Stainless Steel Pool Light Ring Corrosion in a Salt-Water Pool

[Marmeade]

About two years ago, I had my pool light replaced, and a new stainless steel light and ring installed. Since then I've noticed a steady corrosion of the steel (see photo). I had requested a marine grade (314) ring, but I think the Pentair that was actually installed is only 304.

During this time, I've closely monitored my pool chemistries with with the amazing help of this forum and its TFP calculators, maintaining the ranges below. I've used a polyquat algicide for closing, but otherwise have avoided using other pool store chemicals. The pool is regularly scrubbed despite the foul-looking deposits that seem to grow on this light & ring.

Has anybody else had this sort of issue with stainless steel light ring in a salt water pool?

Any suggestions to remedy this situation would be greatly appreciated.

Pool Description: 24K gal vinyl lined pool with SWG
Free chlorine levels: 5.0 to 8.0
CyA: 60-80
pH: 7.8-8.0
Total Alkalinity: 200
Calcium: 225
NaCl: 3000-3400

 

[proavia]

While I don't have an answer to remedy your issue, I do know that your pool being a saltwater pool is not the cause of your issue.

Stainless steel stains less, it isn't stain proof. Rust and corrosion can be caused by low pH over an extended time period. Since you are in Indy, you probably close your pool for the winter. Quite possibly, your pH might be low during the time the pool is closed.

 

[PoolGate]

I would contact Pentair and see what they say. That doesn't look like any grade of stainless perhaps it was only plated? That growth is odd is that anywhere else in your pool?

 

[Marmeade]

While I don't have an answer to remedy your issue, I do know that your pool being a saltwater pool is not the cause of your issue.

Stainless steel stains less, it isn't stain proof. Rust and corrosion can be caused by low pH over an extended time period. Since you are in Indy, you probably close your pool for the winter. Quite possibly, your pH might be low during the time the pool is closed.

Thanks for your thoughts, proavia. The water pH here tends to run high. It's been at 7.8 at the time of opening. I've had to sometimes add dry acid to keep it below 8.0through the season

 

[Marmeade]

I would contact Pentair and see what they say. That doesn't look like any grade of stainless perhaps it was only plated? That growth is odd is that anywhere else in your pool?

No other growth anywhere in the pool. The pool light ring is a standard Pentair, supposedly solid stainless steel, but Pentair doesn't specify the grade.

 

[Bill1974]

I can say that Pentair has changed the material they make their SS light niches out of. I just had a new liner installed and replaced my light niche. Being a mechanical engineer I have a half a clue about materials. the niche that was in the pool had the conduit hub nut come off (looks like it was brazed on not welded on in my opinion). So I bought a new niche to install but could tell it was not the same material. It had a slight difference color and the new one is a bit magnetic too. It's not easy to tell the difference between 316 and 304 stainless steel without special equipment or special chemicals. But it's more common for 304 SS to become magnetic if it is work hardened (bent or formed) than 316. I ask Pentair but never got a response. Since I did not want to have a rusty light niche, I had someone weld on a new 316 ss conduit fitting on my old light niche.

The ring that holds the light niche to the wall is not stainless Steel, it's chrome plate brass. The chrome plating is pretty good a protecting the brass from chlorine and salt. But it should be made from 316 SS as well. I plan to have one made from 316 SS for when mine starts to not look so good.

Unless you let the salt level in your pool get to some insane level neither 304 or 316 should rust. Maybe your pH is out of range too? But you look to have have some serious leaks, new gasket for the light are pretty easy to install. If the seal between the liner and light niche is good and you should only have to seal the conduit if it's leaking.

 

[Marmeade]

I can say that Pentair has changed the material they make their SS light niches out of. I just had a new liner installed and replaced my light niche. Being a mechanical engineer I have a half a clue about materials. the niche that was in the pool had the conduit hub nut come off (looks like it was brazed on not welded on in my opinion). So I bought a new niche to install but could tell it was not the same material. It had a slight difference color and the new one is a bit magnetic too. It's not easy to tell the difference between 316 and 304 stainless steel without special equipment or special chemicals. But it's more common for 304 SS to become magnetic if it is work hardened (bent or formed) than 316. I ask Pentair but never got a response. Since I did not want to have a rusty light niche, I had someone weld on a new 316 ss conduit fitting on my old light niche.

