Solving Stains

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:
• 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

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.

Salt   chlorine

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.

Galvanic   corrosion
Galvanic corrosion occurs when dissimilar metals exist in a high TDS solution   such as a salt chlorine pool.

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:
• Electrochemically dissimilar metals must be present
• These metals must be in electrical contact, and
• The metals must be exposed to an electrolyte (salt in solution)

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
The simple solution to this problem lies in finding a less noble metal to use   as a sacrificial anode that corrodes but doesn’t cause staining. Galvanic   corrosion occurs because, when these two metals are in salt water 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.

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

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