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?
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.
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.
__________________
Dave
Okemos, MI
T.V.:'05 Dodge Ram 2500 4x4 Quad Cab Cummins
AIR#2276
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":
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.
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...
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.
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
Aaron
__________________
....so many Airstreams....so little time...
WBCCI #2449 AIR #2495
Why are we in this basket...and where are we going
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
__________________
Have you never questioned those who travel?
Have you paid no regard to their accounts- Job 21:29
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.
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.
Last edited by Bob Thompson; 04-20-2005 at 07:27 PM.
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
__________________
Have you never questioned those who travel?
Have you paid no regard to their accounts- Job 21:29
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.
Quick Links
any water will do it, and there's always moisture in the atmosphere. 
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 
Linear Mode
