Bimetallic Corrosion Risk in Flaperons

Living in the semi-desert environment of South-Central Washington where we get less than 7" of rain annually, I'm not priming anything more than is absolutely necessay on my RV-12 which will be hangered.

I'm currently muddling through fabrication of the flaperons and would like to hear how some of you kindred non-primers have dealt with the risk of bimetallic corrosion at the leading edge. In the flaperons, a 304 stainless steel counterbalance tube lays in against the inside curve of the leading edge alclad skin. Online information regarding bimetallic corrosion indicates that there's an increased risk of corrosion for the aluminum when in contact with stainless steel in the presence of a catalyst. I suppose one of the surfaces should be primed to prevent this stainless-to-aluminum contact, and am leaning toward priming the inside surface of the skin instead of the tube. Do you think a liberal coating of self-etching primer will provide adequate protection from the potential for bimetallic corrosion? <sorry for the lack of smiley or frowny faces>

Priming

Look VERY closely at any primer you might want to use....most, but not all, primers ARE NOT MOISTURE resistant. They provide a 'tooth' for paint but you will find that few primers actually resist moisture penetration.

You're absolutely right about most primers not forming a moisture barrier. For bimetallic corrosion to occur, the two dissimilar must be in direct contact with each other, accompanied by the presence of a catalyst such as moisture. Even though my self-etching primer may not form a moisture barrier, would it provide enough of a separation of the dissimilar metals to obviate the risk of bimetallic corrosion?

Catbird said:

You're absolutely right about most primers not forming a moisture barrier. For bimetallic corrosion to occur, the two dissimilar must be in direct contact with each other, accompanied by the presence of a catalyst such as moisture. Even though my self-etching primer may not form a moisture barrier, would it provide enough of a separation of the dissimilar metals to obviate the risk of bimetallic corrosion?

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Couple of added points - some primers are porous or permeable. After shooting my steel parts with DP50 and thinking it would seal nicely against moisture, I've got the rust spots on primered steel to prove it does not. As for providing an insulating shield, primer alone would improve the condition 99% over bare metal. But my plane's lived 40 miles from The Gulf for the last 10 years.

For galvanic corrosion to occur, you need an electrolyte (e.g., dirty or salty water) to conduct current between disimilar metals. You're in a dry environment. Shoot the primer, a coat of spray paint and sleep comfortably. Bare or with a coating, I wouldn't sweat it (the galvanic issue) in a dry environment.

POTENTIAL CORROSION

There is an even greater chance of bimetallic corrosion between the bronze bushings where the attach pins go and the wing spars. In the boating industry, aluminum to any copper alloy contact was a real no no! Stainless steel fasteners to aluminum were OK.
Larry

Challenger airplanes (the ultralight not the jet ) air built using 6061 tubing and assembled with stainless rivets, the entire airframe and air commonly operated from floats (we used to have on floats, absolute kick in the butt). Corrosion was almost never a consideration unless salt water operations occurred. I went to ACE in richland next to hastings and got rustoleum clean metal primer and have been using it on the fabricated metal parts and shot the inside of the skin on the flaperon and the stainless tube at its contact points with aluminum just to be picky about it. Our titan tornados use aluminum skins with steel rivets and after 15+ years and pushing 2000 hours on a couple of airframes we built hasn't been an issue.

Thanks for everyone's comments and advice on this. Very helpful. Think I'll rough up the inside surface of the flaperon inboard leading edge skin, spray it with self-etching primer, top it with a thin coat of paint, and move on with final fabrication.

This message board is proving to be extremely helpful.

Jerry Gaulden
West Richland, WA

LIMA HOTEL said:

There is an even greater chance of bimetallic corrosion between the bronze bushings where the attach pins go and the wing spars. In the boating industry, aluminum to any copper alloy contact was a real no no! Stainless steel fasteners to aluminum were OK.

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Yet in the air conditioning industry, copper against aluminum is standard. As in air conditioning coils, for instance. Lots of mositure is present, too. The secret here, is the tightness of the fit between two metals that are very dissimilar. The copper tube is swedged to increase it's outer diameter after it's inserted through the aluminum fins.

Even though the risk of corrosion is high between these two metals, it's very limited because of the tightness that doesn't allow corrosion to begin.

Since I am in the heating and air conditioning industry as a business (almost 40 years), I just haven't found enough cases of stainless steel versus aluminum to ever worry about. Wouldn't bother me to rivet the side skins to the stainless steel firewall at all. I do separate copper from steel or aluminum in the plane...............as in, just laying against the surface.

L.Adamson -- RV6A