Aluminum Alloy in contact with Galv Steel 4

[chasan]

Would the contact of Aluminum Alloy 6061 and Hot dipped galvanized steel result in a corrosion problem if subjected to outdoor exposure. The assembly would be above ground and permit water to drain.

 

[forensiclab]

If zinc is electrically connected to aluminium and both are under water or connected by a film of water, then in theory the corrosion of the zinc will be accelerated. Factors that will influence whether your case will develop a problem would be as follows.

1. Relative surface areas. The greater the aluminium to zinc surface area ratio, the more the corrosion current. If this were a problem, which I doubt, it could be overcome by painting the aluminium!
2. If the items are self draining, then the amount of time they share a water film will only be while it is raining. Avoid pooling.
3. If there is a thin film joining them for longer (dew, monsoon) then the resistance of the thin film would ensure the only part of the zinc that would be affected would be that area next to the aluminium.

 

[StressGuy]

It's been a while since I've worked with an application using aluminum pipe. However, as I recall, we were required to add an isolation pad between the aluminum pipe and the galvanized structural steel. Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[chasan]

Forensiclab,
Thanks for the information. Would SS be a better choice and if yes, what series?

 

[rustbuster1]

Hold on now. In reality there will be no significant galvanic corrosion between zinc and Al (6061) in regular ambient weather exposure - assuming only occassional wetting and moderate time of wetness. This is mostly because the Al alloy forms an oxide surface layer, conditions will not turn alkaline enough at the Al cathode oin the couple to cause corrosion, etc.
Before you rush over to stainless steel, it would help you (and us) if you describe the arrangenment here more clearly, as I see no reason for doing that yet.

 

[chasan]

rustbuster1,
The design involves a truck assembly having a main body made from cast aluminum. A shaft approx .5" dia is to be press fitted into the main body or possibly held in place with snap rings if the press fit concept is not possible.The mass of the steel components is approx 25% of the aluminum mass. The assembly is intended for outdoors and is inserted into an aluminum channel where it is allowed to move on galv steel wheels. The open slot in the channel faces the ground.
The expected life is 15 years.
chasan

 

[SolarWeenie]

Generally speaking - aluminum and zinc are compatible outdoors, as they are very close in the galvanic series. Stainless and aluminum are very far apart and may be incompatible. However, if the zinc layer is depleted and the base steel exposed, galvanic corrosion may occur between the aluminum and the base steel.

 

[PUMPDESIGNER]

I agree with rustbuster1 on caution.
We have been clamping hot dip galvanized steel pipe to aluminum skids for many years and we have never seen any signs of corrosion or deterioration. We service these systems as much as 15 years later and if anything serious were to go on we would have seen it.

PUMPDESIGNER

 

[sunstructures]

From the past articles in this thread it seems my question might be answered, but let me put some specifics and see what you corrosion 'gurus' think about this. It's an already built structure, so I need to know if it is worth the trouble to modify it.
We are currently reviewing an installation that was done on some roof overhang exposed steel framing. Some light architectural glass panels were added. The panels are aluminum framed and attached to support through a prefabricated system of aluminum rails and extruded angle clips. The aluminum is 6105-T5. The aluminum angle clips rest directly on hot dip galvanized structural steel framing, ASTM A501 or A53 Grade B, and are secured to the steel by Type 410 stainless steel self-tapping screws.
The system is well-drained, and not exposed to any particularly harsh environment here in Los Angeles. It seems that galvanic corrosion potential in the dissimilar metals should result in the hot-dip zinc galvanizing acting as the sacrificial element. My check of a galvanic series table shows zinc being more anodic than any aluminum alloy. Also, zinc and aluminum appear closely together on the galvanic series, which leads me to believe that the corrosion potential is low and that the galvanized coating on the steel in the area of the connection would be durable for the life of the structure.
THANKS!