately 1lb of Sika Pro to do fay seal the main spar and rear spar interfaces (not including excess that will be scraped off), and just a fraction of that to run a seam/bead. I guess I've got a night to sleep on it.

This however does raise another possibility, and that is for people that don't/can't prime. If I was living back in the city and didn't have easy access to priming facilities, I'd strongly consider just fay sealing every intersecting joint instead and leave the rest of the skins/ribs bare alclad. In addition to not having to prime (except non alclad components), you also get the added strength of having an adhesive between every layer. Definitely worth considering, as I've spent literally weeks priming (I've not completed the priming of about 99% of the aircraft components as I fabricated/primed as much as possible early on whilst the weather was good, so now I just need to assembly).

Anyway, enough ranting for one night. I hope this is of benefit to someone out there, especially those without the easy ability to prime.

Tom.

VANS air force discussion on Duralac

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