If you own or maintain a saltwater pool, there's a good chance you have seen calcium flakes in it too. This article will explain what the flakes really are and debunk some myths about them. Let's get into it.
Covered in this article:
- How salt chlorine generators work
- Byproducts of electrolysis
- Salt chlorine generators and the LSI
- Stagnant water
- Reversing polarity
- Different types of scale
- Calcium Phosphate
- Calcium Carbonate
- How to get rid of flakes in a saltwater pool
- How to prevent flakes in a saltwater pool
- LSI Balance
- Contain pH with reduced alkalinity
- Flush out the salt cell with circulation
How salt chlorine generators work
We have a more detailed article about salt systems here, but we thought it was important to summarize how salt chlorine generators work. The process–and its byproducts–explain why the calcium flakes occur. Saltwater pools use salt chlorine generators (also called Saltwater Generators, or SWGs) that produce chlorine using electricity. Yes, saltwater pools are chlorine pools. They generate chlorine on site. The process of using electricity to generate chlorine from saltwater is called electrolysis.
Electrolysis has been around for over 100 years. Saltwater can conduct electricity better than fresh water. Salt cells have ruthenium-coated titanium plates (also called "fins" or "blades" that water passes between. One side is positively charged (anode) and the other is negatively charged (cathode).1 See the diagram below:
The electrolysis reaction in a salt chlorine generator looks like this:2
2NaCl + 2H2O → Cl2 (gas) + 2NaOH + H2 (gas)
salt + water → with electricity⚡️yields→ chlorine gas + sodium hydroxide + hydrogen gas
This reaction, however, may be misleading because it does not account for how chlorine gas dissolves into the water. So let's hone in on how chlorine dissolves into water:
Cl2(gas) + H2O → HOCl + H+ + Cl-
Chlorine gas in water yields Hypochlorous Acid + Hydrogen and Chloride
We could expand this even further and show how hydroxide and Chloride react, but it's just getting more complicated than we need to be. What you need to know is chlorine gas is created by electrolysis through saltwater (brine), and that chlorine gas dissolves in water. The reaction also creates byproducts...
Byproducts of electrolysis
As you can see from the first reaction above, electricity in saltwater creates three new products: chlorine gas (Cl2), sodium hydroxide, aka caustic soda (NaOH), and Hydrogen gas (H2). Since chlorine is the primary reason for creating this reaction in the first place, let's designate chlorine as the main product of the reaction, and sodium hydroxide and hydrogen as byproducts. To the chemists reading this, we're just making this distinction for simplicity's sake.
Chlorine is created at the positive electrode (anode), whereas the byproducts of sodium hydroxide and hydrogen are created at the negative electrode (cathode). And that brings us to our next point...
Salt chlorine generators and the LSI
Yes, here we are at Orenda talking about yet another topic tying back into the importance of the Langelier Saturation Index (LSI). Don't forget, LSI balance is our first (and arguably most important) pillar of proactive pool care.
Saltwater pools have to be maintained differently than non-salt pools for a few reasons. For one, saltwater pools, thanks to the salt added to the water, have a baseline TDS of at least 3000 ppm more than other types of pools. Use the free Orenda App LSI calculator and raise the Salt/TDS from 100 up to 3000 and see what happens. You will notice the LSI drops considerably. The TDS lowers the LSI, but the sodium hydroxide byproduct of electrolysis raises the LSI, and that's what actually leads to calcium flakes and scale in the salt cell.
As we know from studying the saturation index (LSI), LSI violations are localized events. For example, carbonate scale always forms in the highest-LSI places first. So when we teach classes, we often ask the question of where scale forms first. Almost everyone says something like "tile line" or "in the spa" or "the spillway". But the truth is, on saltwater pools, the first place to form scale is usually in the salt cell. It's a small chamber where water flows full speed. In that small chamber, a very high pH sodium hydroxide is formed on the cathodes, which raises the LSI in the immediate area substantially. To add to that, there is also heat produced during electrolysis. But thanks to circulation, this is not nearly as significant as the high pH.
