How do I fix a potential galvanic corrosion problem?

[crystalmountain]

I have a situation where an aluminum monorail is bolted to steel plates that are welded to steel pipe hangers. The steel supports and top half of the monorail are hidden in the soffit. The steel supports (plates and pipe hangers) are primed.

I am concerned about potential galvanic corrosion on the aluminum monorail at the faying surfaces with the steel plates. Due to practical reasons, I am unable to access the monorail from the bottom, or remove the monorail and install a third metal between the monorail and the steel plates, or paint the steel plates. I can only access to the monorail from the top.

I am just wondering if, at each support loacation, I bolt a zinc plate to the top surface of the monorail and bolt a steel plate to connect the zinc plate to the existing steel plate holding the monorail, would that be a valid solution to prevent galvanic corrosion? Please keep in mind that the monorail would still have direct contact with the existing steel at the faying surfaces.

Any comment or input is greatly appreciated.

 

[CoryPad]

What does "primed" mean? Can you provide more details on this?

Can the Al monorail be coated at the faying surface?

Can you use some kind of spacer between the steel plate and the Al monorail at the bolted joints? The spacer could be steel and coated with an Al-friendly coating (e.g. one from Magni or Doerken).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[crystalmountain]

Cory, thanks for your reply.

The existing steel was spray-painted with primer, one coat only. I don't think that is a reliable separation between the aluminum and the steel.

It is not quite practical to use spacer between the steel plate and the aluminum monorail because the monorail is located in the soffit some 100 feet above the ground and you cannot not access to it from the ground. One can only access to it from the top though the ceiling space.

If I loosen the bolts and paint the surface, then I have to use new bolts. Again, there is the problem of falling hazards of bolts, hardware or tools.

 

[Metalguy]

Can it get wet and stay wet?

 

[EdStainless]

If it can get wet, your only option may be to try and seal the CS-Al joint from the topside.
If you can keep it dry you should be fine.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[crystalmountain]

Since the connections are inside the soffit, they are probably in a dry environment. However, I am not sure what moisture in the air or even fog will do to them.

 

[EdStainless]

How near the coast are you? How high is the humidity?
If these ring bells then I would look into sealants.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

Can you access it from the top side enough to spray a material like "Corrosion Block" to saturate the joint.
I've been using this on ocean going boats for several years. I've used it on electrical, electronic, and any number of dissimilar metal systems with excellent results.

Lear Chemical makes several variations (ACF-50, Rust Block) of this product so a call might be in order

http://www.learchem.com/Homepage.htm

Anecdotal:
I had bunch of test coupons of various displacement and barrier coatings mounted on our plant wide coatings test racks (5). The unfortunate part is that the coatings test program and racks were unceremoniously disposed of without any waring or discussion. One hell of a lot work on my part and many others lost without one bit of information gleaned. My query as to whys and wherefores was answered with the statement: We don't need to generate this information as it's all available on the Internet.