Scientific Explanation of Metal Corrosion In Indoor Swimming Pools

Almost all natatoriums have challenges with rust and corrosion on metal parts like screws, door handles, hand rails, starting blocks, and light fixtures. On stainless steel, the rust first looks like orange spots, and they continue to grow and spread over time. This article explains the chemistry taking place. An abridged version of this article is reposted on the Orenda blog.

Covered in this article:

• What is corrosion?
• Different causes of corrosion
  • General corrosion
  • Pitting corrosion
  • Atmospheric corrosion
  • Chloride-induced corrosion
    • Chloride vs. Chlorine
  • Sulfate-induced corrosion
  • Galvanic corrosion
  • Crevice corrosion
  • Intergranular corrosion
  • Stress Corrosion Cracking (SCC)
  • Erosion corrosion
  • Microbiologically-Influenced Corrosion (MIC)

• How to mitigate and prevent corrosion
  • Mitigating galvanic corrosion
    • Sacrificial anode
    • Bonding
• Conclusion

What is Corrosion?

Corrosion is an electrochemical reaction when a metal loses electrons.¹ This permanently changes the material itself, 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. These new substances what we refer to as rust, which is orange, brown or red in color.

The metal that loses electrons is called the anode, and the substance that steals the electrons (the corrosive agent) is called the cathode.

In swimming pools, the cathode can be an oxidizing agent, like oxygen, ozone, 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 electrons

2. 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 ion

3. 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 solid

4. 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 + water

Further dehydration can occur, creating iron oxide solid and water.

2FeO(OH)_(s) → Fe₂O_3(s) + H₂O
Iron oxide-hydroxide solid → iron oxide solid + water

5. 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 

Fe₂O_3(s) + nH₂O → Fe₂O_3(s) • nH₂O
Iron oxide solid + water → Hydrated iron oxide solid

As 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.

Iron rust is reddish-brown, but copper or bronze corrosion is green.

The bronze pump housing on the left oxidizes green because bronze is ~88% copper. The cast iron pump housings on the right show the normal brownish-red rust we are all familiar with.

Different causes of corrosion

While all corrosion involves the loss of electrons from an anode, there are several ways this can occur. 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

Does the LSI predict metal corrosion?

For many years, we were taught that the Langelier Saturation Index (LSI) was a predictor of metal corrosion. And while it is true that having a balanced LSI makes corrosion less likely, it's not a direct relationship, because:

• Metal corrosion can still occur in perfectly LSI-balanced water, and
• of all the LSI factors, it's primarily a low pH and high TDS that directly impact metal corrosion.

The likelihood of having a balanced LSI with a low pH and high TDS is very low. The water would need much higher levels of calcium hardness and/or total alkalinity to offset these parameters. So the LSI is a good proxy for making sure these factors are accounted for, but the LSI itself is not the primary predictor of metal corrosion.  

The Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) measure the saturation of calcium carbonate (CaCO₃), which can coat the inside of metal pipes and prevent corrosion. 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 itself. Coating the inside of metal pipes with calcium carbonate was the original purpose of Dr. Langelier's saturation index in 1936. 

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 below -0.31. 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 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.

So with all that being said, the primary corrosion mechanisms in swimming pools are:

• General corrosion
• Pitting corrosion
• Atmospheric corrosion
• Chloride-induced corrosion
• Sulfate-induced corrosion
• Galvanic corrosion
• Crevice corrosion
• Intergranular corrosion
• Stress corrosion cracking (SCC)
• Erosion corrosion
• Microbiologically-influenced corrosion

They are not mutually exclusive!  Most corrosion around swimming pools will be the result of multiple factors simultaneously.

General corrosion

General corrosion is a uniform attack on metals caused by pool water chemistry. Specifically, metals react with dissolved oxygen and water itself, which can form oxides and hydroxides. This especially true for iron, bronze, and carbon steel items.

