Galvanic Compatibility of Aluminum & Stainless Steel

A. Hi Leila. The stainless rods are the cathode and the aluminum is the anode. When the anode is far larger than the cathode as in this case, it minimizes the issue because the dissolution current is spread over a large area. Powder coating the rods will at worst further reduce the cathode area, and at best help to electrically insulate the materials. The balcony railings on most hotels & motels, and probably some other buildings, are anodized or powder coated aluminum held in place with stainless steel anchor bolts. It's probably the same with many traffic lights and street lights. I think you'll be fine without need for an exotic solution.

Luck & Regards,

A. Hi. I might be misunderstanding because I'm not seeing how a sign can be unfinished, but raw aluminum has little corrosion resistance. I think if it's indoors and climate controlled it might be okay, but for outdoors it should be powder coated, anodized, or at least clear coated.

Luck & Regards,

A. There is insufficient information to make a call on this. Will the operating conditions be wet or dry? Will they contain chloride? Will it be hot or cold? etc etc.

Taking this aside, if you weld the metals together, you will get a heat affected zone (HAZ); this can cause many problems as the composition of the HAZ will be a combination of the two metals being welded. If you use fasteners, you will need to make sure they are compatible with the two metals being joined. Furthermore, when you use fasteners you will have to drill the two lengths of metal, so (especially with the galvanised steel) you will be exposing the substrate to the risk of corrosion.

However, with the above caveats, I would suggest you use fasteners, but if possible, ensure they are electrically insulating and therefore minimising the risk of galvanic corrosion because the two metals being joined will be electrically isolated. If this is not possible then I would suggest you use aluminium and stainless steel, as they will generally have the better surface corrosion resistance, but be aware of the caveats above.

A. Hi Simone. They might not be readily available nor cheap, but the ideal would be aluminum coated (Ivadized) fasteners as are used on airplanes. Stainless screws will be more available, and if it is possible to insulate them from the aluminum then galvanic corrosion will not occur.

Luck & Regards,

A. I would suspect that the pins will scratch through the film to create direct contact with the aluminum, which creates a potential for galvanic corrosion. SS is not as active in causing galvanic corrosion of aluminum as is plain steel, but there is certainly a possibility, especially in the crevice of the pin bore where the SS will tend to lose passivity.

You could check test assemblies with a voltmeter to see if you have electric conductivity between the pin and aluminum. If you have a low resistance, there is electrical continuity and a potential for galvanized corrosion.

Good luck.

A. Hi Lou. Mil-C-5541 says to ignore the chem film when considering galvanic compatibility, so I'd say no, it cannot be relied upon as a barrier.

Luck & Regards,

A. Rule of thumb for anodize throw: depth = 4x diameter. In other words, an 1/8" ID should throw 1/2" down. If the hole is a through hole, it should be no problem. Should be OK for coverage in hole. Anodize and stainless should be compatible provided that you're not immersed in acid or direct marine exposure. You should specify a SEALED HARDCOAT as most specifications state UNSEALED UNLESS OTHERWISE SPECIFIED. Sealing will provide additional corrosion protection necessary; someone may counter that you'll lose some abrasion resistance but this is minimal compared to the total lack of corrosion resistance when hardcoat is left unsealed.

A. Dear Peter,
I am NOT as much an expert as some of the others who comment on this site!However I am Hard anodising 6061 Extruded Aluminium to MIL-A-8625 / MIL-PRF-8625 [on DLA], Type III,Class 1 which is eventually assembled along with Stainless Steel.
My opinions on your questions:
1/8" dia and 1/2" bore ?! maybe you got the size mixed up?
Please clarify the size again.
Contact marks on the anodised part need to be specified by you - it will not be anodised -- so it is preferred that you specify it on a portion not in direct contact with the steel.
Galvanic Corrosion does occur between Anodised Aluminium and steel if the anodising has not been sealed properly.
-Passivation does help - Electropolishing can be considered too. Depends on what your application is and whether these additional costs are justified.Since your parts are small in size, you might consider Electroless Nickel Plating the steel as additional insurance against Galvanic Corrosion.
I am sure one of the resident experts will help you to identify which of the above suggestions are preferred depending on the Part and its environment of deployment.
Ravi