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Old 04-19-2005, 11:27 PM   #1
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Dissimilar metal corrosion

I have some awning screws and window frame screws etc. that are rusting. So I have started replacing them with stainless steel.

But I remember something about dissimilar metals causing corrosion to the weaker metal. Is this something I need to worry about with stainless steel and aluminum?
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Old 04-19-2005, 11:55 PM   #2
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hi tim

it's not really corrosion but bonding of the 2 metals that happens very gradually. electrolosis is the word for this i think, as the 2 metal exchange electrons slowly.
i'm sure someone will explain this with greater detail and clarity.

that said i had many screws replaced (using stainless steel) on my class a air with no negatives that i can disern 15 years later. it may be that stainless is less reactive than other steels.

i think the bigger issue with stainless is that they are harder so care must be used to not strip the softer aluminumumumum.

if these are screws that you'd like to remove again someday you could always coat them with thread compound (not locktite) which would provide some buffer while aiding in grip and release.

as a side bicycle spokes are often steel and the nipples brass. no real bonding occurs in this application....but then again a thread compound is used too.

cheers

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Old 04-20-2005, 07:09 AM   #3
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The big problem with stainless steel fasteners is using a SS bolt with a SS nut. If the joint is dry, when you torque it down, the two parts will "weld" themselves to each other and give you all kinds of grief when you go to remove them. The solution is "anti-sieze" compound (aluminum and copper powder or graphite in a vaseline base). The only other problem is price and availability. In small towns, a very limited selection is usually available in the local supplier, but there is always the internet.
In vehicular applications, stainless steel bolts are not as strong as "grade-8" steel bolts, so be wary of changing out anything on any part of the drive train.
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Old 04-20-2005, 09:30 AM   #4
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Hi Tim, not to induce info overload but it depends what kind of stainless steel and aluminum you are talking about. See link to chart explaining "galvanic corrosion":

http://www.pennfast.com/design_info/galvanic.html

My feeling is that if you are not in a constant salty wet environment you'll be fine. Do you camp on the ocean a lot?

This excellent marine and woodworking catalog has a good selection of stainless hardware and it's where I get Sikaflex too. It wouldn't hurt to be on their mailing list.

http://www.jamestowndistributors.com/index.asp

Best of luck,

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Old 04-20-2005, 09:45 AM   #5
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Interesting....their chart raises more questions than it answers, as there are several types of alluminum, steel, and stainless listed, and they're all over the place on the chart.

also, I don't think "salt" is required; but it helps. any water will do it, and there's always moisture in the atmosphere.

the practical answer is that SS screws are the way to go. yes, they will corrode faster by being in contact w/ the alluminum, but the process is still soooo slooooow that it doesn't really matter.

makes me wonder about the belly pans on our trailers. yeah, the factory used alluminum rivets, but isn't the pan itself in direct contact w/ the steel frame? (not to mention the rivet shanks, themselves). probably academic, I guess...
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Old 04-20-2005, 01:06 PM   #6
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OK. It sounds like ss is the way to go and I don't have to worry about it.

Thats good because I have enough to worry about already ;-)

Thanks for the info.
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Old 04-20-2005, 01:23 PM   #7
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There is a risk to using stainless steel. You may end up causing the corrosion to move from the fastener (easily replaced) to the window frame (not so easy). You might get away with it, depending on how wet the location, and worse if you live near an ocean.

I would rather have rusty screws than have the aluminum corrode. If you decide to go with the SS screw anyway, there are a few things you can do to minimize the effects:
1. use aluminum washers between the SS and the window frame. If the washers start to corrode in six months, you can go back to steel without having compromised the frame.
2. use nylon washers to separate the SS from the aluminum.
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Old 04-20-2005, 05:53 PM   #8
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Quote:
Originally Posted by chuck
Interesting....their chart raises more questions than it answers, as there are several types of alluminum, steel, and stainless listed, and they're all over the place on the chart.

also, I don't think "salt" is required; but it helps. any water will do it, and there's always moisture in the atmosphere.

the practical answer is that SS screws are the way to go. yes, they will corrode faster by being in contact w/ the alluminum, but the process is still soooo slooooow that it doesn't really matter.

