Chemist needed - Zinc anode deterioration

dstutto

Member
Feb 25, 2021
19
NY
Pool Size
3000
Surface
Vinyl
Chlorine
Liquid Chlorine
So, this is what I did: In order to minimize harm to my pool walls (behind vinyl liner) and all the stainless steel submerged in my pool water (endless pool), and all the other equipment hooked up to the bonding wire, I installed a zinc anode to the same stainless steel plate that the pool water bonding wire attaches to. See attached picture. At the bottom you will see an actively corroding zinc anode which I purchased from a marine supply store. At the top, you can see a #8 bonding wire entering the pool and attaching to the top of the stainless steel panel. The bonding wire then bonds the pool panels, I-beam, nearby door frame, concrete rebar, and pool equipment (all the metal within 5' as required by the NEC).

Why the zinc anode? That is a very good question. Basically my understanding is that corrosion will occur from the "softest" (my word choice) most willing metal in the loop, by emitting electrons. If that's the pool walls (which was the case when I disassembled the pool from the prior owner), or the pool equipment, etc. then that will be hard to detect and expensive to repair. In corrosive marine environments, the corrosion is directed to zinc anodes, to save more expensive propellers, etc. By placing a willing anode in the loop, I have given myself a way to monitor the extent of corrosion and a way to simply replace the least expensive part (about $2 or something).

So what's the problem? ... Well, it's corroding way faster than expected. The pool as only been installed for a couple weeks, and the anode is corroding to the point that the zinc oxide (I'm pretty sure that the chemical resultant) is floating to the surface (see additional picture). Wow. Luckily the skimmer is located right above the anode and should mostly collect the zinc in the filter.

Question #1: Should this really be corroding this fast? I thought that in a balanced pool, things were in basic equilibrium and shouldn't really decay. Honestly I thought installing the zinc anode was me being way over cautious and that it would probably never show any real sign of decay. Does this indicate that there is some electrical load being applied to the water which is accelerating the zinc decay? I guess I could disconnect the copper wire and put my multimeter on it to see if I can detect a voltage or amperage with various equipment running. I'm not sure my multimeter would be sensitive enough to detect a very small current/voltage.

Question #2: This is much more concerning ... It appears that the zinc (or zinc-oxide) is reacting with the stainless steel and causing it to corrode somewhat. That's exactly what I'm trying to avoid! In the photo, you can see a blackish haze surrounding the zinc anode on the stainless steel panel. The black soot wipes off easily and leaves an inky/ sooty residue on my fingers. You can see where I've wiped around the anode in places. Even more troubling however is the the texture of the stainless steel bench directly below the zinc anode. It feels rough and pitted. Is the zinc-oxide residue also sinking and settling on the stainless steel where it's reacting and causing some sort of corrosion or buildup?

If there's someone who understands the chemistry here, your insight would be appreciated.

The pool is fairly well balanced with PH 7.5, TA 110, CH 150ppm, CL 2-3PPM, CYA (indoor pool) only about 10ppm but working on slowly increasing it to 20-30.

Thoughts much appreciated. Thanks
 

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JoyfulNoise

TFP Expert
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May 23, 2015
18,605
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Definitely need to see better pictures and maybe a simple diagram of your layout.

This is much more of an engineering problem than a chemistry issue which is why I don't recommend people play around with sacrificial anodes, it's not as simple as people think it is. There's actually a lot of sophisticated engineering and design that has to go into anode placement and layout to avoid what you have here which is just a hunk of metal corroding in water because two dissimilar materials are in contact in an aqueous environment. In a very simple sense, yes it is perfectly reasonable to see the amount of material that the zinc is shedding off because you have an ENORMOUS cathodic surface area (all the steel and metallic parts you are trying to protect) connected to a teeny-tiny little chunk of zinc. The mass and surface area ratios of anode-to-cathode are way off. Normally, to protect a metal surface, you want the surface area's balanced which, as you can immediately see, would be impossible in your case because of the amount of steel involved. So one aspect of a proper design would be having multiple anodes connected in close proximity to various parts of the system, ie, having a zinc anode attached to each panel of the steel walls used to construct the pool. As well, you need to know the amount of impressed current you want so that you can accurately gauge the mass of zinc needed so that the anode lasts more than a few months. These design processes are not simple and often require some trial and field-testing in order to get a balanced system in place.

The zinc anode is producing mostly zinc hydroxide that could slowly convert to zinc oxide. There may be a small amount of zinc carbonate as well. Either way, the anodized zinc compounds will slough off the anode and could float if the material is finely grained enough or mixed with air bubbles. This is another reason not to submerge an anode but to properly bag the anode and fill it with a mixture of materials (clay & sulfate based salts) that allows the zinc ions to migrate away from the anode more evenly. Normally one would not want to put the anode in the pool water but have it placed outside the pool and buried nearby in the soil. Again, all of this speaks the engineering needed to get this done properly.

Was the zinc anode welded to the stainless steel? How was it attached?

I really don't think your current design is doing you any good. The zinc is going to eventually corrode away to a small enough mass and area that it will stop protecting anything it's attached to. You're going to get localized corrosion and buildup of material on the stainless steel around the anode due to crevices and microscopic cracks in the surface.

My suggestion to you is to not play around with sacrificial anodes in the pool water. You could consider burying an anode outside the pool area and attaching it to the bonding wire if you really feel that it is necessary. But even a single buried anode is only going to offer limited protection to a layout as complicated as a pool structure.
 
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