aluminum - steel contact considerations

huangyhg

aluminum - steel contact considerations
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.  (
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.
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.
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:
thanks all.  appreciate it.