Guest Column - April 2010
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Pools/Aquatics

The Case of the Mystery Stain
Working With Salt Generator Pools

By Terry Arko


In a swimming pool all three of these exist due to the high TDS from the salt content of the water. The electrochemical cells in most chlorine generators are made of titanium which is listed on the Galvanic Corrosion Chart as a nobler metal. Most pools contain some copper in the system as well in the heat exchanger or in any brass fittings or pipe that may be in the system. Copper is a less noble metal than titanium, and thus it corrodes as a result of the electrolysis in the high salt solution. This leaves black stains and debris in the pool. Copper is also rendered insoluble in the water. Copper in the water will appear as a green translucent color.

The simple solution to this problem is to find another less noble metal to use as a sacrificial anode that corrodes but doesn't cause staining. When differing metals are added to saltwater, one metal acts as a cathode. This is the nobler of the two. Titanium would be one example of a more cathodic or noble metal. The other metal may be more anodic or less noble. An example of this would be copper. Galvanic corrosion occurs because when these two metals are in saltwater with an electrical current, the weaker, less noble metal (copper) will corrode faster than normal. Also the stronger, more noble metal (titanium) will corrode much slower than normal.

It has been found in various marine industries that the addition of zinc in these types of systems prevents the corrosion of copper and stops the staining. Zinc is very low on the galvanic chart and is one of the most anodic metals found. In salt chlorine pools zinc can be added as a solid weight into the skimmer or attached in the circulation system. This slows or stops the corrosion of copper. If the water is discolored from copper, it is advised to use a metal removal product along with the zinc to remove the current discoloration and prevent reoccurrence. Most metal products on the market tend to be phosphate-based, and this too can cause problems in a salt chlorine generator. When selecting a metal product, use a phosphate-free product.

Another mystery in both salt generator and regular pools is the occurrence of a strange purple coloring and debris. This is due to high levels of cyanuric acid and insoluble copper in the water. If pH and alkalinity go low, then copper cyanurate is formed leaving a purple residue along the water line and around lights and steps. The solution here is to lower cyanuric acid down to 35ppm to 50ppm and adjust the alkalinity and pH upward. Also, the addition of zinc will help keep copper from corroding into the water.

These simple methods should help clear the mystery and remove the stains.

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ABOUT THE AUTHOR

Terry Arko has over 30 years experience in the swimming pool and spa industry. Terry is both a Certified Pool Operator (CPO) and CPO Instructor through the National Swimming Pool Foundation (NSPF), and has served in many positions with the Association of Pool and Spa Professionals (APSP). He currently is the product specialist and Northwest territory sales representative for SeaKlear Pool and Spa Products. For more information, visit www.seaklear.com.