Galvanic corrosion between aluminum and stainless used for the firewall

[Gerhard Rieger]

Hi to all, what did you do to prevent galvanic corrosion between the aluminum angle and the stainless firewall?

[Wyo Johnson]

Gerhard,

Galvanic corrosion happens between to different materials when touching and if an electrolyte (water, etc) is present. If you isolate them with a good paint, this should do the trick. This is a common practice on general aviation and large commercial. Same goes for carbon graphite and aluminum...sometimes you can find specs for it online

Andy

[Sir Newton]

100% correct.

[alaskabearhawk]

Nothing...left it as is.

[svyolo]

I think the quantiity of aluminum in the boot cowl and wings makes the anode (aluminum) big enough to far outlive me. Galvanic corrosion is occurring, but at a slow enough rate that I time out before the boot cowl or wings. I am more worried about a situation where me, the wings, and the boot cowl all become un-airworthy simultaneously. If the size of the anode (aluminum) was much smaller, it would corrode away sooner.

You can use steel fasteners in aluminum, but not vice versa. Big cathode/little anode, and the little anode won't last very long. It seems like nobody else worries about it, so I guess I won't either. I thought about it myself a while back.

[Nev]

I've noticed what looks like a tiny amount of corrosion around pull rivets on my front door skins, under the paint. I assume it's from dissimilar metals. I need to check it more thoroughly but I'm guessing I may have used aluminium rivets. If I replaced them with stainless steel rivets is that likely to stop or reduce the corrosion?

[Battson]

Let's start by talking about the galvanic series. Pictured below.

Galvanic corrosion happens two metals with different electro-potential are joined in the presence of an electrolyte (often water / condensation, or salt water), which allows an exchange of electrons between the two metals. The least noble metal (anode) corrodes.

For example, you'll notice cadmium is right next to aluminum alloy, which is partly why they are a great pairing for aviation applications.

Titanium, stainless steel, 4130 steel, and copper / brass are all used on the Bearhawk and can all cause cathodic corrosion in the aluminium parts (or magnesium, zinc plated parts) which are also used.

For outdoor applications a 0.15 voltage difference is generally the limit to ensure corrosion will not occur, otherwise some kind of covering (paint, duralac, etc) or sacrificial coating (zinc galvanizing or cadmium plating) are required.


galvanic%20series%20noble%20metals.jpg

If we take Nev 's question for example, installing an aluminium or stainless steel rivet through aluminium and 4130 steel, we have a great galvanic cell ready to go, it just needs water to start.
If we installed the rivet dipped in duralac or something similar, that reduces the chances of corrosion occuring.

Back to the OP's question, there is normally enough heat and oil spray around the engine firewall flange to keep it from corroding, it is also buried under a few layers of metal so should stay dry (mostly). For safety's sake, I painted mine...

[svyolo]

Electrons go from the anode (-) to the cathode (+). The anode (AL) oxidizes and is "sacrifical", the cathode (SS) is unharmed. Big anode, small cathode is OK, as it takes a very long time. Little anode, big cathode is catastrophic in a few months/years. I did this by accident 40 years ago.

I was a bit alarmed with the dissimilar metals in tube/fabric/al aircraft but had no experience. But the firewall? SS, connected thru AL rivets to the AL flange, connected with metal fasteners to the boot and engine cowl electrically. That is a big anode, to a little cathode. Thumbs up.

I have had the unpleasant experience of removing many SS fasteners from very old Al boat parts that were 20-30 years old. Not a lot of fun. Corrosion in the threads that could usually be cleaned up with a tap, sometimes you had to go one size up.

I also had the misfortune to install AL rivets in a steel van body 40 years ago. They all turned to white powder within 6 months. Completely gone.

I was concerned as well, but there are lots of Cub type aircraft out there. The ratio of AL/Steel must be OK. I have seen both extremes, and now the middle.

[Battson]

Big anode, small cathode is OK, as it takes a long time.

Tell that to my aluminium wings with stainless steel screws in them. They might have a different opinion :P

SS, connected thru AL rivets to the AL flange, connected with metal fasteners to the boot and engine cowl electrically. That is a big anode, to a little cathode. Thumbs up.

It might be OK, but it depends on a lot of factors. It isn't so straightforward when we have electrical currents flowing around to assist with the process.

[svyolo]

I wouldn't even try to analyze what is going on in a Cub or Bearhawk galvanically, but it seems like it works long enough. That is all I have to go on.

[noema]

One thing I do with fasteners and dissimilar materials is to fill the gap with torque seal. It almost adds no additional work and effectively fills the gap where otherwise water could cause a galvanic reaction.

It is actually a nice way to install rivets because they become sticky and don't fall out before squeezing.

[Nev]

A very good idea. Thanks for sharing !