Fighting Corrosion

Skybolt Al · #1 10-30-2009, 11:25 PM

Just took a look at a 2004 RV-6A that is in the shop to have the wings re-painted. There is extensive corrosion around all the fuel tank screws (which are stainless). There is also corrosion in various other places in the wing skins. This plane has been hangared for 4-1/2 years! The workmanship and paint is beautiful.

In the marine industry they make extensive use of sacrificial zinc anodes to protect the aluminum parts from corrosion in the harsh saltwater environment.

Has anybody tried bolting on zinc anodes to aircraft? I would be bummed to see that kind of damage to my project after just 5 years.

frankh · #2 10-31-2009, 12:02 AM

I must say say that extensive corrosion is almost never seen..sure it wasn't left outside on the coast?

I have a zenair zodiac for 7 years, no paint inside or out...light random patches of dusting but no significant corrosion whatsoever.

If it was going to corrode than the steel screws is where it would do it..But usually this needs an electrolyte.

Curious to see what others have seen

Frank

jsharkey · #3 10-31-2009, 05:56 AM

I can understand why the aluminum around the screws is corroding. The stainless steel and aluminum are galvanically incompatible - the aluminum is the less noble material and it acts as the sacrificial anode protecting the stainless steel. Rain water could act as the electrolyte. That's why all the structural nuts, bolts and washers specified are cadmium plated. Aluminum and cadmium have similar galvanic potentials.

Also perhaps the paint prep - acid etch and/or alodine - wasn't well rinsed or neutralized, especially in gaps, before painting and has lead to premature corrosion.

It's good practice during the build to prime both sides of mating contact surfaces - even if the whole interior or exterior isn't primed. That will help to minimize the effect of moisture that can linger in the gaps.

The airliner manufacturers go to great lengths to ensure galvanic compatibility of materials in contact and to seal gaps against moisture ingression.

Jim Sharkey
RV6 - Phase 1

Captain Avgas · #4 10-31-2009, 06:21 AM

Quote:

Originally Posted by jsharkey

I can understand why the aluminum around the screws is corroding. The stainless steel and aluminum are galvanically incompatible - the aluminum is the less noble material and it acts as the sacrificial anode protecting the stainless steel. Jim Sharkey
RV6 - Phase 1

In reality, when the area of the less noble material (aluminium sheet in this case) is large compared to the more noble material (stainless steel screws in this case) the corrosive effect is greatly reduced, and may in fact be negligible.

Having said that it is a very bad idea to use stainless screws for RV fuel tanks from a structural point of view (although many builders do use them). AN stainless screws are only rated at 80,000 psi ultimate whereas the cad plated screws specified by Vans are rated at 120,000 psi ultimate. Thats a very serious reduction in design strength. Vans would certainly not approve of this mod.

chaskuss · #5 10-31-2009, 06:40 AM

Quote:

Originally Posted by Skybolt Al

Just took a look at a 2004 RV-6A that is in the shop to have the wings re-painted. There is extensive corrosion around all the fuel tank screws (which are stainless). There is also corrosion in various other places in the wing skins. This plane has been hangared for 4-1/2 years! The workmanship and paint is beautiful.

In the marine industry they make extensive use of sacrificial zinc anodes to protect the aluminum parts from corrosion in the harsh saltwater environment.

Has anybody tried bolting on zinc anodes to aircraft? I would be bummed to see that kind of damage to my project after just 5 years.

Al,
Jim and Bob have already outlined WHY this corrosion happened. Regarding your idea about a "sacrificial" to prevent corrosion, one is already available.
All the Mil-Spec epoxy primers contain strontium chromate [replacement for zinc chromate]. This is actually a finely powdered metal which is suspended in the primer. This powdered metal IS the sacrificial.
I have in the past had trouble explaining to some listers why it is not a good idea to use an automotive epoxy primer. They were using an automotive top coat [finish paint] and wanted to use the primer recommended by the manufacturer for that top coat [on cars]. The automotive epoxy primers do NOT contain strontium chromate or any other sacrificial metal.
Use a good Mil-Spec epoxy primer like Akzo or Sterling's U-120/U-1202 to help prevent future corrosion.
It is critical when using these primers to adequately stir or shake the primer prior to use. All that strontium chromate will sink to the bottom of the can. You need to stir it up to put it back into suspension, prior to painting. I like to use one of those steel "egg beater" style stir sticks which mounts on an electric drill. I got mine from the local Home Depot.
Charlie Kuss

Mel · #6 10-31-2009, 06:54 AM

Quote:

Originally Posted by frankh

I have a Zenair zodiac for 7 years, no paint inside or out...light random patches of dusting but no significant corrosion whatsoever.

The Zenair aircraft are built with 6061 aluminum. This is the same allow used for all the tubing and extrusions within RVs. 6061 is very corrosive resistant by itself. Most Canadian kit manufacturers use 6061 because of this.

chaskuss · #7 10-31-2009, 07:28 AM

Quote:

Originally Posted by Mel

The Zenair aircraft are built with 6061 aluminum. This is the same allow used for all the tubing and extrusions within RVs. 6061 is very corrosive resistant by itself. Most Canadian kit manufacturers use 6061 because of this.

While 6061-T6 is more corrosion resistant than 2024-T3 [which needs to be Alclad], it is not as strong or resilient. This downside of 6061-T6 often requires using thicker cross sections to achieve the designer's goals. Van uses the stronger, more expensive 2024-T3 alloy [as do almost all certified and kit manufacturers] because it allows greater performance with less weight.
6061-T6 is less expensive than 2024-T3. Van would certainly use this less expensive material if it could equal the performance of 2024-T3.
Charlie Kuss

Mel · #8 10-31-2009, 07:45 AM

I was just pointing out that the Zenair aircraft are designed around 6061 and it IS more corrosive resistant.

rv72004 · #9 10-31-2009, 09:01 AM

Quote:

Originally Posted by Captain Avgas

In reality, when the area of the less noble material (aluminium sheet in this case) is large compared to the more noble material (stainless steel screws in this case) the corrosive effect is greatly reduced, and may in fact be negligible.

Having said that it is a very bad idea to use stainless screws for RV fuel tanks from a structural point of view (although many builders do use them). AN stainless screws are only rated at 80,000 psi ultimate whereas the cad plated screws specified by Vans are rated at 120,000 psi ultimate. Thats a very serious reduction in design strength. Vans would certainly not approve of this mod.

Youre right about the screws and I had a similar discussion with a fellow RV builder. Fact is the aluminium skin will never be as strong as any screw ,steel or s/steel. What we do is put a dab of grease under each screw head. This helps some with corrosion. Dont have any visible on my plane...yet.