Christer:

A chromate conversion coating (one brand name is Iridite) does not
change the electropotential of the material.  Rather the substrate
(aluminum) is coated with a non-conductive layer similar to immersion
plating.  Another possibility to avoid galvanic corrosion might be an
anodizing process.  In any case, the coating is quite thin and an
adverse environment will cause corrosion eventually.

Hope this was of some help.

Don Vischulis
[log in to unmask]

[log in to unmask] wrote:
> 
> Subject: corrosion
> From: [log in to unmask]
> To: [log in to unmask]
> 
> 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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