As water circulates at, say, 50-80 gallons-per-minute (on a residential pool), minimal (if any) scale should form in a salt cell if the LSI of the pool is balanced slightly below 0.00 ( yellow on the Orenda App). Scale really forms most immediately after the pump shuts off, and water stops flowing through the cell.
Yes, scale is most likely to form immediately after the salt cell and pump shut off. Think about it: there is high-pH sodium hydroxide is still in the chamber, and the water just stopped moving. Stagnant water means enough time to cause calcium carbonate to fall out of solution. This is when salt cells scale the most. We have told some of the major manufacturers of salt chlorine generators that calcium flaking would be reduced if there was a 'cool down' cycle for the salt cell, much like a heater has. Not so much for heat, but for flushing out sodium hydroxide. We'll discuss more on this later when we cover prevention measures.
Most saltwater pools do not run the salt chlorine generator 24/7. Instead, it's usually more like 50-80% of the time. So if your circulation pump is running for 12 hours a day, and your salt system is operating at 100%, it is producing chlorine 12 hours a day. But if it's operating at 75%, it's only operating 9 hours a day (0.75 x 12 hours = 9 hours). And that's not 9 hours straight, then 3 hours off. Most salt chlorine generators divide up the day evenly in clusters of, say, 5 or 10 minutes. So you would be producing chlorine 7.5 minutes out of every 10 minutes, with 2.5 minutes off.
Related 'From the Experts' Article: How to Avoid Salt Pool Maintenance Issues
This is important, because many salt systems reverse polarity, meaning the positive electrode becomes the negative, and vice versa. In other words, the electricity switches directions. This occurs many times a day, and the process tends to loosen and fracture carbonate scale that has adhered to the metal blades. The water flowing through the cell then carries those flakes of carbonate scale into the pool. This is why you see the flakes on the floor or benches near return inlets. And in pools with attached spas, flakes are often found in the spa more than the pool because of how the plumbing is designed.
Different types of scale in saltwater pools
Every time we have encountered the flakes, they turn out to be calcium carbonate (CaCO3). No, we have not tested all of them, but have tested some, and with our knowledge of the LSI, calcium carbonate makes the most sense.
However, there are several sources online that talk about these flakes like they are not carbonate scale, but instead, calcium phosphate (Ca3[PO4]2). So we looked into it.3 We have so far concluded that while calcium phosphate might be possible in salt systems, thankfully, it is rarely the case. And we say "thankfully" because calcium phosphate is extremely hard to remove and clean off.
We have seen calcium phosphate in commercial filters and heaters before...it's like concrete. Pools have literally had to cut open their filters and use jackhammers to get the sand out because of calcium phosphate formation in the filters. See the photo of the sand-boulders that were removed from a commercial sand filter only after being broken apart by a jackhammer.
If you really had calcium phosphate scale on your salt cell, it would be time to buy a new salt cell. That is, unless you like using a hammer and chisel.
In all of the pools we have seen and heard of calcium flakes, we have yet to see one saltwater pool with calcium phosphate. Ever. And we get calls every week about calcium flakes in saltwater pools. Sure, maybe that's 'anecdata' instead of actual data, but it's the truth. So to figure out where this myth came from, we did some digging. We found several sources (which we will not link here out of respect) that make the case that calcium flakes are calcium phosphate, and therefore you need to buy a phosphate remover.4
As you know, we are a manufacturer of a phosphate remover. While phosphates may seem like they are reducing the performance salt chlorine generators, it's not a direct relationship. Nor are phosphates a driving factor for salt cell scale formation and flaking. Sure, it's possible to have calcium phosphate if you have levels high enough, but it's rare. Calcium phosphate is more likely to be found in a heater because of added contact time with heat.
