I didn't notice any mention of eliminating the electrical connection between the anode and cathode. This should cause a substantial reduction in galvanic action.
I kind of touched on this in my reply to Ed. Yes, if you eliminate the conductive path between the metals concerned, you will eliminate the effects of galvanic corrosion. However...
a) Given that we're discussing, in most cases, two lumps of metal connected by a path of liquid (see also 'connections' below), it is likely to be prohibitively difficult or expensive to make it totally non-conductive - also, even if you make your liquid non-conductive you are not necessarily making it non-corrosive or non-oxidising, which brings me to...
b) You may not want to
eliminate the effects of galvanic corrosion. You won't necessarily eliminate the effects other corrosion (see above), and you can use the galvanic corrosion effect to your advantage by setting up a 'sacrifical anode' (a lump of disposable metal that is more anodic than the ones you want to preserve), that will corrode faser than - and thereby protect - your important lumps of metal.Connections
Can the electrolyte serve as the return also? Or does GC require a separate connection? Charliek's references indicate the second
That's a good question, and I'm not 100% sure of the whole answer (bear in mind that I'm no expert on any of this - I've just been reading)
What I am sure of is the fact that you don't need a 'loop of water' to create a 'send and return' circuit like you'd expect in a traditional electrical circuit.
If you get a bucket of tap water, and lob in a lump of zinc and a lump of silver, there'll be an electrical current between them and the process of galvanic corrosion will corrode the zinc faster than it would have normally corroded
, and the silver slower than normal.
The process involves the exchange of metallic ions in the water (the electrolyte), there are positive 'cations' and negative 'anions' that are attracted to the anode and the cathode, respectively, through the electrolyte. This is in danger of getting over-technical, and it has been a long time since my 'O'Level chemistry so, suffice it to say that the current flows in both directions through the water.
Galvanic corrosion, remember, is in addition to
all the normal reactions one would expect. A lump of iron in water will rust, because water is an oxydising agent - it forms iron oxide, rust, on the iron, corroding it. Chuck a lump of magnesium in, and there develops a galvanic relationship between the two - the zinc is more anodic, and so will corrode faster than it would have on its own, the iron is more cathodic, so its rusting will slow down. Substitute silver for the zinc, and the opposite will occur - the iron is now the more anodic, and will rust like anything while the silver will remain smug and shiny for longer than it would have on its own. These processes all use the same water - it is at the same time an oxydising agent, and a conducter (an electrolyte).
I'm guessing that you found the reference to the sacrificial anode, in which it was attached to the metal that it was there to preserve by a wire, as well as by the water. I am unclear as to why this should be necessary, there is already a connection because they are in the same electrolyte after all. My guess is that the wire is less resistive than the water - i.e. it makes the path between the two metals the 'path of least resistance' - and thereby guarantees that the anode-cathode relationship between them is as efficient as possible.
To put it another way, because the zinc is connected to the copper by a wire, the copper will concentrate all of its cathodic energy on eroding the easily-available sacrifical zinc, and wont touch the aluminium at all.
And now I think I need a lie down.