Conflicting problem...galvanic corrosion and floating metal!

[philharm]

I am running into a conflicting problem. I am trying to bolt down a small aluminum casing on a large nickel plate....due to the fact that we need to ground this aluminum casing to the Ni plate. In order to accomplish that, I need to rid of the anodized surface of the Al.... now I got a new potential problem--galvanic corrosion. How can I resolve this? If I prime the Al surface with some sort of conductive primer such as the black BR127 esd or a conductive paint of some sort, will this help with the galvanic corrosion issue? Or anyone has any other ideas?

Much thanks in advance for your help!

 

[SJones]

Conductive paint on the aluminium will not help matters. Aluminium is the anode in the couple and it would be best to paint the cathode, nickel, with a protective coating that is insulating if possible leaving only the smallest area at the fasteners exposed for contact. What bolting material is proposed and what will be the operating environment of the assembly?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[philharm]

Thanks for your response! Unfortunately painting the cathode is not an option. The operation environment is not really a concern here.....the problem is the very strict contractual requirement from our customers. We have strict requirement that no ungrounded metal is allowed, and all disimilar metals have to have less than a potential difference of 0.50....

Right now the Mechanical folks decided instead of getting rid of hard anodized on the bottom of AL in contact with Ni, they decided to rid of the top layer. bolt it down with a CRES washer in contact on the top surface. Now I am more worry about the huge EMF difference between CRES and bare Al. Thus a new question arise...what is a good surface ratio of cathode(CRES) vs anode(Anode) where you are comfortable to say that your risk of anode corrosion is going to be small?

Thanks!

 

[TVP]

In what environment (temperature, chemical species, etc.)?

 

[philharm]

Very benign. Say 60%RH usually, 110F max. very infrequent exposure to humidity by the ocean, thus low salt level.

Thanks in advance for anyone's input!

 

[libriut99]

philharm,

If the ref. I am looking at is correct then you have a potential difference of over ~0.8 [V SHE] between Ni and Al.

Also, is it possible to insulate the Al casing from the Ni plate with a non-conductive coating or plate ? and use ceramic washers and / or bolts. Its not an ever lasting solution since over time you will still get galvanic coupling but as a last resort it could help.

libriut99