There are plenty of reasons to remove phosphates, but scale in a salt system is not high on that list. But we agree it cannot hurt, and it can help prevent calcium phosphate from forming. Thanks to Richard Falk, the chemist we cite in many of our articles, below is a formula that shows how to calculate the level of phosphate needed for calcium phosphate to precipitate.5
PO4 = 10^[11.755 - log(CaH) - 2log(t) - (0.65 * pH)]
In his example, the pool has 375 ppm calcium hardness, 7.5 pH, and 80ºF (26.67ºC). In that scenario:
10^[11.755 - log(375) - 2log(26.67) - (0.65 * 7.5)] = 28.44 ppm = 28,440 ppb phosphate
28,440 ppb phosphates is really high and unlikely, but it is possible. We have seen a pool with over 40 ppm (40,000 ppb) before...once. Higher levels of calcium hardness, a higher pH and a higher temperature can all reduce the needed level of phosphate. So on paper, yes it is possible for calcium phosphate to form on salt cells...you just need really high levels.
Expanding on the relationship of phosphates and saltwater pools, salt and phosphates do not bind together. This is another myth that came from who-knows-where. Indeed, high phosphates do contribute to added chlorine demand, which is perhaps what people notice...but the phosphates themselves do not have anything to do with salt. Look at the formula above, and see if you can find the sodium (or the chloride) in it. It's not there.
Because we have so much information already about calcium carbonate, we won't exhaust you here. Just know that it is by far the most common form of scale, and it is what every white flake in saltwater pools is made of...at least in our experience.
How to get rid of flakes in a salt pool
After the white flakes are blown into the pool through the returns, they can get dirty, they can tumble around, and they can begin to look more translucent. They are still calcium carbonate. When you vacuum them up, they may not look as crisp as when they first show up in the pool, but we believe they are the same flakes after being in the pool for several days.
Getting rid of the flakes can be done a number of different ways. You can chemically dissolve them with the right pool chemistry, but this takes time and might not be entirely practical. SC-1000 can help, but in the short term, you might be better off vacuuming the flakes and removing them from the pool. We of course need to get rid of the cause of the flakes too, which means you will need to clean the salt cell following the salt system manufacturer's instructions.
How to prevent white flakes in a salt pool
There are proactive strategies to prevent calcium flakes from forming in the first place. We just need to prevent scale from forming in the salt cell. In order of importance, the strategies are LSI balance, containing pH, flushing the system out, and if needed, using SC-1000. And while this can help prevent scale, nothing is ever 100% with pool chemistry. This will not prevent all salt system maintenance issues, but it should help you reduce the frequency of cleaning.
1. LSI Balance
LSI balance is critical here. Adjust your water to around -0.25 to -0.20 each week; not low enough to etch, but with plenty of space for the rising pH so it does not lead to scale. This strategy is probably the most important way to prevent scale flakes, and there are several ways you can do it using the Orenda App calculator. We recommend a strategy of lower alkalinity and higher calcium to allow you to have a higher pH without forming scale. So instead of lowering your pH to 7.4 every week, consider lowering it to 7.6 or 7.7...maybe even 7.8. Thanks to physics, we know the pH will rise up to its equilibrium point (which we call the pH ceiling). Use this to your advantage! We'll write another article detailing more of the strategy for how to manage saltwater pool chemistry.
2. Contain pH with reduced alkalinity
One of the best things you can do for a saltwater pool is to manage your carbonate, or corrected alkalinity. If you have enough calcium hardness in your pool, you can afford to reduce your Total Alkalinity (TA) below 80 ppm in most cases. Note: if going below 80 ppm TA causes an LSI violation at 7.6 pH, you need more calcium hardness in your water for this strategy to work! Never let your LSI go red on the Orenda calculator.
Reducing your TA also reduces your carbonate alkalinity. Another way to reduce carbonate alkalinity is to increase cyanuric acid (CYA), though we strongly advise against too much CYA in your water. A lower carbonate alkalinity means a lower pH ceiling, which also means Henry's Law of physics will help slow the rise in pH. So our advice to saltwater pool owners and operators is to maintain somewhere between 60-70 ppm TA, which means an even lower carbonate alkalinity. But again, this cannot be accomplished successfully without enough calcium hardness in your water.