makes me wonder about the belly pans on our trailers. yeah, the factory used alluminum rivets, but isn't the pan itself in direct contact w/ the steel frame? (not to mention the rivet shanks, themselves). probably academic, I guess...
Chuck,
The frame on mine corroded...along with the belly pan I suppose you could include a tape of some type to keep the pan and the frame apart, but I think a decent coat of POR-15 should serve the purpose My orginal rivets lasted close to 30 years so I guess I will go back with the same type

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Old 04-20-2005, 05:58 PM   #9
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Quote:
Originally Posted by markdoane
There is a risk to using stainless steel. ...
1. use aluminum washers between the SS and the window frame. If the washers start to corrode in six months, you can go back to steel without having compromised the frame.
2. use nylon washers to separate the SS from the aluminum.
Good points Don. If you don't mind my beating this dead horse...what we are taking about is actually part of battery engineering. I'm facinated by the little research I've done. What you have in dissimilar metals is what 2air' said about the metals exchanging electrons. The farther apart on the chart you get the greater the movement of electrons (electrolysis, is the same as our concern: galvanic corrosion). The electrons will move toward the more "noble" metal and deteriorate the lesser one. That's why aluminum will suffer when next to stainless steel. With respect to the belly pan it's the steel frame that will suffer next to the aluminum. Boat owners are very concerned about this in salt water because it will destroy the innards of an outboard motor so they have a sacrificial metal attached to the motor that gives off it's elecrons. This science also allows you to make potato batteries with zink coated nails and copper plate.

Sorry so way off topic but I thought it was fun to learn. Now you know the rest of the story...

Steve
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Old 04-20-2005, 06:31 PM   #10
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Steve,

Great points. It gets a lot deeper than just the galvanic series. There are surface area effects, polarization effects, and the amphoteric nature of aluminum.

It's very difficult to predict what will happen, especially for general cases. At least in Tim's case, there are only two metals involved. When you have three metals, like using stainless fasteners to attach the aluminum belly pan to the mild steel frame, it gets more complex.

I used to build implantable defibillator batteries.
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Old 04-20-2005, 07:17 PM   #11
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The one thing I do know is that the less noble metal will corrode. Nearly 30 years back I studied this phenomena in college. I don't exactly remember exactly how the process works, but I do remember that free electrons flow to the less noble metal causing it to corrode. Therefore, it makes sense to attach a sacrificial anode to protect the two previously mentioned metals, aluminum and stainless steel. The most common sacrificial annode is zinc and ready to use blocks of zinc are available at most outboard boat shops. For clarity, the process of applying zinc is called galvanizing. For galvanized steel, the zinc oxidizes instead of the steel, producing a whitish powder on the surface.

As someone mentioned earlier, the marine zinc anodes are to protect the aluminum outboard engines. Attaching multiple zinc anodes to an aluminum trailer only makes sense, to protect the aluminum as well as any steel.
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Old 04-20-2005, 08:13 PM   #12
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OK, I'm a simple person and need a simple answer

I have holes where awning brackets used to be and I may put one back on in the future.

What type of screws should I use? And don't tell me olympic rivets because I just mailed back the shaver I borrowed!
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Old 04-20-2005, 09:05 PM   #13
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Hey Tim, If it's non structural they sell aluminum screws at the hardware store made for screen doors and such. Just put a dab of vulkem under the head to seal each well.

BTW, I wouldn't worry a wink about using stainless though.

Steve
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Old 04-20-2005, 09:31 PM   #14
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Thinking about dissimilar metals and galvanic action, I've had a moment of inspiration. I painted my Excella a year ago and the painted portion is doing great but oh my....its hard to keep the cast aluminum pieces polished and shiny. I'm thinking the more noble "painted aluminum" is dumping electrons to the unprotected aluminum castings like the front clearance lites, door frame, etc. I can polish them and 3 weeks later they're corroded. I treated the castings with Zoop Seal, but so far success has been limited. I think I'm going to go out and buy some solid zinc washers and put them between the stainless steel mounting screws and the aluminum body of the trailer to see if I can cut down on the corrosion and thus, the aluminum polishing.
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Old 04-21-2005, 12:05 AM   #15
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I've been getting a lot of advice from a buddy who is an A&P mechanic with the airlines. For years he worked in a refit facility doing to 727's/737's etc metal work similar to what we are all doing to our Airstreams. He has suggested that any time I put a fastener (that is not aluminum) thru an aluminum part on the trailer, to use an aluminum washer. The idea is that you sacrifice the washer instead of the fastener or the aluminum part on the TT. Somebody suggested that above. In the case of my TT, this will only be a real problem around the perimeter in the U channel as the new floor goes in. The bolts and screws that are there now will need to be replaced. It appears that all are steel, and all have corroded to some extent. I say corrosion, because a large portion of it is due to water leaking in.

