Why you should never, ever put a "protective anode" on your pool light.

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wgipe

wgipe

Gold Supporter
Jul 4, 2020
490
Fletcher, OH
Pool Size
20000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Oh, what can it hurt? That was the thought when I built the pool. I noticed the rings tarnishing and pulled them tonight to check them out and found this mess. Guess I answered my own question. An hour of scrubbing later, I've got the niches and lights cleaned up and put back together. I only got a picture of the second one. The forst one i pulled had stuff crystallized all over the back. I don't know if it was salt or CYA that crystallized all over everything in there, but I'm thinking the stupid anodes actually attracted the stuff and caused galvanic corrosion of some sort. Oh well - lesson learned. 20220622_195832.jpg
 
They don't call it a sacrificial anode for nothing! I've used them things over many applications in a prior career, and some applications burn away cleanly and some just make a mess on everything. Some are self-fulfilling, as they can create the action that wasn't there in the first place.
 
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wgipe said:
It’s zinc. Model 104F on this page:

PoolTool Anodes- Manufacturer of specialty pool and spa tools

www.pooltool.com www.pooltool.com
Click to expand...

Yeah. So the white crystals you see are a combination of zinc carbonate and zinc hydroxide. It’s similar to the “white rust” see on pool pump bodies when the galvanized coating on the steel undergoes galvanic corrosion. Zinc is not a good choice for submerged application as it will create lots of shedding (many zinc compounds have low solubility in water). In a submerged application it’s better to use magnesium.

Given that the anode was connected to polished stainless steel using a tinned copper wire, you likely created a localized galvanic couple which is what caused the mess. I suggest not doing that.

You may have also degraded the protective passivation coating (chrome oxide) on the steel light housing. I suggest you pull that light housing again in a few weeks, maybe at the end of the season, and check it for corrosion. If it starts to rust, you will need to replace the light.
 
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Something has to be pulling all of the electrons away from the anode and the light (cathode).

Something has to be getting reduced at the light or somewhere else.

Was the light corroded (losing metal) or was something accumulating on the light?

I suspect that there is something more going on.

Something has to be attacking the light and the zinc is pushing electrons as fast as possible, but it cannot keep up with whatever is attacking the light and pulling out electrons.

I would suspect a stray current.
 
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Anodes are a gimmick when used in a pool. There is not enough salt content in a pool. Now if you have a boat or something you run in the salt filled ocean, they are needed.
 
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wgipe said:
I noticed the rings tarnishing and pulled them tonight to check them out and found this mess.
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Can you show how it looked before cleaning?

Keep a close eye on all metal especially weaker metals like the aluminum tracks for the autocover and the natural gas lines going to the heater.

Check for AC and DC voltage between the bond wire and each piece of equipment by disconnecting the bond wire and checking from the bond wire to the bond lug.
 
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JamesW said:
Can you show how it looked before cleaning?

Keep a close eye on all metal especially weaker metals like the aluminum tracks for the autocover and the natural gas lines going to the heater.

Check for AC and DC voltage between the bond wire and each piece of equipment by disconnecting the bond wire and checking from the bond wire to the bond lug.
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I unfortunately did not get a before picture, but it was a light coating of black tarnish which was quite easily removed by a magic eraser. The much bigger issue was getting all the bits of anode cleaned up that had crystallized.

I also did just check for AC and DC current at each exposed bond wire connection by disconnecting the wire and measuring across and got 0.0V each time. I actually checked my multimeter against a battery and AC voltage about halfway through just to make sure it was operating correctly, and it was. I did the bonding myself and bonded the pool shell at 8 points in addition to everything around the pool. I drilled holes (as opposed to using clamp-style lugs) and used stainless bolts and lock nuts at every bonding connection, and tested continuity on all items before backfill and concrete so as to be sure I had no regrets later, so I'm very confident in the bonding grid.

Sounds like Matt's suggestion of a localized issue is the most plausible one - yes?
 

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JoyfulNoise said:
Yeah. So the white crystals you see are a combination of zinc carbonate and zinc hydroxide. It’s similar to the “white rust” see on pool pump bodies when the galvanized coating on the steel undergoes galvanic corrosion. Zinc is not a good choice for submerged application as it will create lots of shedding (many zinc compounds have low solubility in water). In a submerged application it’s better to use magnesium.

Given that the anode was connected to polished stainless steel using a tinned copper wire, you likely created a localized galvanic couple which is what caused the mess. I suggest not doing that.

