FreeLists / si-list / [SI-LIST] Re: Metal to Metal Question

[Chris Padilla <cpad@xxxxxxxxx>]

Galvanic Series

Group I Group II        Group III       Group IV        Group V
Magnesium       Aluminum 2S     Lead-tin Solder Copper-Nickel   Graphite
Zinc            Cadmium         Lead            Monel           Gold
Galvanic Steel  Aluminum 17ST   Nickel          Silver Solder   Platinum
                 Steel           Brass           Nickel (passive)
                 Iron            Copper          Stainless Steel

Galvanic Action

If dissimilar metals are used, a noise voltage may be induced due to the 
galvanic reaction between the two metals. Moisture or water vapor in 
conjunction with the two metals produces a chemical wet cell. The voltage 
developed depends on the two metals used and is related to their positions 
on the Galvanic Series Chart. The farther apart the metals are on the 
chart, the larger the induced voltage.

The use of dissimilar metals can also produce a corrosion problem. Galvanic 
corrosion causes positive ions from one metal to be transferred to the 
other. This gradually causes the deterioration of the anode material. The 
farther apart the metals are in the galvanic series, the faster the rate of 
corrosion. An undesirable but common combination of metals is aluminum and 
copper. The aluminum will eventually be eaten away. Since aluminum and 
lead-tin solder are closer in the galvanic series, the reaction time can be 
slowed down by coating the copper with lead-tin solder.

When dissimilar metals must be combined, try to use metals from the same group

Hope this helps.


----->Chris



>Hello group.
>I have a customer who has an RF metal can made of
>machined Aluminum that bolts up against a PCB. At
>the mating locations between the PCB and the aluminum
>is selective plated Nickel Rails. That multiple
>surface finish requirement has a premium ($$$$)
>and I was wondering what is normal (or typical) in
>the EMC industry. Do most people bolt up against Tin
>Lead (against aluminum), or do they normally have the
>aluminum plated with Nickel?
>
>I did check the electromotive force charts and quite a
>few finishes appear to be "out of the question".
>
>(This is what I have found si far)
>Aluminum (pure)         -0.80
>Nickel                  -0.24
>(Copper                 -0.34)
>
>
>Tin                     -0.14
>Lead                    -0.13
>
>(Possible issues due to fretting or galvanic reaction)
>Silver                  +0.80
>Gold    (I didn't find gold but I know it's a + value)
>
>Let me know what is typical in the RF Industry.
>
>Thanks for any and all replys,
>
>David Hoover
>Field Application Engineer
>Data Circuit Systems, Inc.
>
>
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