It should be noted, surprisingly, that for some metals and alloys, some corrosion on their outermost layer is a good thing that helps resist further corrosion underneath.  Specifically for aluminum, the oxide formed protects the aluminum from further oxidation and corrosion. The other metal alloy of note is stainless steel, which has chromium and molybdenum in it. When stainless steel begins to corrode, it forms a chromium oxide layer that helps prevent further corrosion underneath...even though it looks bad.⁵

Regular iron and carbon steel, however, lack such protective layers. As mentioned above, when iron is corroded into iron oxide or iron hydroxide (i.e. rust), those molecules do not bond fully to the rest of the iron, so they flake off easily. This is called deleterious corrosion. As the rust flakes off, it exposes the next layer of iron or steel for further corrosion.

Pitting corrosion

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, like copper 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.⁶

Pitting corrosion also occurs on metals not submerged in the pool. It is accelerated by chlorinated water and moisture in the air. Trichloramines and other disinfection byproducts create chloramine vapor that goes into the air, and when moisture condenses on metal surfaces (because their temperature is lower, making condensation more likely than other surfaces like brick). The chloramines in that condensed moisture attack the chromium oxide layer on stainless steel and can create spots of rust over time.

Related: Why do indoor pools rust and corrode so easily?

Notice the 'sprayed on' look. These spots of rust are where moisture condensed on the metal surface, then evaporated away, leaving behind its corrosive chemistry. This type of corrosion in prevalent in almost every indoor pool in the world, and it is preventable! If your pool has corrosion like this, we may be able to help.

While chlorides themselves are not oxidizers, they interfere with the passive layer of stainless steel (the chromium oxide layer). These ions disrupt that layer and penetrate deeper, allowing for pitting corrosion and micro-cracking through the layer to the rest of the steel underneath. This type of corrosion has a distinct look to it, like rust is lightly spray-painted on the metal. 

Atmospheric corrosion

The air around a pool, and especially in a chemical storage room, tends to be loaded with chlorine byproducts. Acid fumes, for instance, either contain sulfates (sulfuric acid), or chlorides (hydrochloric acid, aka muriatic acid). And both acid products off-gas a high concentration of Hydrogen, making these fumes very acidic in moisture.

These fumes can condense in humid air and land on metal surfaces. Look no further than a commercial pool pump room and chemical room to see extreme examples of this:

Stainless steel shelf in an acid storage room. Note the dark blisters of rust on the material.  This photo was taken within the first year of the facility being opened.

It's one thing for spotted rust to be on starting blocks and other rail goods that get wet with pool water. But what about other components that remain dry? Stainless steel towel hooks (shown above), door fixtures and other hardware in natatoriums are all vulnerable to atmospheric corrosion, thanks to the chloramines in the air. And speaking of chloramines in the air...

Chloride-induced corrosion

Muriatic acid, salt, and all chlorine products leave chloride ions (Cl^-) in the water. Chlorides are not oxidizers, and are relatively inert in pool chemistry...except when it comes to corrosion; especially corrosion of stainless steel. Chlorides do two things that impact corrosion:

• Chlorides break down the passive oxide layer that protects specific metals (i.e. the chromium oxide layer on stainless steel), exposing the steel so it can rust. Stainless steel isn't stain-proof, it just stains less.
• Chloride and sodium ions (and most TDS) are basically electrolytes in water. Both will increase the conductivity in the water, which can increase the rate of corrosion. See galvanic corrosion later in this article.

Chloramines, as you know from many of our other articles, are disinfection byproducts (DBPs) that can eventually off-gas into the air. The main one is an inorganic byproduct called nitrogen trichloride (NCl₃), aka trichloramine. Let's take a look at how chloramines in the air interact with ferrous metals (like iron and steel).

1. Formation of inorganic chloramines in water

HOCl + NH₃ → NH₂Cl + H₂O
Hypochlorous acid + Ammonia → Monochloramine + Water

HOCl + NH₂Cl → NHCl₂ + H₂O
Hypochlorous acid + Monochloramine → Dichloramine + Water

HOCl + NHCl₂ → NCl₃ + H₂O
Hypochlorous acid + Dichloramine → Trichloramine (aka Nitrogen trichloride) + Water
 

Trichloramine (NCl₃) is highly volatile and it off-gases, creating the known pool smell, along with many other off-gassing byproducts. But once it's in the air, it can condense with moisture on metal surfaces. So let’s see what happens when it dissolves in water again (condensation).

