Metal corrosion is common in and around swimming pools, but what is corrosion? This article explains the chemistry and offers some examples. For a deeper dive into the science, you can read the full article here, where the focus is on indoor swimming pools and stainless steel corrosion.
Corrosion is an electrochemical reaction when a metal loses electrons.1 This permanently changes the material itself, often making it weaker and discolored. Rust is among the most common forms of corrosion, when ferrous metals (anything containing iron, Fe) lose electrons to form iron oxides and hydroxides.
The metal that loses electrons is called the anode, and the substance that steals the electrons (the corrosive agent) is called the cathode. A cathode can be an oxidizing agent, like oxygen or chlorinated water. It can also be a dissimilar metal or anything else that wants electrons. Let's show an example of corrosion reactions in water, using iron (Fe).
1. Oxidation at the anode
Iron (Fe) in the steel is oxidized to form ferrous ions (Fe2+), because two electrons are stolen:
Fe(solid) → Fe2+ + 2e-
Solid iron loses 2 electrons, creating Ferrous ion + 2 electrons2. Further oxidation
Ferrous ions (Fe2+) from the anode can be further oxidized into ferric ions (Fe3+) in the presence of an oxidizer (like oxygen in water, ozone, hydroxyl radicals, or Hypochlorous acid, HOCl):
Fe2+ → Fe3+ + e-
Ferrous ion loses another electron, becoming ferric ion3. Precipitation
Ferric ions (Fe3+) then react with water and/or hydroxide ions to form ferric oxides and hydroxides, like solid iron hydroxide, which is loose and porous. These ferric oxides and hydroxdes are the primary components of rust:
Fe3+ + 3OH- → Fe(OH)3(s)
Ferric ion + 3 Hydroxide ions → iron hydroxide solid4. Dehydration
Iron hydroxide solid can dehydrate over time to form iron oxide-hydroxide solid + water.
Fe(OH)3(s) → FeO(OH)(s) + H2O
Iron hydroxide solid → iron oxide-hydroxide solid + waterFurther dehydration can occur, creating iron oxide solid and water.
2FeO(OH)(s) → Fe2O3(s) + H2O
Iron oxide-hydroxide solid → iron oxide solid + water5. Hydration
Iron oxide solid + water creates hydrated iron oxide solid. The 'n' value in the formula below indicates these substances can be multiply hydrated.2
Fe2O3(s) + nH2O → Fe2O3(s) • nH2O
Iron oxide solid + water → Hydrated iron oxide solidAs you can see, rust can both hydrate and dehydrate. Changes between these states accelerates the damage and loss of material. This is why iron and steel that go between wet and dry conditions tend to corrode faster.
Corroding iron turns reddish-brown, but corroding copper and bronze turn green (bronze is ~88% copper). In the photo below, check out the bronze pump housing on the left compared to the cast iron pump housings on the right.
Corrosion in swimming pools can be caused by various chemical, electrochemical and environmental factors. The main water chemistry variables that impact corrosion in swimming pools are the pH, conductivity (TDS is a decent proxy for this), sulfates, and especially for stainless steel, chlorides.3
For many years, we believed that the LSI could predict metal corrosion, and this was the information we shared with you (because it was what we were taught in the pool industry).While maintaining a balanced LSI does reduce the likelihood of corrosion, it is not a direct correlation because:
Achieving a balanced LSI with both low pH and high TDS is quite rare. The water would require significantly higher levels of calcium hardness and/or total alkalinity to counterbalance these conditions. Therefore, while the LSI serves as a useful indicator to ensure the pH and TDS are considered, it is not the primary determinant of metal corrosion.
The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) measure the saturation of calcium carbonate, which can coat the inside of metal pipes and prevent corrosion. Coating the inside of metal pipes with calcium carbonate was the original purpose of Dr. Langelier's saturation index in 1936. And while having aggressive water on either index makes it easier to have corrosion (because there is no thin calcium carbonate layer coating the inside of the pipes), they do not directly predict corrosion.
It should be noted that Dr. Langelier's strategy of calcium carbonate in the pipes is no longer widely used in municipal drinking water systems. Nowadays, most tap water intentionally has a higher pH (~8.0) and low calcium hardness, and there is widespread use of orthophosphates and phosphate-based sequestering agents.4
The LSI in most tap water is well below -0.30. We know this from years of experience doing the Orenda Startup®. This is why it's so important to get chelated calcium in the pool while it's filling, because the tap water itself will dissolve calcium from the fresh cement in the pool finish.
