Need isolation plate material between aluminum and concrete 1

[maxwolf]

A streetlight base made of aluminum (357-T6) is to be bolted to a reinforced concrete foundation. The 4 foundation bolts are hot dipped galvanized steel.

The outside of the light base is subject to urine, deicing salts and sea air.

I first considered some kind of plastic plate (ABS for example). But am wary of a plastic cracking. A stainless steel plate seems a mistake given the large contact area with the aluminum, their galvanic potential difference and the probability of long term wetting, at the edges and from capillary action between the base and the plate. Also, some streetlights are vandalized and poorly maintained so the interior may be exposed to the elements as well at times if the access door is damaged or left open.

I've heard of bituminous coatings for aluminum, but I think I need to ensure that the coating will not be scratched thru during transport or installation.

Any suggestions for an isolation plate material? Please see attached sketch.
Many thanks.

 

[racookpe1978]

Isn't the greater problem going to be between the 4 galvanized bolts/washers/nuts and the AL base?

I haven't heard long-term (or seen) Al - concrete problems?

 

[CoryPad]

Concrete can be basic (high pH). In general, aluminium is passive in acidic conditions, but corrodes in basic conditions.

Al pole bases regularly are in contact with concrete, galvanized fasteners, and stainless fasteners. All of these conditions can lead to Al pole base corrosion.

The plastic plate is a bad idea because it will creep.

Coating the aluminium pole base is an option. An epoxy basecoat for corrosion resistance with a urethane topcoat for ultraviolet radiation protection would be a good idea.

 

[maxwolf]

Thanks for the comments. I did a little more reading.

For the HD galv. steel bolts in contact with the aluminum base:

"...Doyle and Wright [2]show that the rate of corrosion of aluminum does not increase from being in contact with galvanized steel. They conclude: "zinc is very compatible with aluminum in all atmospheres, in several cases showing that aluminum was actually cathodically protected by zinc."
Codes have incorporated the above results. For example, the Aluminum Association in its structural code states that there is no need to separate or paint galvanized steel surfaces in contact with aluminum. (Aluminum Design Manual [3], pg. I-B-62)."

[2] Doyle, D.P. and Wright, T. E., "Quantitative Assessment of Atmospheric Galvanic Corrosion", Galvanic Corrosion, ASTM STP 978, H.P.Hack, Ed., American Society for Testing and Materials, Philadelphia, 1988, pg. 168.


For the Aluminum base in contact with the concrete:

CoryPad, your suggestion of a coating, epoxy... would that be for the bottom of the base in contact with the concrete foundation?

For the exterior (and possibly interior) of the aluminum base, they will probably do some kind of conversion coating. Something like a chromate based process, but more environmentally friendly.

Thanks again.

 

[CoryPad]

Yes to coating the contact surface with the concrete. Conversion coating is the most basic surface treatment and improves corrosion resistance, and it is a good pretreatment to enhance coating adhesion.

 

[EdStainless]

And make sure that this can drain. trapped water will destroy and good steps that you have taken.

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