Carbon steel and Aluminum

[CANPRO]

Hello, I have a situation where an aluminum track will be fastened to an existing structure with grade 5 bolts. The contractor installing the bolts said he will use a copper anti corrosion inhibitor between the steel and aluminum. The aluminum track is being used to clean windows on a high rise. I've done some research into how these metals react with each other, but chemistry has never been my strong point. I'm just looking for some disussion on how these metals react and what is typically used to prevent corrosion. Thanks in advance for the help.

 

[MiketheEngineer]

Usually a "gasket" of some sort is inserted between the AL and FE - or something along those lines - so that they cannot touch.

"Painted on" materials will probably wash away after a few years - I would think.

 

[CoryPad]

Copper is a horrible choice. You can research "galvanic corrosion" to learn more. Depending on your exact configuration, you may be able to coat one or both of the parts to reduce corrosion.

 

[Guest102023]

Whatever you do, have the equipment inspected regularly, including NDE, although I am sure that is a legal requirement everywhere.
This would not be the first such structure, find out how other people have solved it. Google?

 

[racookpe1978]

Would he gain any advantage by using stainless steel fasteners, and stainless steel strip between the Al and the structural steel? The SS would be less aggressively attacked than the copper by both Al and steel..

 

[MintJulep]

This is one of those areas where I only know enough to know that I should consult with someone who knows more than I do.

Definitely something that needs to be engineered, not left to the installation contractor to slather on some goop. And yes, copper is a horrible choice here.

The aluminum is probably anodized.

The galvanic potential of aluminum varies quite a bit with grade, and so far we don't know the grade.

The bolts are probably plated, and the plating is probably zinc.

Since this is a window washing track there will be chemicals involved. Those chemicals will most likely be alkaline, and if intended to be mixed with water will be mixed at a much higher concentration than instructed on the bottle.

When the probably zinc plated bolts are tightened down on the probably anodized unknown grade aluminum the anodized film will probably crack locally, causing potential sites for highly localized galvanic corrosion to occur.

The probably zinc will corrode preferentially to the aluminum, which is probably the preferred situation. However eventually enough thickness may be lost to allow the bolt preload to go away, which is probably not a really desirable failure mode.

 

[CANPRO]

Hey,thanks for the info. The contractor has since changed the bolts to stainless steel, and we're going to use a teflon or some equivalent as a washer. I was researching galvanic corrosion when I realized that the copper could be an issue, but its always assuring to get a second opinion. Thanks again for the input.

 

[KENAT]

Stainless and aluminum aren't always the best play mates, depending on environment, grade, treatment on the AL etc.

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[MintJulep]

Why are you allowing the contractor to make engineering decisions?

 

[CANPRO]

It wasn't the contractors decision. Stainless steel was originally specified, and when they used standard bolts I started to research how these metals would react. I realize stainless steel and AL aren't great together, which is why we're using the teflon washers to reduce the contact area.

 

[Kwan]

You can cadmium plate the bolts. Cad is closer to alum than stainless in the galvanic series.

I would also wet install the fasteners with sealant (look for a sealant that contains chromates like PS 870 from pro seal).

If someone's life depends on this joint then you want to ensure inspections occur (remove bolts and inspect for cracks and corrosion).

 

[MintJulep]

Soft teflon washers in a structural joint aren't such a good idea.

 

[KENAT]

Cadmium plating is over rated for some applications in my opinion/experience, and by many definitions nasty stuff to be avoided, it wouldn't be my first choice.

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[Kwan]

Kenat does have a point. There are other choices that would be even better such as aluminum coated fasteners. I would stay away from zinc because it would be more corrosive compared to cadmium.

Teflon washers might help but it seems to me that you will still have moisture enter the joint. Note that teflon is a bad choice if you have a structural joint. The washers will deform over time and let in moisture. High rise, window cleaning, and teflon washers are words I wouldn't want to mix together.

I would definetly wet install with sealant. Further, you can coat the heads and nuts with sealant. If you want to add even more protection to the joint you can coat with corrosion protection compound.