Related: CO2, pH and Henry's Law
3. Flush out the salt cell with circulation
Part of the problem with salt cells forming so much scale was described earlier in this article. Namely, salt cells generate a high pH byproduct, sodium hydroxide (pH 13+). Most scale forms right after the salt cell and circulation pump shut off. So to fix this problem, simply run the pump for a few minutes after the salt cell shuts off. This will flush the high pH chemistry from the cell, and the circulating water will help cool it down too, though the latest experiments are showing heat is not a driving factor for flakes. This dramatically reduces the chances of scale formation by rapidly lowering the LSI back to the normal range (assuming your pool is LSI-balanced, of course).
Another strategy to help with this is to reduce the output percentage of the cell. Often times we hear of residential salt systems being run wide open (100% output) all the time. This not only maximizes chlorine production, it also maximizes the production of high-pH sodium hydroxide and minimizes the amount of water flushing out the salt cell. We mentioned this earlier in this article, but reducing the salt system output to 50-80% is a good way to reduce the likelihood of scale.
4. Use SC-1000
If LSI balance and flushing out the cell are not enough, SC-1000 can help. Many pools are treated with SC-1000 weekly simply as a preventative maintenance dose to help keep calcium in solution, especially salt pools. Here's a video showing what it can do to an already-existing scale line:
If you have scale issues, here's our procedure for how to soften it and remove it chemically. Additionally, if you're relying on your salt chlorine generator to handle all the non-living organics in the pool, there's a good chance it will eventually fall short. This is because chlorine is not really designed to remove the oxidant demand; it's designed to kill germs. You can reduce the oils and other non-living organics that can coagulate in a salt cell and contribute to the problem.
The white flakes in salt pools are almost always calcium carbonate. Calcium flakes occur because salt chlorine generators create a very high pH byproduct (sodium hydroxide) which lead to scale formation in the salt cell. When the salt cell reverses polarity, the carbonate scale fractures and loosens, and flakes are carried by the circulating water out into the pool. Voilà, calcium flakes.
Cleaning flakes out is pretty easy, but preventing them takes skill. Prevention requires an LSI strategy to not only balance the pool today, but for the entire week until the next service visit. Then, find a way to cool the salt cell down with added circulation time after the salt system shuts down. Think of this like a cool-down cycle. Finally, if you want extra help preventing scale, you can use SC-1000 on a weekly maintenance dose to prevent scale formation. We hope this article helps bring clarity to this topic. If you have more questions, feel free to contact us.
1 Titanium is used because it is less susceptible to corrosion. If the salt cell had iron or steel fins in them, they would not last very long. The ruthenium coating is used to maximize the production of chlorine gas from chloride ions. The activation energy required to produce chlorine is lower when using ruthenium as opposed to steel or other metals. Using a metal like iron, for example, would require much higher salt levels in order to produce chlorine.
2 Lowry, Robert W. 2009. IPSSA Intermediate Training Manual. Pg. 39
3 Lei, Song, Weijden, et. al.. 2017. Electrochemical Induced Calcium Phosphate Precipitation: Importance of Local pH.
4 This evidently came about because people dissolved samples of these calcium flakes, then tested that water sample for phosphates. Indeed, phosphates were present. It's not yet clear to us how the phosphates got in there, but then again, we did not take the samples or do the experiment ourselves. All we're saying is we have yet to find actual calcium phosphate flakes in a pool, or even still stuck to a salt system. Every single case of flakes we have seen has been calcium carbonate. If that changes, we will update this article and talk about it in a future one.
5 Lenntech Technical Manual. FILMTEC Membranes Water Chemistry and Pre-treatment: Calcium Phosphate Scale Prevention. Pg. 2.