You need to be concerned about more than just the EMF differences. Be careful
of
the area effect. Making the surface more noble, and therefore more corrosion 
resistant, is only effective as long as the coating is continuous and intact. 
Any small defect, that will inevitably develop, causes and accelerated attack
on
the aluminum, since its small exposed area is now linked to a much larger more 
noble surface.



     
     Andy Magee - Applications Engineer
     Rogers - Circuit Materials
     Tel: (602) 917-5237
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     Website: http//www.rogers-corp.com/cmu
     
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Subject: corrosion
Author:  Christer.Marklund::(CHRSTR) at ~FABRIK
Date:    2/27/97 4:50 AM


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Date: Thu, Feb 27, 1997 4:50 AM
Subject: corrosion
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Dear technetters.

A few days ago i asked You a question about stainless steel of type 303.
We had some problem soldering a nickel plated stainless connector to
ceramic. As many of You mentioned the problem with that soldering was a
material in the connector that didn`t match the ceramic.
The connector supplier will change to kovar or some other material with
coefficient of thermal expansion at 10-11 ppm or lower.


A new question:
I sometimes come into contact with galvanic corrosion at different
moisture tests.
We normally use a lot of aluminium in our designs. Aluminium in contact
to other material (nickel, gold etc) in severe environments will make
corrosion damages to the aluminium.
The potential for different material (EMF) could be found in different
litteratures.
Aluminium is normally given a potential of -0.6 to -0.8 V depending of
alloy type.
Stainless steel is about -0.35 V, nickel is about -0.15 V.
A rule of thumb is that the difference between two materials that is in
contact to each other shall not be more than 0.25 V in severe
environment.

My question is: Do any of You have any value of how much the potential
of aluminium is raised when the aluminium is chromated? 
We normally use a cromate that gives a color to the aluminium surface of
golden to tan.

Regards

Christer Marklund

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