Star ground with dissimilar metals: concern?

[serengetiplains]

I'm wanting to attach a star ground to my aluminum chassis. Should I be concerned about the electro-chemical effects of dissimilar metals? The star-ground chassis bolt will be non-ferrous, either stainless steel or brass. Wires to the ground will be silver and copper. Should I be concerned about all these dissimilar metals, or any combination of them? Should I solder or bolt the wires to the ground? TIA!

[EC8010]

Keep your hands clean and dry when handling the star ground. Dissimilar metals need some moisture (ideally slightly acidic and salty) to form a battery and corrode. Avoid brass, use passivated screws.

[zigzagflux]

If you are landing all your grounds on top of one another, they are suitably bonded through the silver/copper/tin/nickel surfaces. It sounds like the bonding bolt goes through the aluminum, and all grounds attach to that point. Therefore the connection to the aluminum chassis is just for potential control, not to carry any current. You could probably get away with this, due to low ampacity requirements, but I would recommend an internal tooth washer to dig in to the aluminum.

Does your 3rd prong also bond to this same point? That point would be recommended to have some current carrying capacity, for safety purposes.

FWIW, aluminum is downright nasty as a landing pad. The power industry uses it all the time due to lower cost, but it requires careful attention for a durable connection (indoors or outdoors).

Raw aluminum is VERY reactive with oxygen. Net result is that after you polish aluminum down to a conductive surface, it immediately begins oxidizing. Aluminum oxide is a very poor conductor. Attempt to bolt a copper busbar to aluminum busbar, and you have a disaster waiting to happen. We infrared scan electrical connections; these problems are common.

All of this is exacerbated by the different coefficient of expansion between copper and aluminum. As the temperature changes, the metals expand differently, eventually loosening the connection. Belleville (conical) washers are used to address this expansion (I think the NEC requires them).

Electricians are supposed to wire brush all aluminum surfaces to a very deep and rough texture, following up IMMEDIATELY with a good amount of antioxidant schmutz. Then a very strong mechanical connection, almost to the extent of deforming the aluminum.

We routinely plate our aluminum surfaces to avoid the oxidation problem. Etch the aluminum, then immediately copper plate. Wash the surface, follow up with a different etching solution, then silver plate. Silver oxide is relatively conductive.

Moral of the story: if you know you are going to use aluminum top plate, I would plate the underside prior to assembly. No worries. It's pretty cheap to do, just gotta find the solutions.

[Eli Duttman]

While no simple method for dealing with the self protecting nature of Aluminum exists, a fairly non-fussy approach is available.

Prepare a saturated solution of Zinc sulfate whose pH has been lowered by the addition of a SMALL amount of sulfuric acid. Any place on Aluminum that solution comes into contact with will get a thin plating of Zinc.

The Zinc layer can be soldered to or plated over. If you want to chrome Aluminum, start with the Zinc "tweak". Lay Copper down over the Zinc. Lay Nickel down over the Copper. Finally, lay the Chromium down.

[sorenj07]

Originally posted by Eli Duttman
Prepare a saturated solution of Zinc sulfate whose pH has been lowered by the addition of a SMALL amount of sulfuric acid. Any place on Aluminum that solution comes into contact with will get a thin plating of Zinc.

The Zinc layer can be soldered to or plated over. If you want to chrome Aluminum, start with the Zinc "tweak". Lay Copper down over the Zinc. Lay Nickel down over the Copper. Finally, lay the Chromium down.

Great advice! I remember making a "silver penny" in this way in Chem class. Then we heated the penny on a hot plate to mix the zinc and copper to make them into "golden" brass plating. I think we used a boiling solution of something though. Something to do with saturation? I can barely remember and it was only one year ago

[Looneytunes]

"Prepare a saturated solution of Zinc sulfate whose pH has been lowered by the addition of a SMALL amount of sulfuric acid. Any place on Aluminum that solution comes into contact with will get a thin plating of Zinc.

The Zinc layer can be soldered to or plated over. If you want to chrome Aluminum, start with the Zinc "tweak". Lay Copper down over the Zinc. Lay Nickel down over the Copper. Finally, lay the Chromium down."

Very nice, Eli.

Do you have any information on the other plating solutions? DIY chrome plating would be very cool.

Thanks,
Looney

[piffpaff]

Originally posted by EC8010
... Avoid brass, ...

Why avoid brass? I only ask because I know the pins on old Siemens Ba tubes are made from what looks like machined brass. They are highly spec'd tubes and surely the engineers back then knew what they were doing. Probably made them this way for a reason. Intriguing.

[serengetiplains]

Thanks for your replies, guys. Particularly informative EC, zigzag and Eli.

[EC8010]

Avoid brass because it's a long way away from aluminium on the electrochemical table. According to a book I bought when I was a lad:

Silver 0V
Copper 0.18V
Austenitic stainless steel (0.2V)
Brass 0.3V
Stainless steel (12% chrome) 0.45V
Solder 0.48V
Dural 0.6V
Aluminium sheet 0.76V
Zinc plate 1.1V
Magnesium alloys 1.6V

I discovered the brass/aluminium problem at school when I machined a brass insert, wiped it with my sweaty fingers, then interference fitted it into an aluminium sleeve. Within minutes there was a black discolouration at the interface, so I wiped it off (with aforesaid sweaty finger). It returned. A few months later, I bought the book that explained why this had happened.

As has been mentioned earlier, aluminium oxide is a good insulator (it's the dielectric in electrolytic capacitors, after all). When you make your ground, use a star washer between your earth tag and the aluminium so that it cuts into the aluminium and cuts through the inevitable oxide. The bigger the screw and star washer, the better.

Those Siemens "brass" valve pins are more likely gold plated (to reduce contact problems due to surface oxidation).

[Eli Duttman]

There's always a "rub". Read this Copper plating page. Notice the discussion of cyanide "strike" solution. That POISONOUS stuff is necessary to start the Copper layer over Zinc.

Bottom line, plating can be a very bad idea for the DIYer. Pay the man his money and have a professional plating shop do the work. The toxic stuff involved is (IMO) to be avoided.