Pool corrosion occurs when water is under-saturated with calcium carbonate and seeks to dissolve minerals from any available source — pool surfaces, metal fittings, and equipment. Low pH is the most common cause.
Key Facts
- An LSI below -0.3 indicates aggressive (corrosive) water.
- Low pH (below 7.2) is the most common cause of pool corrosion.
- Soft water (low calcium hardness, below 150 ppm) is inherently corrosive to pool surfaces.
- Metal staining — green, blue-grey, brown — is often the first visible sign of corrosion.
What Causes Corrosion
Water naturally tries to reach equilibrium with calcium carbonate. When pool water is under-saturated (LSI negative), it dissolves minerals from the nearest available source. Plaster pools lose surface calcium to the water, resulting in etched, rough, pitted surfaces. Metal equipment — pump housings, heat exchangers, ladder rails, handrails — releases metal ions into the water when the water is corrosive. These dissolved metals then deposit elsewhere in the pool or on equipment as stains. The primary causes are low pH, low alkalinity, and low calcium hardness.
Signs of Corrosion
Early signs of corrosion: rough plaster texture that feels like sandpaper (in plaster pools), green or blue-green staining on pool surfaces near metal fixtures (copper corrosion from heat exchangers or fittings), light brown or rust-coloured staining near ladder anchors (iron corrosion), pitting on concrete deck or coping, and discolouration or roughening of vinyl liner seams. Advanced corrosion: visibly etched plaster surfaces with pit marks, leaking heat exchangers due to metal loss, cracks in grout lines, and metal staining spread broadly across the pool floor.
Stopping Corrosion
The treatment for active corrosion is raising the LSI into the acceptable range: raise pH to 7.4–7.6 (largest single impact), raise calcium hardness to 200+ ppm for plaster pools, and raise alkalinity to 80–100 ppm. Corrosive water damage to surfaces is typically permanent — etching and pitting already present cannot be reversed chemically. However, stopping the process preserves the remaining surface life. For staining from dissolved metals, use a metal sequestrant product after balancing chemistry. Shocking a pool with active metal staining can worsen it — hold off on shock until metals are sequestered.
Examples
A pool owner notices blue-green staining appearing along the waterline and on the pool floor after using a copper-based algaecide. Testing shows pH 7.1 and Ca hardness 100 ppm — an LSI of approximately -1.0. The corrosive water is dissolving copper from the algaecide and it is plating out on pool surfaces. Step 1: Add a metal sequestrant product immediately to bind free copper in solution. Step 2: Do not shock until metals are under control. Step 3: Raise pH to 7.4 and hardness to 250 ppm over 48 hours. Step 4: Run the filter 24 hours. The staining fades over several days.
Common Mistakes to Avoid
- Using a copper-based algaecide in soft water (low hardness) — the aggressive water dissolves the copper rapidly, causing widespread staining.
- Shocking a pool with active metal staining — the sudden high chlorine oxidises dissolved metals and causes them to precipitate out as permanent stains.
- Filling a plaster pool with soft municipal water without immediately raising calcium hardness to at least 200 ppm.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference