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Aluminum - Steel Contact Considerations

Aluminum - Steel Contact Considerations

Aluminum - Steel Contact Considerations

(OP)
I was always taught to avoid contact between aluminum and steel in that they would initiate rusting due to their differences in the galvanic scale.  (http://www.corrosion-doctors.org/Definitions/galvanic-series.htm)

In dealing with conditions with aluminum and steel, we've called out thin gaskets between the materials and specified stainless steel fasteners.

However, in looking at the scale, I see that stainless steel is typically farther away from aluminum than plain steel is.  This suggests that rusting is MORE likely between aluminum and stainless when compared with aluminum and mild steel.

What do you typically do with this situation?

RE: Aluminum - Steel Contact Considerations

JAE...I think what you're doing is exactly right.  Even though aluminum is anodic to steel, you'll typically find that it's the steel that corrodes more when in contact with aluminum.  The reason for this is that initial corrosion occurs first in the aluminum and forms a passivating oxide layer, then the effect reverses and the aluminum stays intact while the steel's oxide layer (rust) doesn't passivate the surface and general corrosion takes place.  Aluminum's corrosion products (generally aluminum oxide except when there's other influence like chlorides) tend to protect the surface of aluminum form further corrosion; whereas, steel's corrosion product exacerbates corrosion.

If you want to see a good example of this, look at the cast aluminum accessories on a chain link fence gate.  The carbon steel screws will be corroded while the adjacent aluminum only has a dull gray surface.

Typically stainless steel and aluminum fair alright together.  Some alloys of stainless will start to discolor and corrode.

I deal with aluminum structures routinely and use the same technique you described when contact is necessary.

RE: Aluminum - Steel Contact Considerations

I agree with Ron.  What you've done seems right.  The SS fastener marginally increases the corrosion of the base metal (aluminum and bare steel in this case), but since there is so much more surface area of the base metal it's typically not an issue.

RE: Aluminum - Steel Contact Considerations

Also, make sure that you are using a table for the proper electrolyte. The one that you have referenced is for seawater which is appropriate for a coastal seawall, etc. However, the document states "A 'galvanic series' applies to a particular electrolyte solution, hence for each specific solution which is expected to be encountered for actual use, a different order or series will ensue."

For a less aggressive environment, perhaps the material separation in the table is less dramatic.

Here is a nice summary of Ron's and SEIT points:
http://engineer.cmiwaterfront.com/whitepapers/aluminum_corrosion.pdf

Note that this paper states that the Aluminum Association approves of stainless steel fasteners for use with structures (see page 4).

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RE: Aluminum - Steel Contact Considerations

(OP)
Thanks all.  appreciate it.

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