Understanding Metal Corrosion

If you've ever walked into an indoor pool facility and noticed rust seemingly "spray painted" across metal fixtures, or discovered flaking metal on a heat exchanger, you've witnessed the destructive power of corrosion. While most pool professionals focus on water balance and chemical levels, understanding the underlying science of metal corrosion can be the difference between profitable operations and costly equipment replacements.

In episode 164 of the Rule Your Pool podcast, host Eric Knight tackles one of the most complex yet crucial topics in pool chemistry: metal corrosion. With insights from metallurgist Alex Wensley, this episode breaks down the electrochemical processes that turn your pool equipment into expensive liabilities—and more importantly, how to prevent it.

The Electrochemical Foundation of Corrosion

At its core, corrosion isn't just "rust happening"—it's a sophisticated electrochemical reaction that follows the same principles powering batteries and electrical systems. Understanding this process is crucial for any pool professional serious about equipment longevity.

Corrosion is an electrochemical reaction. What does that mean? An electrochemical reaction means that electrons have to move for corrosion to occur. This is the same mechanism that makes batteries work. It's the same mechanism that makes electricity move. Electron transfer.

— Eric Knight, Rule Your Pool Podcast

Every corrosion event requires two players: an anode (the metal that gives up electrons and gets corroded) and a cathode (the oxidizer that steals those electrons). In swimming pools, the anode is typically your expensive equipment—stainless steel components, heat exchangers, ladders, and galvanized hardware. The cathode? Usually chlorine or chloride ions lurking in your water and air.

This electron transfer explains why corrosion accelerates in certain conditions. When metals lose electrons to oxidizers like chlorine, they literally break down at the molecular level, creating the rust and flaking we see on corroded equipment.

Why Indoor Pools Face Unique Corrosion Challenges

Indoor pool environments create a perfect storm for accelerated corrosion, and it's not just about the water chemistry. The real culprit often lurks in the air itself.

When chlorine oxidizes organic matter (like swimmer waste), it creates disinfection byproducts including nitrogen trichloride—commonly known as chloramines. These compounds don't just create that distinctive "pool smell" and respiratory irritation; they carry chlorides into the air where they become invisible corrosion agents.

Indoor pool operators, when you see corrosion that looks like it's spotted, almost like it was spray painted on, that's telling you that there are chlorides in the air from the pool, off-gassing, that condenses on those metals. And because there's chlorides in there, it's very corrosive.

— Eric Knight, Rule Your Pool Podcast

The process works like this: chloride-laden moisture condenses on metal surfaces (which tend to be cooler than surrounding materials), creating highly acidic conditions. Testing has revealed that condensate from dehumidifier systems can have a pH as low as 2—acidic enough to literally dissolve concrete over time.

This explains why uncoated evaporator coils fail so quickly in indoor pool environments, and why modern dehumidification systems require specialized coatings to survive.

The Real Impact of Water Balance on Corrosion

While aggressive water (LSI below -0.30) accelerates corrosion, it's important to understand that water balance alone doesn't cause corrosion—it simply makes existing electrochemical reactions more likely to occur.

The Langelier Saturation Index (LSI) indicates whether water is aggressive (seeking to dissolve minerals) or scaling (wanting to deposit minerals). Aggressive water creates ideal conditions for the electron transfer that drives corrosion, but you still need those chloride cathodes to actually steal electrons from your metal equipment.

This is why some pools with perfect LSI readings still experience corrosion issues—especially in indoor environments where airborne chlorides create additional pathways for metal degradation.

Chlorides: The Hidden Corrosion Accelerator

Every chlorine product introduces chlorides into your pool system, and unlike the active chlorine that gets consumed during sanitization, chlorides accumulate over time. Salt water pools are particularly susceptible since they rely on electrolyzing sodium chloride to generate chlorine.

Chlorine can be incredibly aggressive to metals, as it has a very strong desire to pull electrons from metals and thus causing corrosion... 2% chlorine causes accelerated corrosion. There are entire labs devoted to salt spray testing for research on which materials can resist chlorine exposure the best.

— Alex Wensley, quoted in Rule Your Pool Podcast

The concentration matters enormously. Ocean water contains roughly 2% chlorine by weight, which creates the aggressive corrosion environment coastal communities know well. While pool water typically contains much lower chloride concentrations, the enclosed environment and constant exposure create similar challenges over time.

In indoor environments, the problem compounds as these chlorides become airborne and condense on every metal surface, creating corrosion even on equipment that never directly contacts pool water.

Practical Prevention Strategies

Understanding corrosion science translates into specific preventive actions that can save thousands in equipment replacement costs:

• Maintain proper LSI balance: Keep your Langelier Saturation Index between 0.0 and +0.3 to prevent aggressive water conditions that accelerate existing corrosion processes.
• Control indoor air quality: Invest in properly designed ventilation and dehumidification systems with coated components to handle chloride-laden moisture.
• Monitor chloride accumulation: Test for chloride levels periodically, especially in salt water pools, and consider partial water replacement when levels climb too high.
• Protect exposed metals: Use marine-grade coatings, stainless steel rated for chloride exposure, or non-metallic alternatives where possible.
• Address the source: Minimize disinfection byproduct formation by maintaining proper bather loads, shower enforcement, and avoiding problematic algaecides.

The Human Health Connection

The same airborne chlorides causing equipment corrosion also impact human health. As Knight notes from personal experience with lung scarring from years of competitive swimming, the respiratory effects of breathing chlorinated air compounds can be severe and permanent.

And think about it this way. If the air is doing that to steel, what is it doing to you? It's not good to breathe this stuff in. And there's tons of studies about it.

— Eric Knight, Rule Your Pool Podcast

This connection underscores why addressing corrosion isn't just about protecting equipment—it's about creating healthier environments for swimmers and staff. Proper air handling systems that minimize chloride condensation also reduce respiratory exposure to harmful disinfection byproducts.

Moving Forward: Prevention Over Reaction

Metal corrosion in pool environments is largely predictable and preventable when you understand the underlying electrochemical processes. Rather than treating corrosion as an inevitable cost of doing business, forward-thinking pool professionals can implement systematic approaches to minimize electron transfer opportunities.

The key lies in recognizing that corrosion prevention requires a holistic approach: balanced water chemistry, proper ventilation design, appropriate material selection, and regular monitoring of both water and air quality parameters.

As Knight emphasizes throughout the episode, a swimming pool can either be "a profit machine or a big liability and a pain." Understanding and preventing corrosion is fundamental to ensuring your facility falls into the former category rather than the latter.

Episode Chapters

• 00:00 Course Announcements and Corrections
• 05:30 Introduction to Metal Corrosion
• 08:15 Electrochemical Basics of Corrosion
• 12:45 Salt Cell Example of Electron Transfer
• 16:20 Causes of Corrosion in Swimming Pools
• 22:10 Indoor Pool Air Quality and Corrosion
• 28:30 Chlorides as Corrosion Catalysts
• 32:00 Real-World Examples and Prevention