With an understanding of galvanic corrosion, stains in salt-chlorine pools don’t have to be a mystery.
|Salt chlorine pools have become popular over the past few years. There are no hazardous chemicals on site, and water is sanitized and oxidized automatically. Also, there are little to no chloramines. And many users of salt generators claim softer-feeling water with less chemical odor and no dry or irritated skin.
Yet in spite of all these benefits, some have also reported strange phenomena. These include discolored water and unusual stains that are hard to remove and prevent.
Stains appearing in salt pools include:
While these stains seem to be a mystery, typically they are a result of the high TDS in salt pools and a simple chemical reaction known as galvanic corrosion.
To understand this electro-chemical reaction, a simple grasp of the technology of chlorine generators is first needed.
Chlorine generators work through a process known as electrolysis.
In nature, chlorine is found primarily in the chloride ion, a component of salt found in the earth or the oceans. Electrolysis is the means of generating chemical products from their native state. A salt generator works by passing electricity through a solution of sodium chloride to produce chlorine as a disinfectant or sanitizer. The most commonly used chlorine generators are the in-line type. In these systems, salt water is circulated over electrochemical cells. The cells used in these systems typically are made of titanium, and they convert the sodium chloride to free available chlorine. Now it’s important to understand how this metal relates to galvanic corrosion.
Some metals are nobler and more cathodic, meaning positive currents flow from them. They also tend to steal electrons from the less noble anodic or negative metals. A galvanic corrosion chart is used in industries that work with fluids and metals, such as cooling towers. The chart shows that the “anodic” or “less noble” metals at the negative end of the series — such as magnesium, zinc and aluminum — are more likely to corrode than those at the “cathodic” or “noble” end, which include gold and graphite.
There are three things needed in order for galvanic corrosion to occur:
In a swimming pool, all three of these exist due to high TDS from the salt content of the water. Most pools contain some copper in the system as well as in the heat exchanger, or in any brass fittings or pipe that may be in the hydraulics. As discussed earlier, the electrochemical cells in most chlorine generators are made of titanium. Copper is a less noble metal than titanium, and thus it corrodes as a result of the electrolysis in the high salt solution.
This electrolysis leaves black stains and debris in the pool. The copper also is rendered insoluble in the water, which may create a green translucent color.
Solutions to staining
However, the addition of zinc in these types of systems can prevent corrosion and stop 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 recommended to use a metal-removal product along with the zinc. This removes the current discoloration and prevents 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, make sure it’s phosphate-free.
Another mystery in both salt 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 drop too low, 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 up the alkalinity and pH. Also, the addition of zinc will help keep copper from corroding into the water.
These simple methods should help solve the mystery…and remove the stains.
Source: Terry Arko- Pool and Spa News | 8.14.2009