Risk of galvanic corrosion of dissimilar metals in bonding the chassis to Earth?

[loudthud]

A place I used to work that used aluminum chassis pieces usually specified chromate and etch as a finish. They used stainless Pem nuts. Does the finish have any effect?

Edit: A couple of refugees from that place and I had a rude awakening when we found out how much that finish cost at another company.

[SoulFetish]

A place I used to work that used aluminum chassis pieces usually specified chromate and etch as a finish. They used stainless Pem nuts. Does the finish have any effect?

Yeah, that works too and would be fine. Chromate works to passivate aluminum (and other metals). I believe it also maintains electrical continuity as well. The idea is to prevent all three materials from direct contact with each other. It is the the combination of the two dissimilar metal (anodic and cathodic) and an electrolyte which cause the electro-chemical reaction. All three are needed, so break any link from direct contact and you can prevent the reaction. However, with chromate, the zinc element becomes the sacrificial metal and acts to protect the aluminum.

[J M Fahey]

A place I used to work that used aluminum chassis pieces usually specified chromate and etch as a finish. They used stainless Pem nuts. Does the finish have any effect?

You bet

I paint my aluminum panels with wash primer, a 2 component paint base which does a lot of good things:

1) hardens by chemical reaction (has a base paint and a catalyzer) which is *good*

2) the base contains phosphoric acid which etches aluminum surface and makes it porous, for way better finishing paint adhesion.

3) itīs yellowish because it also contains chromates.

Look who else uses Sherwin Williams wash primer :

[SoulFetish]

Ken, you make a great point about the way they are used as well, ie. Stainless fasteners on aluminum is not nearly as problematic as aluminum fasteners on stainless!

[Chuck H]

I recognized the bi metallic "it's going to be a battery" thing. Since I build on aluminum, and I've seen too many rusty (steel) or powdery (aluminum) chassis connections I looked into it and went with stainless nuts and bolts with a galvanized tooth washer. No, I didn't grease it after the connection. But I'm fairly sure it would only improve longevity based on some older, high quality electronics I've seen that DID use goop of one kind or another. This from a practical, not an EE and never worked in Naval aviation perspective All the info about dissimilar metals is readily available now on line. Not so when I started.

P.S. If you want to see a first hand example of what dissimilar metal contacts can do in a corrosive environment, try storing tomato sauce based leftovers in a steel pan and cover it with aluminum foil. Check in on it after three days

[Enzo]

The chassis is only supposed to be a shield NOT the audio signal interconnection medium.

Of course this is true. However, At various times, I have cured amps of making noise by refreshing the contact of the transformer mounting bots with the chassis surface. Noise can wind up in the audio chain without being generated in it.