Consulting the Metallurgy Oracle

[Nanohedron]

Fed up with the regular recurrence of slimy gunk in my ultrasonic humidifier's working end (lowermost section in the pic)...



...and remembering that copper is well-documented for its antimicrobial properties, I tried putting US pennies in its reservoir (blue section, about 1 gallon), and it definitely made a difference. I found that it took ten to properly do the trick; slime cleanup went almost to nil. A couple of pennies more might have been better yet, but the old horse had three years in and finally broke down, and I already had other ideas. The drawback to pennies is that they are loud and rattly when you have to refill the reservoir, and for me it's irritating enough to make me cast about for other ways to use copper without the noise. The most obvious solution: Copper mesh. It's the same thing as those copper pot-scrubbers you can buy in the cleaning section at a grocery store; I wager those stand to have way more surface area than a dozen pennies, and something spongy-sproingy like a copper mesh scrubber isn't going to rattle.

The concept sounds good on the face of it, right? So I made sure I got a 100% copper scrubbie; plated would probably have done, but for my purposes, better to err on the side of overkill. I popped it in the new humidifier's reservoir and congratulated myself for being a smart cookie. But then I found that in this case the scrubbie soon fouls the water with a reddish haze that looks like iron rust, which of course it can't be. Wherever the water rests inside the humidifier, it leaves a rust-red film.

Obviously this cannot stand. Any ideas about what I'm dealing with, here? The pennies never did that in the slightest.

[DrPhill]

Are american pennies copper?

https://www.vedantu.com/chemistry/copper-oxide

Edit: https://en.wikipedia.org/wiki/Copper(I)_oxide

[Nanohedron]

Are american pennies copper?

Not 100%, anymore; modern mintings are just copper-clad. Since 1982 to present, "They are composed of an inner core alloy of 99.2% zinc and 0.8% copper with an outer plating of pure copper, for an overall composition of 97.5% zinc and 2.5% copper ..." (Wikipedia)

In any event I should think that the copper plating and the mesh product would behave similarly in water, but clearly this is not the case.

I was hoping that maybe after a few refilling cycles the fouling would abate, but so far it's not happening.

[Dan A.]

I'm hardly the world's foremost authority on metallurgy, but I can see a few possible reasons the pennies were more effective than the scrubbie. One, the zinc had some role in prevention of slime buildup. Two, the difference in construction of the scrubbie and the pennies somehow had something to do with the problem. Three, the 100% copper scrubbie was not as advertised. Four, something in your water supply (assuming you use city or well water in the humidifier) caused an adverse reaction with the scrubbie but not the pennies. This mystery has me greatly intrigued.

[daveboling]

Your red residue could be copper chloride (CuCl2). If your water is chlorinated, the pure copper scrubbie could provide enough copper for the salt of copper to precipitate. You said the precipitate was reddish, which would strengthen the possibility. The scrubbie has many times the surface area of a few pennies, so more copper is available for recombination.

dave boling

[DrPhill]

Or the red could be copper oxide.
I wonder if there is any possibility that the zinc/copper combination in the pennies is forming an electrolytic cell and causing zinc salts to form instead of copper? It is ages since I was a chemist, but I vaguely remember an 'electrochemical series' of metal(loids) that can be combined to generate current. Zinc and copper
My memory may be faulty, or the nature of chemistry may have changed in the eons since I was at school....

https://en.wikipedia.org/wiki/Lemon_battery

[Nanohedron]

Thanks for the input, folks. It's evident that whatever the cause, an entire copper scrubbie is in this case going to mean scads of that red precipitation (yes, my tap water's chlorinated, if that plus an overabundant copper surface area is the culprit), so I threw in the towel, took out the scrubbie, cleaned everything up yet again (just one day passed and it looked like a year's worth of neglect), and went back to pennies - a dozen, this time. So far the water appears crystal clear.

After that, I can put up with the rattling.

I suppose the thing to do would be to calculate the surface area of a dozen pennies, calculate its equivalent for the copper gauze, and try that. Alas, I am not a mathlete, so instead I'll have to settle for putting a dozen 1¢ coins out of circulation. I await a Federal knock at my door.

[Dan A.]

I suppose the thing to do would be to calculate the surface area of a dozen pennies, calculate its equivalent for the copper gauze, and try that. Alas, I am not a mathlete, so instead I'll have to settle for putting a dozen 1¢ coins out of circulation. I await a Federal knock at my door.

The U.S. Mint has a page on coin specifications. With that information (and a calculator) at hand, I calculated a penny's surface area to be nominally 0.88 inches.

Nano, I don't believe you need to worry about the Feds coming after you...but I apparently have way, way too much time on my hands.

[Nanohedron]

With that information (and a calculator) at hand, I calculated a penny's surface area to be nominally 0.88 inches.

A most helpful beginning! At twelve pennies, that would be 10.56 inches of surface area. Now, how to find that same surface area in a section of copper gauze? That's going to be the real work, I should think. This particular iteration of copper gauze is woven (knitted, actually) in tubular form, BTW.

You know, even if we get this figured out, if I'm to give it a try there still remains the problem of getting all those damned pennies back out of the reservoir, now. Gonna be a whole lotta shakin' goin' on...