We at Orenda have been teaching the importance of the LSI since we learned about it in 2016. There is no doubt aggressive water damages cementitious materials (due to the loss of calcium). But even with perfect LSI balance, there are several different types of metal corrosion that can still occur.
Before we briefly touch on all these, know that none of these are mutually exclusive. Many of these are occurring simultaneously. Swimming pools–especially indoor pools–create the perfect environment for corrosion. To go deeper into all of these types of corrosion, you can read the full article here.
General corrosion is a uniform attack on metals caused by pool water chemistry. Specifically, metals react with dissolved oxygen, chlorine, chlorides, and water itself, which can form oxides and hydroxides.
Pitting is a more localized corrosion that can form small holes (or pits) in the metal. In swimming pools, this most often happens inside a heater (specifically the heat exchanger). It can also occur in metal pipes and housings, like copper, bronze, or cast iron.
Pitting corrosion in heat exchangers is usually caused by acidic water. And while unlikely, it can occur when the water is LSI-balanced. If/when strongly acidic chemicals flow through the pool equipment, locally inside the heater, the LSI will be very low. Think trichlor tabs in the skimmer (2.8 pH), or column pouring acid, which sinks to the floor and gets pulled into the main drain.5
The air around a pool, and especially in a chemical storage room, tends to be corrosive. Acid fumes, for instance, either contain sulfates (sulfuric acid), or chlorides (hydrochloric acid, aka muriatic acid). Both have a high concentration of Hydrogen, making these fumes very acidic when they combine with moisture and condense. Look no further than a commercial pool pump room and chemical room to see extreme examples of this.
All chlorine products leave behind chloride ions (Cl-). Chlorides are relatively inert in pool chemistry...except when it comes to metals. Chlorides are not oxidizers, but they do two things to impact corrosion:
It's worth taking a moment to distinguish between chloride ions and chlorine.
A chloride is a chlorine atom with a -1 charge (Cl-), while a regular chlorine atom has a positive charge due to having one less electron (Cl+).
Because of its negative valence, chlorides cannot steal electrons from metals (or anything else), and they are not oxidizers.6 Our article and podcast about Oxidation-Reduction Potential (ORP) explains this in more detail. Chlorine steals electrons, which in turn reduce it into chloride ions that can no longer oxidize.
Chlorides alone do not cause corrosion. Instead, chlorides interfere with the chromium oxide layer of stainless steel, which allows other oxidizers to penetrate and interact directly with the steel underneath.
So when we see chloramine corrosion, it's actually chlorine doing most of the corrosion. And this confused us at first, because nitrogen Trichloride makes it sound like it contains three chloride ions. But in fact, these are positively-charged chlorine atoms (Cl+) bound to nitrogen. So trichloramines do not have chlorides in them.
In contrast, the vapors from muriatic acid do contain chlorides, as Hydrochloric acid (HCl) has the chlorine atom in a negative state:
H+ + Cl- → HCl
Hydrogen ion + chloride ion → Hydrochloric acid
Chloride-induced corrosion goes hand-in-hand with atmospheric corrosion. Just look in a chemical storage room or pump room, and you might see severe corrosion on metal components that look like this:
Like chlorides, sulfates (SO42-) accelerate corrosion on metals by interfering with the passivity layer of specific metals. This is especially true when both chlorides and sulfates are present in the water.7 Sulfates are actually worse than chlorides in this regard.
Most swimming pools have sulfates from products like sulfuric acid, dry acid (sodium bisulfate), non-chlorine shock (potassium monopersulfate), algaecides (copper sulfate, ammonium sulfate, etc.). While their acidity is the major corrosion catalyst, sulfates are corrosive on their own.
Below is a photo of a commercial pump room that has an open drum of sulfuric acid (hooked to a feeder) in the same room. The fumes lead to rampant corrosion, just like muriatic acid fumes. Look closely at the screws on the multi-port valve and backwash valve.
That said, sulfates also attack cement. And they can also bind with calcium to form calcium sulfate scale crystals. It's best to keep sulfates below 300 ppm8, or better yet, keep them out of your pool entirely.
