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Guest Column - April 2010

Pools/Aquatics

The Case of the Mystery Stain
Working With Salt Generator Pools

By Terry Arko


S
alt chlorine pools have become popular over the past few years. They provide great convenience. There's no hazardous chemicals on site, water is sanitized and oxidized automatically. There are little to no chloramines. Many users of salt generators claim softer-feeling water with less chemical odor and no dry or irritated skin.

Along with the report of these benefits, some have also reported strange phenomena as well. These include things like discolored water, strange stains throughout the pool that are hard to remove and prevent. Stains appearing in salt pools include:

  • Black flecks on pool bottom
  • Black staining on ladders and light rings
  • Reoccurring stains and discoloration on light rings around steps or rails and discolored water.
  • Purple haze and debris in pool water

These stains seem to be a mystery, however in salt pools with high TDS (total dissolved solids), they are due to a simple chemical reaction known as galvanic corrosion.

To understand this electrochemical reaction, a simple understanding of the technology of chlorine generators is needed.

Chlorine generators work by using a process known as electrolysis. In nature chlorine is found primarily in the chloride ion which is 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 popular chlorine generators are the in-line type. In these systems saltwater is circulated over electrochemical cells. The cells convert the sodium chloride to free available chlorine. The cells used in these systems are typically made of titanium. Though it may seem new, the technology of splitting molecules via electrolysis goes back all the way to the 1700s.

Galvanic corrosion occurs when dissimilar metals exist in a high-TDS solution such as a salt-generator pool. Some metals are nobler and more cathodic, meaning positive currents flow from these and they 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 Galvanic Corrosion Chart shows that the "anodic" or "less noble" metals at the negative end of the series such as magnesium, zinc and aluminium 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:

1. Electrochemically dissimilar metals must be present.
2. These metals must be in electrical contact.
3. The metals must be exposed to an electrolyte (salt in solution).