Galvanic Corrosion

[Psychohillbilly]

mlhwild asked about aluminum bolt together frame and sence the responces got a little of subject I thought I would readdress it. My concern in bolting together aluminum is galvanic corrosion
Galvanic corrosion occurs when two dissimilar metals or alloys are
in contact with each other and exposed to water. Most metals have different solution potentials, in a water environment. When two metals are physically touching, the difference in potential results in a current flow. The rate of corrosion increases when the potential between the two metals is high. Sence the trailer frame is expossed to water I am concerned that corrosion would set in between the bolt and frame resulting in the conection loosening up over a period of time. I am not an expert in this field but I thought I would pass on this info.

The psychohillbilly

[ceebe]

That would be my concern as well. Its a constant problem on airliners. The floorbeams under the galleys require several layers of protection to prevent corrosion caused by dissimilar metals and electrolyte (coffee and other spills). I helped restore a Budd railroad passenger car . The stainless skin was perfect, but around the windows and other leak areas, there was only a shadow of the underlying steel structure. The corrosion that might happen on a trailer would not take very long to set in either.

[Chris C]

Psychohillbilly,

You're correct, galvanic corrosion could be a real long term problem. The only way I can think of which would come anywhere near solving the problem would be to line the holes between the two dissimilar materials with a grommet and then placing some sort of washer both top and bottom to keep the connecting bolt from touching either material. I know back when I had a sailboat, we had to put sacrificial annodes under the water, connected to metal components of the structure to allow them to erode before the metal on the boat. But in the real world, there is very little we can do to completely eleviate galvanic corrosion in trailers. I haven't started on my tear yet, but plan to address the problem as best I can and not worry about it too much. I imagine, with a little care one could expect 20 years out of a trailer structure before such corrosion would pose much of a problem................and who on this forum plans on using the same trailer for longer than that? Just my opinion, of course.

[asianflava]

Psychohillbilly,

The only way I can think of which would come anywhere near solving the problem would be to line the holes between the two dissimilar materials with a grommet and then placing some sort of washer both top and bottom to keep the connecting bolt from touching either material. .

You can also coat the fastener (both sides) with something to seal out the moisture. No electrolyte, no corrosion.

[angib]

My concern in bolting together aluminum is galvanic corrosion

I doubt this is really a practical concern, as it requires the bolts and frame to remain immersed in a conductive liquid. If you're gonna park your trailer in a salt lake, then you may well have a problem. Otherwise, no problem. Rusting of steel bolts in a steel trailer seems a much more likely problem to me and no-one is reporting freqent failures of HF trailers for this reason.

If you want to guarantee against galvanic corrosion, you need to apply a zinc chromate corrosion inhibitor such as Duralac. We used this on S/S screws into an aluminium insert on yacht deck fittings and the screws would come out perfect years later. The inhibitor effectively insulates the bolts from the structure and so breaks the galvanic cell. Not cheap (maybe $20 for a toothpaste-size tube) but it works.

Andrew

[Psychohillbilly]

As I said I'm not an expert. Just thought I would throw it out there sence I new about it and it could be something to be concerned about.

The psychohillbilly

[gyroguy]

My concern in bolting together aluminum is galvanic corrosion :shock: Galvanic corrosion occurs when two dissimilar metals or alloys are in contact with each other and exposed to water. Most metals have different solution potentials, in a water environment. When two metals are physically touching, the difference in potential results in a current flow. The rate of corrosion increases when the potential between the two metals is high. Sence the trailer frame is expossed to water I am concerned that corrosion would set in between the bolt and frame resulting in the conection loosening up over a period of time.
The psychohillbilly

Having flown hang gliders and ultralight aircraft since 1981, I've got some experience with galvanic (or electrolytic) corrosion.

Most hang gliders and ultralight aircraft are made from 2024-T6 or similar aluminum alloy seamless tubing. The tubing is usually held together by 1/8" thick aluminum plate of similar alloys. AN (aircraft) bolts and hardware are used to secure the plates and tubing.

The AN bolts are steel, plated with cadmium, and in theory the aluminum contacts the cadmium instead of the steel. In practice, airframe vibration eventually wears off the cadmium plating so the steel does touch the aluminum. I have removed such bolts from 20-year-old aluminum airframes and found no corrosion on the aluminum. The corrosion is on the steel bolt instead, in the form of rust! The rust is often confined to the portion of the bolt that is inside the tubing, where humid air is trapped and the moisture remains as the air cools.

I have found corrosion on aluminum in two instances, neither related to contact with steel. The first instance was a piece of sealed aluminum tubing, a wing strut. It had whitish/grayish corrosion inside that appeared to have been formed when wet, salty air got caught inside as the tubing heated and cooled outdoors. The second instance was on an aluminum keel mounted to the bottom of a pre-World War II Army target drone that had been sitting behind a house in the dirt for 30 years.

Based on ultralight experience, my guess is that corrosion in the form of rusty bolts is more likely in teardrop trailers than corroded aluminum. Many teardrop chassis designs use steel tubing, which in theory offers the same safe haven for moisture as aluminum tubing on ultralights.

But it's a guess on my part. Has anyone taken apart a teardrop from the 1940s or 1950s and specifically looked for corrosion?

[Spadinator]

Some of the worst Galvanic corrosion we would encounter was around the cockpit and canopy of Phantoms. I don't know why these areas were worse that others but our fix was to remove the damaged area and replace with aluminum that was alodined and as was said above, zinc chromated. You have to remember that these bird had been in service for 30 or more years when we would encounter the corrosion.

[GeorgeTelford]

Hi all

Andrew the disimilar metals do not need to be immersed in liquid to corrode, where steel touches aluminium even when just in normal atmosphere it rots, that duralac sounds good amazed that anything coulod stop the rot between mated surfaces.

On me old bus all the steps had rotted but only where the Aluminium touched the steel (well it was spreading away from the steel structure) that was after 15 years service (the aluminium turns to crumbly white powder.