Stainless + Aluminum = Corrosion

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
16,058
Display Name
Display name:
Sixer
My airplane is made from Aloomennyum as the British would call it.
Most of my hardware could use a refresher.

I found this delightful package of shiny new hardware pictured below.

I have been told by "Airplane super experts" that mixing stainless with aluminum will make my plane fall out of the sky in tiny little pieces.

1. Is that true?
2. If so, then why make stainless?
3. What is the alternative?


Screenshot_8.jpg
 
SixPapaCharlie said:
1. Is that true?
Click to expand...
Yes and no. In general, aluminum and certain stainless steels, in the presence of an electrolyte, will produce galvanic corrosion. However, if no electrolyte is present or there is a barrier between the metals it will not. Stainless hardware on painted aluminum is a common upgrade with usually no issues. Now if you're still concerned use aluminum washers with the stainless screws.
 
Bell206 said:
Yes and no. In general, aluminum and certain stainless steels, in the presence of an electrolyte, will produce galvanic corrosion. However, if no electrolyte is present or there is a barrier between the metals it will not. Stainless hardware on painted aluminum is a common upgrade with usually no issues. Now if you're still concerned use aluminum washers with the stainless screws.
Click to expand...

But wouldn't the stainless screw penetrating the aluminum beyond the washer still be a contact point?
 
Smart to ask the question. Strange things can happen with dissimilar metals.

There are screws and bolts that work great with Aluminum - but you might find it a little expensive. Titanium works great.

Here is a google picture of stainless bolts and an aluminum plate.

R.50e85ebf76fdf4596a7843d74cf977ae
 
As mentioned above, "it depends".
You need the "correct" alloys since not all of them are subject to galvanic corrosion. You need moisture, and the metals need to be in contact. I opine there are enough sources of electrolytes (ionic compounds of some kind) available to dissolve in the moisture that they are likely to be present.
 
SixPapaCharlie said:
But wouldn't the stainless screw penetrating the aluminum beyond the washer still be a contact point?
Click to expand...
Sure. But without an electrolyte present nothing happens. Even steel and aluminum, which are close to each other on a noble chart, will corrode into a ball of dust if a strong enough electrolyte is in play. And every nut-plate on your aircraft is some sort of steel. For example, if your plane lives on the beach you will need to take certain measures that a plane from Arizona does not. But even at the beach not all stainless vs aluminum is bad as we routinely used a mixture of stainless and cad plated hardware on helicopters that lived in the GOM. Keep your aircraft clean and you shouldn't have any problems. As to the pic above, 10:1 it was in a wet boat bilge that never dried out, ie. submerged in an electrolyte. However, for reference the corrosion on the left stud is intergranular and not galvanic corrosion which happened during the plate manufacture process.
 
Years ago I used stainless bolts on aluminum intake manifold bolted to cast iron heads on a V8 engine.
The bolts were used stainless bolts that were used on fertilizer bagging scales. I think the fertilizer was a contributing factor? The bolts turned to white powder within a couple months and was eating the aluminum intake manifold.
 
Last edited:
I used the same brand stainless screws on my fuel tank cover. I used anti seize on every screw. 1.5 years later they are holding up well. But the plane stays inside and doesn’t see much rain. I do use water to clean the bugs off.
 
I always assumed stainless steel would be better than mild steel. Is that not the case?
 
Take them off and reinstall every year or later (or the next person) will have fun drilling them out.
 
Our new service truck has aluminum doors with stainless hinges. The John Deere dealer near us has the same manufacturer and we noticed each door had small areas of corrosion. We put helicopter tape between the doors and hinges when it was wrapped. No such problem. Told the manufacturer and they do that with all new builds as well.

Anti-seize is my go to. It's zinc based. On marine outboards they use a zinc anode as a sacrificial metal. Obviously theyre subject to a constant electrolyte.
 