You may have also degraded the protective passivation coating (chrome oxide) on the steel light housing. I suggest you pull that light housing again in a few weeks, maybe at the end of the season, and check it for corrosion. If it starts to rust, you will need to replace the light.
Click to expand...
Thanks so much, Matt. I'll pull them again and check them for sure. Thank you for the suggestion. I'm so frustrated with myself - I don't usually fall for this sort of snake oil....
 
wgipe said:
Sounds like Matt's suggestion of a localized issue is the most plausible one - yes?
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This type of thing is super complicated and difficult to determine exactly.

Probably just need to carefully watch for corrosion or other issues to see if they continue or if they were just a onetime thing.

You won’t really know until you wait the same amount of time to see if the problems reoccur.
 
In your setup, the zinc slug was the anode (-) with electrons flowing from the anode to the steel light right and house which served as the cathode (+). Oxidation, or an increase in the positive charge of the Zn metal ion, occurs at the anode -

Zn0solid ----> Zn2+aq + 2 e-

In reality, zinc ions are too reactive to remain in water and so there is the formation of zinc oxides and hydroxides and that is all the crusty, crystalline junk you found in the light niche and on the surface of the zinc slug itself.

The stainless steel metal becomes the cathode and that is where reduction happens, that is, the electrons from the zinc metal travel to the iron and charge it up. The problem with steel is that it is a super-complicated alloy surface and stainless steel has an exceptionally thin but rigidly adherent chrome oxide passivation layer. This is what protects stainless steel from rusting (and why an anode is not necessary). So the stainless steel body kind of acts like a giant capacitor that presents a charge to the water solution around it. When it does that, the extra electrons will actually combine with dissolved metals in the water and those metals will plate out onto the surface of the steel. In fact, the galvanic corrosion process and the electrochemical plating process for coating metals are one and the same. So basically all of that "black" coloring on the steel retaining ring was just plated out metals ... probably a very thin layer of iron oxide or some mixed iron/copper oxide. Plated metals don't adhere to stainless steel cathodes very well and are easily scrubbed off ... this is why in the metal plating industry, steel racks, hooks and pins are used to fastened oddly shaped objects in a plating bath. The object you want to coat will get a nice adherent coating and all the mounting hardware to hold it in place will be coated as well but easily reused by simply cleaning off all excess metal. When I did gold plating on contact parts, the "cathode basket" was made out of stainless steel so that the excess gold could be stripped off and saved. We would send the spent gold coated hardware back to the chemical supplier for a discount on gold cyanide salts.
 
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JamesW said:
The question is, did the black come from the light or did it just deposit on the light?
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The technical term for that stuff in the metal plating world is - "smut"

www.pfonline.com

Smut and Desmutting

Question: I am new to this industry and have heard about smut and desmutting operations.
www.pfonline.com
 
Most all zinc anode applications are in raw water, waste, or open body, where the byproduct wastes are not contained within the desired closed system, especially not within a small niche.
 
JoyfulNoise said:
In your setup, the zinc slug was the anode (-) with electrons flowing from the anode to the steel light right and house which served as the cathode (+). Oxidation, or an increase in the positive charge of the Zn metal ion, occurs at the anode -

Zn0solid ----> Zn2+aq + 2 e-

In reality, zinc ions are too reactive to remain in water and so there is the formation of zinc oxides and hydroxides and that is all the crusty, crystalline junk you found in the light niche and on the surface of the zinc slug itself.

The stainless steel metal becomes the cathode and that is where reduction happens, that is, the electrons from the zinc metal travel to the iron and charge it up. The problem with steel is that it is a super-complicated alloy surface and stainless steel has an exceptionally thin but rigidly adherent chrome oxide passivation layer. This is what protects stainless steel from rusting (and why an anode is not necessary). So the stainless steel body kind of acts like a giant capacitor that presents a charge to the water solution around it. When it does that, the extra electrons will actually combine with dissolved metals in the water and those metals will plate out onto the surface of the steel. In fact, the galvanic corrosion process and the electrochemical plating process for coating metals are one and the same. So basically all of that "black" coloring on the steel retaining ring was just plated out metals ... probably a very thin layer of iron oxide or some mixed iron/copper oxide. Plated metals don't adhere to stainless steel cathodes very well and are easily scrubbed off ... this is why in the metal plating industry, steel racks, hooks and pins are used to fastened oddly shaped objects in a plating bath. The object you want to coat will get a nice adherent coating and all the mounting hardware to hold it in place will be coated as well but easily reused by simply cleaning off all excess metal. When I did gold plating on contact parts, the "cathode basket" was made out of stainless steel so that the excess gold could be stripped off and saved. We would send the spent gold coated hardware back to the chemical supplier for a discount on gold cyanide salts.
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That's really wild and makes total sense now that you explain it so plainly. Thanks for taking the time to do it.
 
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