The ring that holds the light niche to the wall is not stainless Steel, it's chrome plate brass. The chrome plating is pretty good a protecting the brass from chlorine and salt. But it should be made from 316 SS as well. I plan to have one made from 316 SS for when mine starts to not look so good.

Unless you let the salt level in your pool get to some insane level neither 304 or 316 should rust. Maybe your pH is out of range too? But you look to have have some serious leaks, new gasket for the light are pretty easy to install. If the seal between the liner and light niche is good and you should only have to seal the conduit if it's leaking.

Thanks for the information, Bill1974. The pool pH hasn’t been below 7.2 for several years, and then only when I was dropping it with dry acid as part of a SLAM two years ago. It seems like these rings just aren’t made to last. I’ll have the rings and gaskets replaced next week. If you want to go into the ring manufacturing business, I’ll be a customer!

 

[yblaser]

Could there be galvanic corrosion going on due to dissimilar metals?

 

[JoyfulNoise]

>>>> I've had to sometimes add dry acid to keep it below 8.0through the season <<<

THIS

is really not good. Dry acid is sodium bisulfate. Over time, the bisulfate anion decomposes into sulfates. Sulfates are very bad for pool equipment and high levels of them can lead to corrosion issues if you are not careful. Sulfates are also really bad for SWG's as they enhance the degradation of the transition metal catalyst coating. Most SWG manuals will say not to use dry acid for pH control in an SWG pool. Switch to muriatic acid.

Sulfates don't go away, they can only be diluted by fresh water.

Also, check that the replacement process did not sever the bonding wire to the light niche. Loss of bond can cause stray voltages to build up and galvanic corrosion.

 

[Marmeade]

THIS

is really not good. Dry acid is sodium bisulfate. Over time, the bisulfate anion decomposes into sulfates. Sulfates are very bad for pool equipment and high levels of them can lead to corrosion issues if you are not careful. Sulfates are also really bad for SWG's as they enhance the degradation of the transition metal catalyst coating. Most SWG manuals will say not to use dry acid for pH control in an SWG pool. Switch to muriatic acid.

Sulfates don't go away, they can only be diluted by fresh water.

Also, check that the replacement process did not sever the bonding wire to the light niche. Loss of bond can cause stray voltages to build up and galvanic corrosion.

Wow! I never knew that! Good to know! I’ll change to muratic acid immediately. Is there a way to measure sulphate levels?

The 110 AC light was changed to a 12 VDC light by a pool electrician two years ago, so I assumed that he did it right (famous last words) The light seems to work OK. How would I check for the bonding wire issue?

 

[Marmeade]

Could there be galvanic corrosion going on due to dissimilar metals?

Thanks ybaser. The only metal contact for them seems to me to be the base light ring, and the overlying light ring. Both from Pentair for this purpose. But I’ll definitely check this when I have them replaced next week. Unfortunately, they’ll be replaced by the same products from the same company.

 

[Marmeade]

Wow! I never knew that! Good to know! I’ll change to muratic acid immediately. Is there a way to measure sulphate levels?

The 110 AC light was changed to a 12 VDC light by a pool electrician two years ago, so I assumed that he did it right (famous last words) The light seems to work OK. How would I check for the bonding wire issue?

Intrigued by your sulfate angle, I measured my pool water’s sulfate level today using test strips by Quantofix. It was less than 200 mg / L (200 ppm), essentially undetectable with the semi-quantitative test strips I used. My records indicate that I’ve added Leslies Dry Acid (93% Sodium bisulfate) at 40 oz in 2021, 38 oz in 2020, and 106 oz in 2019. During this time I’ve turned the water over approximately once from pool closings, and filter back washings.

The pool guy I‘m going to us to replace the 2 year old pool rings simply says that ”pool lights are a problem” without going into details.