Galvanic corrosion occurs when two dissimilar metals are in contact, and one steals the other's electrons. Normally this is when metals are touching directly, but it can also occur through water with sufficient conductivity (like saltwater pools). Salt chlorine generators work by sending electricity through saltwater to create chlorine. It works because the salt (sodium and chloride ions) are electrolytes. More salt means more conductive water.
We have seen galvanic corrosion affect heaters in saltwater pools and spas in a major way, degrading the copper or cupronickel to a point where it flakes off and flows into the salt system itself, where it gets oxidized. This creates dark green/black flakes of copper oxide that can blow into a pool or spa:
This galvanic corrosion can occur if there is an electrical connection between the salt cell and the heater. Salt systems is rare, and would be a problem with the salt cell's power supply. Consult your equipment manufacturer(s) if you see such issues in your pool.
Another example of galvanic corrosion is when bolts and screws are in direct contact something made of a different metal. Like galvanized bolts securing a copper pipe, or a cast-iron pump housing.
Crevice corrosion is also localized corrosion that occurs in confined spaces or crevices where water can accumulate and stay for a while. Stagnant water in crevices creates a micro-environment that can accelerate corrosion, especially if the water has chloramines in it.
Crevice corrosion is most often around bolts, weld joints, or puddles where water sits long enough to evaporate. Think of the anchors in the pool deck for rail goods (sometimes made of brass, which can also cause galvanic corrosion). These anchors are essentially holes that can fill up with stagnant chlorinated water, which is perfect for crevice corrosion.
Intergranular corrosion occurs along cut edges that expose metal grain. This sometimes happens at weld joints in pool equipment. Similar to wood grain, exposed metal grain is more prone to corrosion. Stainless steel's chromium oxide layer protects smooth surfaces but not all exposed grain, making these areas more susceptible to corrosion, especially near incomplete welds.
Corrosion weakens metal, and if that metal is under tensile stress, it can eventually crack.
This can lead to catastrophic failures., like the air duct collapse in an indoor pool at a resort in Colorado in 2024. Metal railings can also fail, as shown in the photo below.
Water friction can also accelerate corrosion, especially in high-velocity or high turbulence environments like inside a pump or its cast iron volute. Friction helps break the rust free, which speeds up the rate of material loss (deleterious corrosion).
We see this on older commercial pools that have cast-iron pump components. In the photo above, the destroyed brass impeller is just 7 months old! And notice the intact impeller is already showing a green patina because brass is a copper alloy.
Certain bacteria can produce corrosive substances like hydrogen sulfide, which attacks metals. Hopefully you're keeping your pool properly disinfected so these germs cannot survive.
Biofilms can also contribute to corrosion. Remember our podcast with one of the world's leading biofilm researchers, Dr. Darla Goeres?
Several scientific studies discuss the acidic and corrosive nature of biofilms too.9 Biofilms in commercial pool filters corrode carbon steel and other metals, highlighting the need for annual cleaning or purging.
Because there are so many types of corrosion, it may seem impossible to prevent all of them. And in some cases, that can be true. But do not lose hope! There are ways to mitigate corrosion and prevent most of it.
Coatings can protect against most corrosion, but if two different metals are in direct contact with one another (even if they are both spray coated on the outside), galvanic corrosion can still occur. The main thing is to not let dissimilar metals stay in contact with one another. Once it begins, the oxide molecules do not bond as well to the metal itself, which makes them easier to flake off. So what can be done about galvanic corrosion?
A sacrificial zinc anode is more reactive than many other metals, so it can donate electrons if needed. As a result, the zinc corrodes first, instead of the other metals, effectively 'sacrificing' itself. This is a form of cathodic protection, which we'll elaborate on in a moment. Ask your pool equipment manufacturer for details.
Bonding connects all metal components around the pool to a common ground, ensuring they share the same electrical potential and reducing the risk of electrical currents. This safety measure, required by law, prevents electrical shocks and prolongs pool equipment life.
A zinc anode is a type of cathodic protection, sacrificing electrons to protect other metals. Another method, not suitable for pools, involves running a low electrical current through metal to provide electrons for cathodes.11 This is not viable in swimming pools because we do not want to electrically charge metal components (outside of the salt cell itself).
