steel/aluminum connection

[broekie]

I am working on a building design. The owner wants the stairs made from aluminum members. These members will be in contact with some steel supporting members. Do I have to treat the steel members with any special primer, paint, etc?

I thought I remembered that you don't want aluminum to come in contact with galvanized steel, but some stuff that I have read in the last day or so seems to contradict that.

The "AASHTO Specifications for Strucutral Supports for Highway Signs....." says that, "Steel surfaces to be placed in contact with aluminum shall be painted with good quality, non-lead-containing priming paint, such as zinc molybdate, alkyd type primer in accordance with Federal Specification TT-P-645B, followed by two coats of paint consisting of two pounds of aluminum paste pigment (ASTM Specification D962-88, Type 2, Class B) per gallon of varnsih meeting Federal Specification TT-V-81, Type II, or equivalent." Does something like this apply? It goes on further to seem to say that it is OK for galvanized steel to be in contact with aluminum.

Thanks in advance for any help/clarification you may be able to offer.

 

[FFIELD]

When dirrerent metals are in contact with each other there is the possability for electric currents to pass through the interface. This depends on the moisture content at the surfaces and the different types of material surfaces. Something like a battery. If there is a current the metals can corode at the interface. Not being an electrical expert I can only give you the basics. In critical work such as refineries, gas lines, etc. a non conducting gasket or insert is placed between the two surfaces.

 

[JAE]

We would always use neoprene gaskets between the planes of contact.

 

[ERV]

broekie,

The concern may be oxidation, if so, use a stainless steel isolation plate between the two materials.

 

[Thane]

I always take the better safe than sorry approach. Whenever placing aluminum on steel, I place two coats of primer on both the steel and aluminum to create a barrier to prevent galvanic corrosion.

As long as you have a solid barrier (paint, adhesive, primer, some platings) that prevents moisture from contacting both surfaces, you will prevent a chance for galvanic corrosion.

Thane Arheart
Mechanical Engineer

 

[Ron]

broekie...they must be isolated. I like solid isolation such as a gasket, as compared to a coating. Coatings get scratched during assembly.

Don't rely on the nobility charts to back up your argument for isolation if you need it. The charts say that aluminum will corrode when in contact with steel. True, but the first thing that occurs is a surface oxidation of the aluminum which quickly causes passivation, then the process is reversed and the steel corrodes. To show you a common example, look at a chain link fence with cast aluminum features on the gate....all the steel hardware in contact with the aluminum is likely rusty, while the aluminum only has a dull finish.

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[bubb375]

broekie-

Looks like everyone has you starting down the right path. In the aerospace industry, we typically will seal both surfaces with an RTV material to prevent corrosion. Also keep in mind that any fasteners joining the two materials must prevent corrosion as well. So if you are using steel bolts, you may want to paint/seal them as well.

Brian.