[Nanohedron]

I wonder if there is any possibility that the zinc/copper combination in the pennies is forming an electrolytic cell and causing zinc salts to form instead of copper?

On strictly no basis of expertise, I would assume not; US pennies are 100% copper-clad. This is done by electroplating, so on a proper penny no zinc will be directly exposed to the elements, and quality control at the US Mint is pretty tight. Could water leach through the copper surface? The cladding is around 20 microns thick - about the width of a fine human hair - so although the layer may be regarded as solid, I know nothing of its permeability, if any, at that thickness. But I can tell you that when left to oxidize far enough, modern copper-plated pennies display verdigris no differently than solid copper. So, I'm not too convinced of the possibility of a copper-zinc interaction at work, at least on the penny's surface.

[chas]

When I saw the OP a few minutes ago, I was going to say, this isn't a metallurgy question, but other posters have gotten to the chemistry angle before I did. What would happen to copper would be very dependent on the particular chemistry of your water. I had a problem with a cooling system for a magnet, where the water was ultrapure, and the filters were being clogged with copper dust. Turns out ultrapure water is quite corrosive. The dust clogging the filters was really copper, which somehow was being knocked off the walls of the magnet tubing; we're still not sure how. The way we combatted it was to bubble in some oxygen gas to increase the pH.

The other angle is acoustics. Ultrasound can affect the situation more than most people are aware. I've had thin coatings blown away by ultrasonic cleaning. DrPhill's suggestion that zinc could play a role is quite possible with ultrasonic input. The electroplated copper could have thin spots or pinholes where the ultrasound creates higher pressure and blasts particles away. And zinc is also an antimicrobial and a very powerful antifungal. I suspect the copper scrubbers are of pretty low quality, and could definitely see them being blown apart by the ultrasound.

[Andro]

Why not use distilled water?

[Nanohedron]

Why not use distilled water?

Because of the 3 Es: economics, environment, and entanglement (in no particular order). Off the top of my head, 1) I would have to pay a lot more for just one gallon, never mind the parade of gallons to come, than my one-time investment of 12¢; 2) I would have to lug it, which is more lugging and logistics than I have to physically contend with in using pennies just the once; at an average of 2 gallons a day, over time that's a lot of lugging and spondulics, and I have neither the storage space, the disposable income - nor, I must admit, the youth - to go there; 3) My recycling waste would be driven to a ridiculous level - a spot my fellow tenants already occupy apparently without much thought, and that is a club I do not care to join; 4) Even if refilling used jugs were an option, we're still not out of the lugging and cost aspect, not to mention the added monetary/environmental cost of burning petrochemical car fuel to do it; and 5) My city's water quality is actually pretty good; it's why we have quite a few local breweries. All I'm doing is knocking out a few rogue but otherwise-harmless slime-producing biota for my personal convenience, and since the pennies are already sitting there looking at me (they like to hang out on the table), it's an easy decision.

The only way distilled water makes sense is if I could get it free out of a tap in my kitchen; if I set up a still it would be for more entertaining things than water, I can tell you that.

There's always the option of a tank additive - which I have done in the past - but you have to measure it into its woefully shallow little cap, and since the setup's ungainly from the start (so much for forethought in design), the additive overflows onto your fingers every time, so that's not only waste, but a possible hazard to me. Plus, again, an unnecessary cost when a mere several pennies will do the trick indefinitely.

When I saw the OP a few minutes ago, I was going to say, this isn't a metallurgy question, but other posters have gotten to the chemistry angle before I did. What would happen to copper would be very dependent on the particular chemistry of your water. ... The other angle is acoustics. Ultrasound can affect the situation more than most people are aware.

Thanks, chas. This whole episode has opened my eyes to a host of factors that I hadn't ever considered. It's become abundantly clear that what has worked for me won't necessarily apply to all. Still, if you have humidifier slime, whether it's an ultrasonic unit or not I think the copper approach, in some guise, is always worth a try just in case.

[an seanduine]

Just a couple of thoughts. The ´slime´ you object to is often a biofilm caused primarily by fungi and/or algae. A change of ph will often disrupt this. Your ´pennywise´ solution may be accomplishing this. Sometimes a very slight change can do this. Two other things to try is adding a dilute solution of peroxide. . .often concentrations as low as .01 % will disrupt these simple organisms. Another possibility would be adding very small amounts of baking soda. . .although, like ´hard´ water, this can lead to troublesome deposits.
Good luck, stay well, and stay hydrated.
Bob

Edited to add: Perhaps a tiny amount of silver. . .dropping the cost (at face value to 10 cents) by using a pre-1960´s dime?

[Nanohedron]

Two other things to try is adding a dilute solution of peroxide. . .often concentrations as low as .01 % will disrupt these simple organisms. Another possibility would be adding very small amounts of baking soda. . .although, like ´hard´ water, this can lead to troublesome deposits.

Thanks! I knew Chiffers would come through.

Edited to add: Perhaps a tiny amount of silver. . .dropping the cost (at face value to 10 cents) by using a pre-1960´s dime?

I'll leave the further research to you. I've hit upon a viable solution for my circumstances, and I think I'll stick with it.