Sequence of chemicals and possibly any effects on what happened to screws at light.

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Good screws generally do not rust, but manufacturers insist on using cheap products so lower quality stainless steel such as 304 instead of 316 or 316L. Also, if one is not consistent in using the higher quality stainless steel, then if it is in contact with lower quality metal, galvanic corrosion will accelerate the corrosion of such lower quality metal near the point of contact. The screws in my light rings have not corroded and they are now 12 years old. I have no sacrificial anode.

The little Zinc Anode at the light in the water WILL work if the anode is electrically connected to the light metal if that is connected electrically to the screws. When I said not to put the zinc into the water it wasn't because it wouldn't work but rather to avoid adding zinc ions to the water.

Very strange as the screws never started deteriorating near the point of contact, but just the heads, inside where the Philips head goes in. Then dripping on the head and staining the niche ring. My guess is that the pre-drilled holes may have not been sprayed properly (who knows), but for now good luck appears to be on my side. New ones are fine (keeping my fingers crossed).



That is what the old one looked like.

 

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UPDATE:

I wanted to update on this post. After careful inspection, the light niche screws look like they are brand new. There is no rust apparent anywhere in the swimming pool, including the return jets. I would highly recommend this as a secondary way to protect from corrosion in a swimming pool. Only a few parts are needed from Lowes or Home Depot and a Marine Supply Store (Sacrificial Zinc Anode). Total cost out the door is around $30.

A big "Thank You" to Chem Geek for his knowledge and help!

 

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Now, that the pool is closed, I wanted to update the thread. Because I stick my head in the water with goggles as I am clearing the lines, if any corrosion is present, this would be apparent.

Now, almost two years later, I can say that the Sacrificial Zinc Anode that is buried in the moist soil is working out really well. In addition, my heat pump now kicks water out right in the same area, so even during real dry times I do not have to manually add water.

I visually inspected the old screws this year and the rust was only on the heads and not on the remaining part of the screw.

 

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Update for 2018 on the Zinc Anode. The anode was inspected about a month ago, and it appears smaller than the previous years, but still in tact. There is no sign of any corrosion on the screws, especially at the light niche. I can honestly state that after almost 3 years, no problems. The rust dripping on the screws lasted a month before started the thread on 08/20/2015, and the installation was about 15 days later, and since then no issues.