2. Condensation of airborne chloramines

NCl₃ + H₂O → NHCl₂ + HOCl
Trichloramine + Water → Dichloramine + Hypochlorous acid

The above is the reverse (equilibrium) reaction of what we last saw before. So now we have hypochlorous acid which can oxidize iron.  When it does so, it increases chloride ion so corrosion can be accelerated.

Did you catch that? Airborne chloramines, when re-dissolved in water that condenses (usually on metal surfaces), put HOCl in the water droplets. HOCl oxidizes metals and leaves behind chlorides, which further interfere with the chromium oxide layer. It's a vicious cycle that is virtually impossible to stop without being able to remove the chloramine pollution from the air.

3. Corrosion from chlorinated condensation

HOCl + H₂O + Fe_(s) → Fe²⁺ + Cl^- + OH^-
Hypochlorous acid + Water + Solid iron → Ferrous ion + Chloride ion + Hydroxide ion

HOCl + H₂O + 2Fe²⁺ → 2Fe³⁺ + Cl^- + OH^-
Hypochlorous acid + Water + Solid iron → Ferrous ion + Chloride ion + Hydroxide ion

Fe³⁺ + 3OH^- → Fe(OH)_3(s)
Ferric ion + Hydroxide ion → Iron hydroxide solid (loose and porous)

NET: 3HOCl + 6H₂O + 2Fe_(s) → 2Fe(OH)₃ + 3Cl^- + 3H⁺
Hypochlorous acid + Water + Solid iron → Iron hydroxide + Chloride Ion + Hydrogen Ion

 

Note that the net reaction of chlorine (which came from trichloramine) with solid iron not only ends up producing rust, but it also increases chloride ion (Cl^-) and Hydrogen concentration. This degrades the passivity layer of stainless steel and decreases the pH, accelerating corrosion.

Chloride ion vs. Chlorine

A chloride is a chlorine atom with a ^-1 valence, meaning it has one electron (Cl^-), whereas a regular chlorine atom has one proton (Cl⁺).

Much like oxygen, nitrogen and hydrogen atoms, chlorine is inherently unstable, so it binds to another to create chlorine gas (Cl₂⇡). This is why oxygen (O₂⇡), nitrogen (N₂⇡), and Hydrogen (H₂⇡) gases all have 2 atoms. Separating them makes them unstable again, so they seek to bind to something. In oxygen's case, it can lead to the creation of ozone (O₃⇡).

Because of its negative valence, chlorides cannot steal electrons from metals (or anything else), and they are not oxidizers.  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.

The name Nitrogen Trichloride makes it sound like it contains three chloride ions. But in fact, these are chlorine atoms (Cl⁺) bound to nitrogen. So trichloramines themselves do not have chlorides in them.

N^3- + 3Cl⁺ ⥂ NCl₃

Nitrogen + 3 chlorine atoms ⥂ Nitrogen trichloride

The above equation is just to demonstrate the components of nitrogen trichloride. The actual formation of it is shown in the equations section 1 above.

The vapors from muriatic acid, however, do contain chlorides, because Hydrochloric acid (HCl) has the chlorine atom in the 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:

Sulfate-induced corrosion

Like chlorides, sulfates (SO₄^2-) 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.

Virtually all swimming pools have both, thanks to chlorine providing chlorides, and products like sulfuric acid, sodium bisulfate (dry acid), potassium monopersulfate, sodium thiosulfate, and others. It should be noted that the acidity of these products is a larger catalyst for corrosion, but sulfates on their own are still corrosive. Here's a photo of a pump, located in a room that holds an open drum of sulfuric acid on a feeder. The sulfates and hydrogen off-gassing from the acid contribute to atmospheric corrosion.

It also looks like the bolts in the pump flanges may be experiencing galvanic corrosion, being in direct contact with the bronze pump housing. Bronze is ~88% copper. Note the stainless steel screws on the multi-port valve are also rusting, thanks to the aggressive sulfates and hydrogen in the air.

In fairness, sulfates are also aggressive toward cement. They can also pair with calcium to form the insoluble (and sharp) calcium sulfate scale crystals.  It's best to keep sulfates to a minimum (below 300 ppm⁸), and keep them out of your pool if possible.