Jim K said:
I always assumed stainless steel would be better than mild steel. Is that not the case?
Click to expand...
It really depends on what you are trying to accomplish/prevent with the stainless hdw especially if you are replacing a different type. For example, SS kits as mentioned in the OP are good for what they do, but if you were to replace hardware in more structural application with stainless it probably won't work out as well. Another example, over the past 20 years a number of aircraft OEMs have made a big push to use titanium hardware to control corrosion, reduce weight, etc. However, just like some stainless steel properties, titanium can be very brittle and not handle work-hardening. There was a fatal helicopter accident where the initial cause was due to a broken titanium stud for an oil filter housing which broke due to the repetitive removal of the housing for mx. Afterwards, all bolts/studs used in similar applications were changed back to mild steel bolts/studs. Same goes for the use of various compounds to prevent corrosion on threads. As a company my old day job restricted the use of anti-seize on any threaded joint except in high temperature environments like a turbine engine. Instead we used a variety of alternatives like Mastinox to prevent thread issues. The reason was anti-seize can act as a "torque-multiplier" and with little effort you can exceed the recommended fastener torques. So it can really depend if something is "better" or not.
 
The further apart (right to left) the more likely that the right most metal will corrode.
Loctite or anti-sieze. One or the udder.
Warning: I ain't no A&P.

galvanic%20series%20noble%20metals.jpg
 
Bell206 said:
It really depends on what you are trying to accomplish/prevent with the stainless hdw especially if you are replacing a different type. For example, SS kits as mentioned in the OP are good for what they do, but if you were to replace hardware in more structural application with stainless it probably won't work out as well. Another example, over the past 20 years a number of aircraft OEMs have made a big push to use titanium hardware to control corrosion, reduce weight, etc. However, just like some stainless steel properties, titanium can be very brittle and not handle work-hardening. There was a fatal helicopter accident where the initial cause was due to a broken titanium stud for an oil filter housing which broke due to the repetitive removal of the housing for mx. Afterwards, all bolts/studs used in similar applications were changed back to mild steel bolts/studs. Same goes for the use of various compounds to prevent corrosion on threads. As a company my old day job restricted the use of anti-seize on any threaded joint except in high temperature environments like a turbine engine. Instead we used a variety of alternatives like Mastinox to prevent thread issues. The reason was anti-seize can act as a "torque-multiplier" and with little effort you can exceed the recommended fastener torques. So it can really depend if something is "better" or not.
Click to expand...
Fair enough, I was referring to "better" with regards to corrosion. @Capt. Geoffrey Thorpe 's chart answers that question though. I didn't expect ss to be further from aluminum than mild steel.
 
Timbeck2 said:
So our planes need to be built of graphite
Click to expand...
FYI: they already are using graphite as in carbon fiber. Its one of the reasons the move to titanium and other exotic hardware. Carbon fiber being electrically conductive also suffers from galvanic corrosion and has a field day with your standard steel and aluminum alloys, eg., AN hardware, etc. as shown in the chart. Have seen CF cowling mount holes rot away with the wrong hardware installed. Who would have thought.;)
 
Don't forget, SS often is not as strong as the original fasteners - you can't blindly buy a packet of SS (from a well-meaning manufacturer) and have the same strength.
 
98855C98-ADB7-44E7-8C85-C3C417EC1847.jpeg For the AP on here…The screws for the tank covers on a Cessna are structural right? Each replacement screw had markings on the screw head. It was a big job getting those screws out after 30 years. Is it best or necessary to support the wing while replacing the fuel tank?
 
Last edited:
Another question my AP suggested I paint the screw heads, I hate to do that in case I have to take them back out. Should I paint them? TIA
 
Gary Ward said:
Is best or necessary to support the wing while replacing the fuel tank?
Click to expand...
No.
Gary Ward said:
Should I paint them?
Click to expand...
I'm not a fan of painting stainless hardware. I do paint steel hardware though. A quick way is to simply punch X number of holes in a cardboard box, stick screws in holes exposing the heads, and paint away. Then install the screws.
 
Bell206 said:
FYI: they already are using graphite as in carbon fiber. It’s one of the reasons the move to titanium and other exotic hardware. Carbon fiber being electrically conductive also suffers from galvanic corrosion and has a field day with your standard steel and aluminum alloys, eg., AN hardware, etc. as shown in the chart. Have seen CF cowling mount holes rot away with the wrong hardware installed. Who would have thought.;)
Click to expand...

I know. I was being facetious
 
Gary Ward said:
I used the same brand stainless screws on my fuel tank cover. I used anti seize on every screw. 1.5 years later they are holding up well. But the plane stays inside and doesn’t see much rain. I do use water to clean the bugs off.
Click to expand...

the problem with that anti seize stuff is knowing what the new target torque value should be. Or is that data published somewhere for these AN grade fasteners?
oh and the other problem with it.... 1 drop on a hand will soon be spread onto everything you own
 
The standard AN screws on Cessna Tanks are cadmium plated. As the screw moves through the nut plate much of the plating is stripped off. That leaves bare steel in a humid environment for many years. I would suggest spraying some type of corrosion retardant before the end of the screw turns into a Rust Ball.