Corrosion occurs when a metal loses electrons. The metal is the anode and the electron-stealing agent is the cathode. There are many different contributing causes of corrosion, and most of the time, corrosion is the result of several of them.
To mitigate corrosion, use protective coatings, clean regularly with non-chlorinated water, and remove condensation. While low LSI (≤ -0.31) or a Ryznar index above 7.0 doesn't directly cause corrosion, they indicate pH and TDS levels. Proper LSI balance reduces the chance of low pH and high TDS.
However, even with LSI balance, corrosion can occur. So avoid errors like using trichlor in skimmer baskets, pouring acid directly, or failing to bond pool equipment and metal components properly.
1 Corrosion is very similar to oxidation, in that electrons are stolen from something. In a redox reaction, an oxidizer takes electrons from an oxidant. In corrosion, a cathode takes electrons from an anode, electrochemically altering that anode. The loss of electrons leaves the metal in a new chemical state; an oxide state. Ferrous oxide or hydroxide is a larger molecule than iron, and it does not bond well with iron, so it will tend to flake off, exposing the layer underneath to more corrosion. This process is called deleterious corrosion (because material is removed, or deleted), and it is ongoing until the metal is either gone, or sealed to prevent further corrosion.
2 Many thanks to Richard Falk with proofreading this article and helping to ensure these formulas are correct. Prior to publishing, we go through several steps of review and confirmation to guarantee accuracy in our articles. I had these close to correct, based on other sources online, but several different sources had slightly different variants of these chemical reactions. I wanted to be sure what we were publishing was accurate for what occurs in chlorinated swimming pools. Credit goes to Richard for these formulas.
3 As mentioned earlier in this article, chlorides themselves are not oxidizers. They are largely inert, except when there are enough of them in the water to attack the chromium oxide layer of stainless steel. All chlorine products leave chlorides behind. Saltwater pools start with over 3000 ppm salinity (ideally 3400-3600 ppm, depending on the salt chlorine generator manufacturer's recommendations). The big concern with chlorides is as it pertains to airborne chloramines that get into the humid air, then condense on metals around the pool and pump room. In this case, the chloride concentration is extraordinarily high as the water evaporates away (leaving the chlorides behind). This is why chloramine corrosion looks like orange spots sprayed on metal surfaces. The spots are where water droplets condensed, then evaporated away.
4 We have talked about this ad nauseam on our website. Ever since the 2014 water crisis in Flint, Michigan, the EPA has mandated corrosion prevention in drinking water. This is a good thing, as it protects the infrastructure of our water grid. It has mostly been accomplished using phosphate-based sequestering agents, and in some cases, orthophosphates themselves. Learn more about the different types of phosphates here.
5 Ironically, column pouring acid can also cause scale formation in a heater if done repeatedly in a cementitious pool. This sounds incorrect, but let's explain. Acid is heavier than water, and sinks to the floor if not diluted enough. It will dissolve cement, and get neutralized by the basic calcium hydroxide (12.6 pH). This eventually causes white calcium carbonate marks on the bottom of the pool, but in the short term, that calcium-rich 12.6 pH water can get pulled into the main drain. If it does, it can precipitate in the heater because the temperature is high enough to force calcium out of solution. We explain this further in Rule Your Pool Podcast episode 165.
6 When we talk about chlorine being "reduced", it means chlorine's valence gets reduced until it is a chloride. Chlorides cannot take on more electrons, and therefore cannot oxidize or sanitize. See our article on Oxidation-Reduction Potential (ORP). The reduction half of the redox reaction is what we're talking about here. So when chlorine gets used up, chlorides are left behind.
7 Pohjanne, P., Carpén, L., Kinnunen, P., Rämö, J., Sarpola, A., Riihimäki, M., & Hakkarainen, T. (2007). Stainless steel pitting in chloride-sulfate solutions: The role of cations. 071981-0719813.
8 PWTAG Technical Notes. (2011). Sulphate Attack (PDF download).
9 Liu, P., Zhang, H., Fan, Y., & Xu, D. (2023). Microbially Influenced Corrosion of Steel in Marine Environments: A Review from Mechanisms to Prevention. Microorganisms, 11(9), 2299. https://doi.org/10.3390/microorganisms11092299
11 Collins, Tony. (retrieved 12/2024). 5 Different types of corrosion prevention methods. EonCoat.