Bolting Aluminum Beams using Galvanized Steel

[LSUengr2013]

I am having to install an aluminum beam between two steel beams and I have read some conflicting information regarding galvanic corrosion.

Tried to upload some images but doesn't seem to be working. I was proposing to use a galvanized double clip angle to bolt the aluminum beam into the existing steel ones. This was based on the Aluminum Steel Guide 2005 Section 6.7.

"Where aluminum is in contact with or fastened to the materials specified in Sections 6.7.1 through 6.7.3, direct contact between the aluminum and the other material shall be prevented as specified in those sections or by placing a compatible, nonporous isolator between the aluminum and the other material.

6.7.1 Steel
Steel surfaces to be placed in contact with uncoated aluminum shall be painted with a coating suitable for the service. Where very corrosive conditions are expected, additional protection can be obtained by applying a sealant that excludes moisture from the joint during service. Aluminized, hot-dip galvanized or electro-galvanized steel in contact with aluminum need not be painted. Stainless steel (300 series) in contact with aluminum need not be painted except in high chloride environments"

But the more I have dug into the subject it doesn't seem like just having the steel elements galvanized is enough precaution. This will be an indoor application, but large garage doors open it up to quite high levels of humidity at times.

I've also found the suggestion of painting any dissimilar contact surface with one coat of zinc chromate primer and then one heavy coat of aluminum pigmented asphalt paint.

If anyone has any insight they could share to the best path forward the better. Thanks in advace.

 

[Dhurjati Sen]

Once you paint the different metals, you reduce the chances of Galvanic Corrosion significantly. However, a holiday that remained undetected or a paint peel off after a few years could bring Galvanic Corrosion, with a vengeance.

DHURJATI SEN

 

[EdStainless]

What is the risk of it getting wet?
As long as it is dry there is no issue.
If this is someplace where it can see water then multiple layers of sealants would be warranted.

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P.E. Metallurgy, consulting work welcomed

 

[LSUengr2013]

Ed, I believe the chance of it getting real wet are extremely small. This beam is in a covered warehouse, but it does get humid within the warehouse as it is in the Southern US, and the warehouse is not air conditioned, I could see some condensation potential.

 

[rowingengineer]

Generally we isolate the two and use plastic Packers/ wraps and bolt covers to achieve this.

 

[EdStainless]

They will get wet every morning as they will be cool and damp air warms.
Better isolation and cover is needed.
Over time the bird poop would destroy them anyway if left bare.

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P.E. Metallurgy, consulting work welcomed

 

[MegaStructures]

EdStainless would humidity not be enough to support transfer of ions between dissimilar metals? Typically I would always isolate the two metals with a plastic/rubber material as I would be concerned about the aluminum corroding with mild steel, or the galvanized steel corroding for that condition.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[r13]

The paragraphs below is offered by the American Galvanize Association in its "knowledge base". It is offered for information only. Link

"The higher the electrical potential difference the greater the possibility of a reaction. In the case of zinc and aluminum, there is only a slight chance of a reaction because of the relatively small change in potential between the two metals and the formation of an insulating film on the surface of the aluminum.

One of the key factors in the reaction between dissimilar metals is the contact surface area. A severe reaction can occur when a large cathode (higher or more positive potential) is in contact with a small anode (lower or negative potential). In this situation, corrosion rates can increase dramatically.

Another key factor in the determination of corrosion of two dissimilar metals in the presence of conductive substances. In many cases, condensed water does not provide enough conductance to start the corrosion process. Whenever possible, the best solution to this type of corrosion is to prove an insulation barrier between the two dissimilar metals."

 

[EdStainless]

Humidity alone will not support an electrical current, so no corrosion.
But I have never seen an application with high humidity and no condensation.
And in warehouses the condensation along with dust and bird poop will corrode nearly anything.

I could tell you a story about a SS fire protection system in a warehouse, they switched products and stated handling salt in side it. Very spectacular. It was a dry system so pressurized with air. After a couple of years the pitting and external SCC finally caused some leaks. Once the pressure bled down to 20psi a valve opened and filled the line suddenly with 80psi water. Thousands of feet of pipe unzipped and created a deluge.

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P.E. Metallurgy, consulting work welcomed

 

[dik]

I'm not sure (if memory serves), but I think the sparky guys for switchyards I've worked on use silicone-bronze washers to isolate the two.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Continuation of the AGA article.

"The question is frequently asked when two dissimilar metals are placed in contact with each other. The chart below shows the electrode potentials of metals in seawater. If two metals with different potentials are placed in contact and there is a conductive medium, such as sea water or condensation, there can be a reaction, commonly known as a galvanic cell."

 

[LSUengr2013]

Thank you all for the responses, have learned a lot. My current plan is to use a 1/4" thick neoprene gasket material between all aluminum and steel flat contact surfaces and nylon washers and spacers for the bolts.

 

[EdStainless]

Sleeving the inside of the holes is where most people fail at this.
Good luck.

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P.E. Metallurgy, consulting work welcomed

 

[LSUengr2013]

For sleeving the inside of the holes I plan on using a nylon shoulder washer.