Galvanic corrosion

Galvanic corrosion is the result of two dissimilar metals being in contact with one another. This can occur when the metals are directly touching, or via electrolytes in water. Pool water is conductive, especially with higher salinity. Think about saltwater pools.

Salt chlorine generators work by sending electricity through saltwater to create chlorine. The process is called electrolysis. The salt (sodium and chloride ions) are electrolytes. More salt means more conductive water.

An example of galvanic corrosion in pools and spas is when a malfunctioning salt cell corrodes a heat exchanger, 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. Think of galvanized screws or bolts. Galvanized means the metal item is coated in zinc, which is a more willing anode than steel. This makes it take longer for the steel to be corroded. For what it's worth using galvanized hardware in natatoriums is really bad idea. They don't last long.

Crevice corrosion

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 is chlorinated. 

Crevice corrosion is most often around bolts, weld joints, or puddles where water sits long enough to evaporate. This is also common in commercial pools in escutcheons for rail goods being secured into the floor. Think ladders, starting blocks, hand rails, backstroke flag posts, etc. (see the photo below, on the right). These rail goods are secured into the pool deck in metal anchors embedded into the concrete pool deck. These anchors are essentially holes that can fill up with stagnant chlorinated water, which is perfect for crevice corrosion.

Water puddles on metal surfaces can also lead to crevice corrosion, as seen in the photo below (on the left).

Intergranular corrosion

This type of corrosion is along the grain boundaries of metal (i.e. a cut edge). This is usually caused by improper heat treatment or insufficient welding, leaving cut edges exposed. This most often happens along weld joints of pool equipment and other steel items around an indoor pool or pump room.

Think of the grain of wood on the cut edge of a 2x4 beam. The exposed grain on cut metal is very similar, hence the term intergranular. This exposed grain on the edges of metal is easier to corrode. On stainless steel, for instance, the chromium oxide outer layer protects the smooth sides of metal, but may not protect all the exposed grain. This is why intergranular corrosion can occur faster than the rest of the metal. Again, look near weld joints that may not be completely welded.

Stress Corrosion Cracking (SCC)

If corroding metal is under tensile stress, it can crack. The corrosion, of course, makes the metal weaker as it penetrates deeper into the metal. The combined effect of mechanical stress and pitting corrosion (in the case of the photo below) can lead to potentially catastrophic failures. Metal railings can fail, along with support brackets for heavy lights, air ducts, speakers and other objects. In 2024, large air ducts fell from the ceiling and almost killed people when their metal brackets failed.

Erosion corrosion

Erosion is the wearing down of material by friction (usually water). This can accelerate corrosion, especially in high-velocity or high turbulence environments, such as a pump impeller or cast-iron volute. The turbulence from the water accelerates the loss of iron oxides, thereby increasing the rate of material loss (deleterious corrosion).

The brass impellers on the right get corroded quickly. Notice on the intact impeller that the blades are already beginning to show a green pitina. Brass is mostly copper and zinc. Zinc is a better anode than most other metals, meaning it gives up electrons to cathodes easily (see our segment below about sacrificial zinc anodes) The destroyed impeller shown in the photo is only 7 months old! This is a combination of many factors, including chloride-induced corrosion and erosion.

Iron oxide, for instance, will flake off much faster if water is rushing over it constantly, leading to iron getting into the swimming pool. We see this on older commercial pools that have cast-iron components. In the photo above, we see chloride-induced corrosion (not only from the water inside the iron pump basket housing, but from the corrosive air in the pump room), as well as erosion corrosion that wore down the brass pump impeller in just seven months!

Microbiologically-Influenced Corrosion (MIC)

Believe it or not, bacteria can produce acidic, corrosive substances like hydrogen sulfide, attacking metals. Hopefully, such microbes will not survive in your properly-disinfected swimming pool.

Biofilms fall into this category. We spoke at length with Dr. Darla Goeres about biofilms specifically, and we learned they are quite aggressive to surfaces and can corrode metals. Several scientific studies discuss the acidic and corrosive nature of biofilms too.⁹ Here's our Rule Your Pool podcast with Dr. Goeres:

Biofilms that build up in commercial pool filters are especially corrosive. This reinforces the importance of cleaning and/or purging filters annually.

