Aluminum, Galvanized Steel, & Galvanic Corrosion

[Mantooth]

I have attempted to research this to find my answer but, to be honest, it is way out of my area of expertise. I own a nice little 12ft aluminum trailer and I am wanting to add a spare tire mount for obvious reasons. Finding an all-aluminum mount appears to be next-to-impossible. So, I have widened my search to include steel mounts.

What I want to do is make sure I am not setting myself up for galvanic corrosion. Can any steel be used with aluminum? I have heard stainless steel is a big no-no, is that correct? What about galvanized?

TIA,

Mantooth

 

[JimPghPA]

In chemistry there is a thing called the electro-negativity chart. The further apart two metals are on the chart the more electron action there will be. Aluminum and copper are on opposite ends of the electro-negativity chart. So you can make a good battery with those two metals, but if you nail copper gutters up with aluminum roofing nails they will fall down because of corrosion. Also brass has a lot of copper in it so aluminum and brass will interact with each other.

Aluminum use to be used for wire in houses, and if you wanted to connect aluminum wire to copper wire you had to use a steel connector that made electric connection to both but did not allow them to touch each other.

I have not looked, but you could look on the internet and see if there is any information on an electro-negativity chart that shows aluminum and the types of steel you are thinking about using.

 

[Dave9]

In a nutshell, you want metals similarly anodic and when mismatched, for the weaker of the two (structurally) to be less anodic. Carbon Steel can be used with aluminum, is much better than stainless unless you have a big chunk of aluminum and a little bit of stainless where the stainless not corroding (by itself due to environmental factors like salt water and depends on the grade of stainless) is paramount. Galvanized (zinc) will also protect the fastener more (than bare steel, obviously less than stainless), with negligible difference between it and aluminum (negligible corrosion), but anywhere the zinc wears off is a new rust site, just not any worse than if it were bare steel in the first place.

Yes steel can be used with aluminum, and zinc coated aka galvanized will be a little better at preserving the aluminum (which is harder to replace than the bolt, right?), but if the galvanized aren't hardened much, I'd just get a grade 5 or better zinc chromate (yellow) coated bolt, with a big steel wing nut (if it's to be hub mounted instead of lug mounted so no tools are needed for removal (theft risk?)), and slap some marine grease on it, but if this trailer is ever going into saltwater then I'd lean towards galvanized and then paint it. Paint over galvanized is the safest way to reduce corrosion the most, unless you do something really elaborate like using a insulator, like a big (thick) plastic mounting block and then stainless to attach to that.

More info on this page, especially the chart linked on it.

https://pomametals.com/how-to-prevent-galvanic-corrosion/

 

[Boomer]

Get an old inner tube and cut out rubber sheets. Wrap these around the trailer and then put the mount on top so the two metals don't touch. May need replaced every few years (keep what is left of the inner tube) but it will eliminate the problem.

 

[MasterSolenoid]

I'm not an expert, but from what I've read, galvanic corrosion needs three thing to accure.
1) Two different metals
2) They have to be touching
3) Moisture

Like Boomer said, you have to separate the two metals.
If not an inner-tube, how about 1/16" nylon washers.

How does Ford attach the aluminum box's to steel frames ?

 

[Mantooth]

Thanks, for the responses. To answer the saltwater question, no - this trailer will not see any saltwater. I live in Southern Indiana and 99% of the trailer's use is hauling my Honda Pioneer 1000-5 SxS. It sees tons of dirt and gravel, but no saltwater.

 

[Chris142]

Get an old inner tube and cut out rubber sheets. Wrap these around the trailer and then put the mount on top so the two metals don't touch. May need replaced every few years (keep what is left of the inner tube) but it will eliminate the problem.

I was wondering what would be a good way to prevent it. This or plastic truck mudflaps cut to fit.

 

[doitmyself]

Carbon Steel can be used with aluminum, is much better than stainless unless you have a big chunk of aluminum and a little bit of stainless where the stainless not corroding (by itself due to environmental factors like salt water and depends on the grade of stainless) is paramount.

I have aluminum greenhouse cooling pads (lots of water) with stainless fasteners and there is no galvanic corrosion after 20 years. I study this stuff and then this experience makes no sense. In agreement with Dave9 above, it is probably due to "a large anode area (aluminum)in contact with a relatively small cathode area (stainless bolt) is generally not problematic" from my link below. Maybe stainless bolts in his trailer is not a problem?

Pictures of your mount might help with answers. I like the idea of isolating the mount with plastic or rubber.

I also think Dave9 is correct about steel and zinc. Look at the galvanic charts and aluminum is next to zinc and steel: https://galvanizeit.org/design-and-fabrication/design-considerations/dissimilar-metals-in-contact . A zinc or hot dip galvanized unit on aluminum should be o.k..

 

[Kestas]

I'm not an expert, but from what I've read, galvanic corrosion needs three thing to accure.
1) Two different metals
2) They have to be touching
3) Moisture

Like Boomer said, you have to separate the two metals.
If not an inner-tube, how about 1/16" nylon washers.

Well said.

I imagine this trailer will be sitting outside. You don't necessarily need salt water. Given enough time, the weather (rain and humidity) may be enough to corrode the joints. Crevice corrosion enhances the process.

As doitmyself noted, galvanized steel would be an excellent choice. The zinc not only protects the steel, but would protect aluminum as well.

 

[willbur]

In chemistry there is a thing called the electro-negativity chart. The further apart two metals are on the chart the more electron action there will be. Aluminum and copper are on opposite ends of the electro-negativity chart. So you can make a good battery with those two metals, but if you nail copper gutters up with aluminum roofing nails they will fall down because of corrosion. Also brass has a lot of copper in it so aluminum and brass will interact with each other.

Aluminum use to be used for wire in houses, and if you wanted to connect aluminum wire to copper wire you had to use a steel connector that made electric connection to both but did not allow them to touch each other.

I have not looked, but you could look on the internet and see if there is any information on an electro-negativity chart that shows aluminum and the types of steel you are thinking about using.

Interesting-particularly using alum wiring. I always heard alum was not to be used in house wiring because thermal expansion would first loosen connections, resulting in more heat and eventual fire. Thanks for the useful info.

 

[Mantooth]

Well said.

I imagine this trailer will be sitting outside. You don't necessarily need salt water. Given enough time, the weather (rain and humidity) may be enough to corrode the joints. Crevice corrosion enhances the process.

As doitmyself noted, galvanized steel would be an excellent choice. The zinc not only protects the steel, but would protect aluminum as well.

It's not technically outside. I have what we call a shelter house...meaning it is a covered building with no sides. The trailer can, and does, get wet on occasion, but it isn't setting out uncovered per se.

 

[PimTac]

It’s not just the mount but the fasteners also that you must consider. If you have to drill a hole for the mount that will be a source of corrosion too.

Does this aluminum trailer have a steel frame and structure for the shell? For obvious reasons the mount holes will have to go in structural frames or be reinforced. Otherwise the weight will eventually wear out the fastener holes.


I found this. Maybe this will help.

How to deal with dissimilar metals

Trailer manufacturers know that when using two dissimilar metals, precautions need to be taken to prevent two-metal (galvanic) corrosion. That applies

www.trailer-bodybuilders.com