Yet another CSI question for vinyl pool with a heater and SWG

[Elemental21]

I know this has been asked in the past but recently I've been in discussions with 2 pool techs that swear up and down that a significantly negative CSI (LSI is what they use) is aggressive to "pool lights, heater, pump". I know the general view of TFP is that negative CSI isn't an issue for vinyl pools/heaters/SWGs. I have, however, seen at least one old discussion where the debate raged for a bit and ChemGeek left it as "If we find something consistent with metal corrosion in vinyl pools at lower calcium and that such corrosion is stopped or reduced at higher calcium levels, then our recommendation will change.". I assume that has never been found. The reason I am second guessing, or just looking for piece of mind, is because I have very low CH and very low TA (see below). So my CSI in is the order of -1.40. I'd rather not mess with anything if I don't have to though because my pH has been a constant 7.5 for 3 years running. I assume it's related to the low TA and CH after reading an orendatech article about pH ceiling (referenced below). I'd love to hear anyone's thought on the rock solid 7.5 pH level too. Am I right about the low CH/alk keeping my pH stable? I guess my point is, everything is great. I literally have never had to add acid to my SWG pool and I'd love to keep it that way if I'm not causing myself any issues. Thanks!

My current water test:
FC: 4.5
CC: 0
pH: 7.5
TA: 50
CYA: 60
CH: 75
Salt: 3000

Understanding Henry's Law, CO2 and pH

The pH of a swimming pool is determined by how much CO2 is in solution. Henry's Law explains why CO2 leaves the water, and when it does, pH rises naturally

blog.orendatech.com

 

[ajw22]

You are doing fine.

There is no science behind what the pool techs are saying. Especially since the LSI was originally developed for boilers.

How can the LSI/CSI be aggressive to a pump when everything water comes in contact with in a modern pool pump is plastic?

 

[Elemental21]

Agreed, I thought the pump comment was telling but wanted to quote it exactly.

 

[PoolStored]

"If we find something consistent with metal corrosion in vinyl pools at lower calcium and that such corrosion is stopped or reduced at higher calcium levels, then our recommendation will change.".

I am in the middle of this experiment. I purchased a new 316 marine grade ladder 2 years ago. My CSI was running around -.6 last year. I have significant rusting of the ladder at the waterline.
This year, I've raised and maintained TA, pH and CH higher this year. I intend to maintain that this year. My CSI has be running -.2..-.3
I intend to maintain that this year and replace the ladder next year. Will see if maintaining CSI higher impacts the new ladder.
The experiment is obviously not done, but I personally believe that the corrosion is due to my water maintenance. Time will tell.

I had a long private conversation with @JoyfulNoise on this exact topic, in part this is what he said:

One has to be exceedingly careful when throwing around terms like saturation indices and corrosion. There are so many caveats and assumptions that need to be carefully considered before using them as a guide. Historically, saturation and corrosion indices were developed by chemical engineers simply as a means of shorthand so that water managers could have an easy-to-understand number to help drive decision making. These indices are EMPIRICALLY derived and very few of them rest on first-principals derivation. The CSI/LSI is also solely an index used to predict how calcium carbonate scale will react in water ... it says NOTHING about metal corrosion. In water boilers, steam pipes and cooling towers, calcium scale is a big issue for both equipment upkeep as well as the PREVENTION of corrosive liquids and gases from contacting metal surfaces. That is all these indices are good for.

There are other empirically derived indices that are used to PREDICT when corrosion MIGHT be possible. The Pukorius Index (PI) and Ryznar Index (RI) are used in this regard. I looked at your PM Logs and it would seem that based on your latest test results and worst case scenario (CSI < -0.9), your Ryznar Index value is 9.15. Anything over 8 is considered corrosive to steel. Even if your CH were to go up to 250ppm and your TA were 80, the RI would still be 8.2. So your water is corrosive towards steel. But remember, these indices only tell you what COULD happen, not what will happen.

As far as your ladder goes, were the steps anchored to the tubes using stainless steel bolts or cheaper carbon steel? This is often the case where something is called "marine grade" but it only refers to the main components while the fasters and the materials are cheap. Once corrosion of steel starts, it doesn't stop. It will only get worse, not better. So, at some point you need to pull the ladder, take it all apart and clean off the corrosion. You'll probably need to use something like Naval Jelly to restore the metal surface and add back a passivating layer. Then I would go and find marine grade steel fasteners to replace whatever they used.

Your pool water is brackish and so chloride induced crevice corrosion and pitting will always be a failure mode for the steel no matter what your indices are. Anytime you can pull the ladder and rinse it off with fresh water, the better it will be. Saltwater can splash all over the ladder, dry and leave behind an enriched chloride film. Add to that a potent oxidizer (chlorine) and you have a recipe for corrosion. It's all about preventative maintenance.

 

[Elemental21]

Interesting. Curious to hear the results of your experiment. That seems like a very short period of time to show rust.

Here's a pic of my ladder, which shows no signs of rust that I can see and my pool light, which does. It appears something under the light is rusting and the hardware. The other light looks pretty much the same. My pool was built in the 2019 season so this is my 6th season.