How to mitigate and prevent corrosion

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.

• To begin with, using alloys like 316L stainless steel helps. 316L has molybdenum in it, making it more resistant to corrosion than other stainless steels.¹⁰ Galvanized screws, bolts, and other items should not be used in swimming pool environments because they will fail quickly. Especially if they are directly touching a different metal, like brass.
• In indoor pools (natatoriums), the priority should be proper ventilation and utilizing source-capture exhaust to remove chloramines from the space. Chloramines will still get into the air (thanks to disinfection byproducts off-gassing), but capturing them quickly before they have time to condense on metals is helpful.
• Regular rinsing stainless steel with fresh, unchlorinated water also helps, along with stainless steel cleaners and sealants. Try not to let chlorinated water stay on stainless steel long enough to evaporate off. Splash-out from the pool will leave behind salts as the water droplets evaporate.
• Pay extra attention to weld joints. The metal used to weld steel is often weaker than stainless steel itself. Using a protective coating product can help. Along those lines, certain paints and epoxies can also protect metals. In natatoriums, such coated metals almost always last longer than exposed metals.
• 316L stainless steel should be perfectly fine underwater, provided the pH does not get too low (how low is a matter of opinion, but in our experience, don't go below 7.2 pH).

Mitigating galvanic corrosion

Coatings and good water chemistry 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.  

So what can be done about galvanic corrosion?

Sacrificial anode

A sacrificial zinc anode is a piece of zinc metal that is intentionally added to a system to protect other metal components from corrosion. Zinc is more reactive than many other metals, so it acts as the anode, donating electrons if needed. As a result, the zinc corrodes instead of the other metal components, effectively 'sacrificing' itself.

This is a form of cathodic protection, which we'll elaborate on in a moment. Consult your pool equipment manufacturer about where and how to install such an anode.

Bonding

Bonding involves electrically linking all metal components in and around the pool to a common grounding point. This ensures that all metal parts maintain the same electrical potential, minimizing the risk of electrical currents passing between them. This is a legally required safety measure to prevent electrical shocks from ground currents or any electrical faults that might energize the water. To avoid shocks, the metal path must have a uniform electrical potential.

Proper bonding is not only a safety precaution but also a crucial step in extending the lifespan of your pool equipment.

Cathodic protection

While a zinc anode is technically a type of cathodic protection (because it sacrifices electrons to save other metals), there is another form of cathodic protection used in industrial applications. Note that this is not viable in swimming pools.

The idea is to intentionally run low electrical current through the metal so that any cathode wanting to pull electrons can just pull from the electrons in the current, rather than the metal itself.¹¹

This is not viable in swimming pools because we do not want to electrically charge metal components (except a salt cell).

Conclusion

Corrosion is an electrochemical reaction where a metal loses electrons (e^-). The metal being corroded (and losing electrons) is the anode, and whatever is stealing those electrons is the cathode. 

There are many contributing factors to corrosion, and this article outlined the main ones that pertain to swimming pools. We also included examples.

Corrosion can be mitigated by using protective coatings, regular cleaning and rinsing with fresh, non-chlorinated water, and wiping off condensation periodically. Water with a low LSI (≤ -0.31) or a Ryznar index above 7.0 does not directly cause corrosion, but these indices are good indicators that account for pH and TDS (conductivity).  With proper LSI balance, it's unlikely to have a low pH and high TDS simultaneously.

Yet even with LSI balance, certain types of corrosion can still occur. So beware of mistakes like putting trichlor in skimmer baskets, column pouring acid, or not properly bonding all pool equipment and metal components.

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. Read more about the different types of phosphates here. 

⁵  This is counter-intuitive, in that some corrosion on stainless steel can actually be a good thing. Indeed it can protect the steel underneath for a while. But this does not account for chlorides. With chlorides, as in any swimming pool environment, they will continue to attack the chromium oxide layer until the stainless steel is exposed to more serious corrosion. It's just a matter of time.

⁶  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.

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

¹⁰  Industrial Metal Service.

¹¹  Collins, Tony. (retrieved 12/2024). 5 Different types of corrosion prevention methods. EonCoat.