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Aluminum girts and stainless steel screws

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Corrosion is a little like evaporation.  The surface of a metal loses metal ions much like the surface of a pond or container of water loses water molecules.   The rate of corrosion, like the rate of evaporation, depends on ambient conditions - metals typically corrode more in hot, moist conditions; evaporation is typically greater under hot, dry, windy conditions.
 
Different metals corrode at different rates just as different liquids evaporate at different rates. Almost all metals corrode to some extent all by themselves. They can be ranked as to their tendancy to corrode in a particular environment; the ranking is called a galvanic series.  The standard galvanic series is based on the tendancy to corrode in sea water, probably because the maritime industries funded much of the early corrosion research.  Other series have been developed, however, for example tendancy to corrode in soil.  The corrosion is driven by potential differences on the surface of the metal.
 
When dissimilar metals are in contact, it creates a potential difference which can signficantly increase the corrosion of the anodic metal (the one with the more negative electrochemical potential in that environment),  while at the same time decreasing the corrosion of the cathodic metal (the one with the more positive electrochemical potential.)  This is the principal behind galvanizing - the combination of the dissimilar metals increases the rate at which the zinc corrodes, and decreases the corrosion of the carbon steel. 
 
The potential difference, and thus the amount of corrosion, depend on how far apart the two metals are in the galvanic series for that environment.  It also depends on the relative surface areas of the two metals - the rate of corrosion is controlled by the size of the cathodic piece - the one that is not corroding.  The cathodic metal is basically serving as a sink for electrons, which allows the anodic metal to turn into positive ions and go into solution.
 
Aluminum is anodic to stainless steel, and thus will corrode when the two of them are in contact.  If you have a very small piece of stainless steel, though,  you have a very small cathode.  There is not much driving the reaction, so you don't get much corrosion.  There are different aluminum alloys and different stainless steel alloys, each with slightly different electrochemical potentials.  Thus the tendancy for corrosion of the aluminum will depend slightly on which alloys you have used,  but any aluminum-stainless steel combination will have minimal corrosion when the aluminum piece is much larger than the stainless steel piece.
 
To have a practice of not mixing metals because of some fuzzy understanding of corrosion is not real impressive.  It means clients may be getting an inferior design and/or paying way more than they should.
 
Gail Kelley