Just my 2 cents.

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Old 04-21-2005, 09:31 AM   #16
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My gosh, reading this thread made me think at first that I was on the forum for vintage MG sports cars (and their owners). Had to pinch myself to get up to speed: No, dummy, this is about Airstreams!

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Old 04-21-2005, 09:50 AM   #17
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Jim,

What you say makes sense, but then I started thinking--why is the sacrifice of a washer even necessary? Why not use a washer of some other inert substance (nylon, plastic, rubber, copper, brass, something?) that could act as an insulator, stopping the sacrifice of the washer, as well? If there is anything inherently better about an aluminum washer, what about painting them before use (por-15?)? Would you mind asking your mechanic friend about this if you don't know the answer already?

Just curious, since trying to figure this out as well...

Mary

Quote:
Originally Posted by Jim & Susan
He has suggested that any time I put a fastener (that is not aluminum) thru an aluminum part on the trailer, to use an aluminum washer. The idea is that you sacrifice the washer instead of the fastener or the aluminum part on the TT.
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Old 04-21-2005, 03:14 PM   #18
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Mary,
For the most part, the issue here is controlling the corrosion so it doesn't damage important items. Unless the trailer is kept in a perfect environment, it is going to corrode. As soon as oxygen gets to any raw metal, it is going to corrode. Raw metal can be intentional, like aluminum castings, or it can be incidental like rock chips, clearcoat failure, and raw metal edges from fabrication. Add water (humidity, fog, rain, snow, salt water...) and the corrosion is accellerated. By keeping oxygen and water away from the raw metal, thru painting, clearcoating, waxing, etc. you greatly reduce the tendacy to corrode. What we're trying to solve is how to control the corrosion by sacrificing other metals.

When dissimilar metals are in contact with each other, the more noble (less likely to corrode "example: gold or titanium") metal will transfer excess free ions normally part of the corrosion process to the lessor noble metal (example: zinc). Check out the galvanic chart connection given earlier. This causes accellerated corrosion in the lessor noble metal with very minimal impact on the more noble metal.

Zinc is one of the most likely to corrode metals commonly available today. The beauty of zinc is that it can do a lot of corroding before it is consumed, hence my suggestion that solid zinc washers might be used as sacrificial metal to protect the aluminum and steel. I could see how aluminum washers could be consumed quickly, causing connection failure. If you make the washers so they can't corrode by painting them, the corrosion is goes to the least noble metal available which is either the aluminum or stainless steel.

You probably already have an example of zinc in corrosion on your trailer. Check out the safety chains at the hitch connection. If your chains are like mine, they are galvanized steel. In places, the zinc is gone and the steel underneath is starting to rust but not quickly. The zinc is being sacrificed to protect the steel.
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Old 04-29-2005, 03:23 PM   #19
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Stainless Steel

Here is another good source for stainless steel. I used this company many times when I was building wood drift boats in Alaska. Their prices are really good and they will ship. They don't have a website, just call them and they will get you a price.

Northwest Fasteners Inc
PO Box 92038
TillicumWA98492
253-582-1671
Fax 253-581-3131

One more note about this thread, which was not mentioned. That is an electrical current. The tear drop side markers are grounded to the body. So we have a current flowing from the maker lights to a ground through the body. Add dissimilar metals into the formula and it can cause additional corrosion. To fix this in my trailer, I am going to run a ground wire to all the lights that will ground right to the frame. Just food for thought.
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Old 04-29-2005, 03:46 PM   #20
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Geeze! Just use SS and move along!
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