Dissimilar metals don't play well together

[Chad Fabry]

When I was performing QA inspections for a major PV manufacturer, I always took a photo of dissimilar reactive mixes of materials. Every time I included a photo and a description of the issue, they told me it wasn't a problem and to stop reporting the condition.

Well, their competitor just recognized the problem and announced it here.

Thanks to Brian Connely for bringing this to my attention.

[kurt]

Surprise. Who'da thought using copper was wrong?

A long while back, you described the phenomenon where the material with less mass, or some similar size relationship, was the one to endure, with the smaller mass material being the sacrifice. It was in relation to copper/galvanized pipe connections, and why it doesn't always mean deterioration of the less noble (or either) metal.

Or something like that. What was that?

[Marc]

Anodic Index

The greater the difference in anodic index values, the greater the galvanic reaction between them will be, provided that the conditions for it are present.

It's not about size.

For example, the difference in AI is 0.45 for aluminum/brass contact but only 0.10 for tin-plate/aluminum.

In offshore jackets used for supporting production/operating oil platforms in the Gulf of Mexico, large bars of zinc are attached to the underwater portions of the unpainted, uncoated steel jackets. The zinc corrodes by galvanic reaction, sparing the steel structure from such damage. Every once in a while divers go down to replenish the zinc.

Marc

[Chad Fabry]

It's not about size.

I'm pretty sure everyone here is familiar with the galvanic scale and electrolytic reactions but what Kurt was talking about is that mass matters- the more noble material doesn't always go unscathed. Area and shape of the materials, mass, what acts as an electrolyte and the type of connection

between the materials all play into which piece fails first.

[kurt]

Right. That thing.

Is there a name for it, or a unifying principle, or something? I seem to recall you describing the anodic relationship to another principle.

I ask, because I see old galvanized/copper connections all the time that are just fine. Not necessarily taped joints either; plain old doped joints, and even bare....they don't go bad. Not that I can see anyway, even with dismantlement.

Does the integrity of the bonding/grounding have anything to do with it? I've heard that theory proposed with no underlying reference or support.

[Jim Katen]

If there is such an explanation, relating to relative mass, then I'd like to hear about it. My understanding of galvanic deterioration is that mass shouldn't matter.

I also see copper-to-steel connections fairly often and deterioration of either metal is the exception, not the rule. But I don't think that it has to do with the relative mass. I think it's more likely that fresh water is just not a particularly good electrolyte and scale on the inside of the pipes might provide some electrical insulation to slow down the reaction.

[John Kogel]

The only thing I can come up with is that if there is a poor connection between galvanized steel and copper, then the copper will tarnish as if the zinc or steel was not there.

How about a bare copper wire on the negative terminal of a battery, left hanging in salt water beside an aluminum boat? Would the copper be eaten up?

[kurt]

I don't know, but I'm always amazed how the sacrificial anode on a boat will go totally to blazes.

I thought Fabry had an explanation the intricacies of which metal tends to degrade, where it's not necessarily always the less noble metal.

How about the copper-aluminum flashing thing....the aluminum step flashing with the copper counter? I've seen them go quickly, and I've seen them withstand a couple decades of exposure.

I'm just trying to figure out how it actually works; I know the theory, but the theory doesn't always play out.