Understanding Metal Corrosion in Pools
- aquasplashmarketin
- Apr 6
- 3 min read
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).
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 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:
metal corrosion can still happen in water with a perfectly balanced LSI, and
among all LSI factors, low pH and high TDS are the main contributors to metal corrosion.
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
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.