Make sure you use a Respirator when spraying from the bottom. Chemical pneumonia suxx.
 
I use nylon washers under SS screw heads and use a little Never Sneeze on the threads. Everything is still where I originally put it.
 
With stainless bolts and nuts, if you don't use anti-seize, you may eventually have a lot of trouble getting them apart. I don't think it's corrosion, I think it's galling, but small stainless fasteners can bind together just as solid as rusted plain steel does. Antenna installers, where the antennas were all held together with small (<1/4") 30x stainless fasteners ran into this, and many just switched to galvanized rather than use anti-seize.
 
LesGawlik said:
I use nylon washers under SS screw heads and use a little Never Sneeze on the threads. Everything is still where I originally put it.
Click to expand...
Nylon washers will allow relative movement of the parts. If this occurs on the Cessna fuel tank covers, for instance, you soon have serious structural damage. That cover is part of the "box" that stiffens the wing torsionally.
 
Capt. Geoffrey Thorpe said:
Stainless in stainless is galling. Stainless (or regular steel) in aluminum is corrosion.
Click to expand...
I have seen stainless screws gall into steel anchor nuts, too. Stainless is soft, making it easy to "tear" inside the threads. The Phillips socket, or any other type of drive, is easily torn out when trying to remove stainless screws. Stainless is tough to drill, making removal of such damaged screws really miserable.

Electrolytic corrosion? The cadmium plate wears off steel screws and they rust, and that rusting causes easily as much damage to the aluminum as any stainless/aluminum corrosion.

The stainless screws are fine for non-structural inspection covers. Those are the screws that look like wood screws. Structural covers have machine screws, a fine, non-tapered thread. I'd avoid stainless there.

Most mechanics know that any loose item in an airplane's belly presents a risk to control cables and pulleys and other stuff. I have often found screws dropped into bellies, and I always get them out and look around to see what else is waiting in there to make trouble. Steel screws can be extracted out of some difficult place with a magnet; stainless screws can't. So now you're fooling with little finger-grabber tools and sometimes flashlights and mirrors. Fun? No.

In the flight school I just replaced lots of worn steel screws often. They're cheap enough and they ensure that I won't have trouble with them later. The labor to repair the damage caused by one seized-in screw can add up to many bags of screws. The heads of the rivets for the anchor nuts for the Cessna fuel tank covers that run along the front spar are underneath the leading edge skin, making removal and replacement of one nut a long and difficult job.
 
Dan Thomas said:
I have seen stainless screws gall into steel anchor nuts, too. Stainless is soft, making it easy to "tear" inside the threads. The Phillips socket, or any other type of drive, is easily torn out when trying to remove stainless screws. Stainless is tough to drill, making removal of such damaged screws really miserable.

Electrolytic corrosion? The cadmium plate wears off steel screws and they rust, and that rusting causes easily as much damage to the aluminum as any stainless/aluminum corrosion.

The stainless screws are fine for non-structural inspection covers. Those are the screws that look like wood screws. Structural covers have machine screws, a fine, non-tapered thread. I'd avoid stainless there.

Most mechanics know that any loose item in an airplane's belly presents a risk to control cables and pulleys and other stuff. I have often found screws dropped into bellies, and I always get them out and look around to see what else is waiting in there to make trouble. Steel screws can be extracted out of some difficult place with a magnet; stainless screws can't. So now you're fooling with little finger-grabber tools and sometimes flashlights and mirrors. Fun? No.

In the flight school I just replaced lots of worn steel screws often. They're cheap enough and they ensure that I won't have trouble with them later. The labor to repair the damage caused by one seized-in screw can add up to many bags of screws. The heads of the rivets for the anchor nuts for the Cessna fuel tank covers that run along the front spar are underneath the leading edge skin, making removal and replacement of one nut a long and difficult job.
Click to expand...

Screws are cheap. I keep plenty of every type in the plane on hand and replace screws that show any wear. With the type specified by the manufacturer (kit producer). I figure the engineers who designed it know more than I do and had a reason for using a specific fastener.
 
You must log in or register to reply here.
Back
Top