Preventing saltwater scale

by Sally Bouorm | January 1, 2011 2:36 pm

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By Bob Harper

Scale is one of the most common and serious challenges in saltwater pool maintenance. It typically forms first on the cell plates of the electrolytic chlorine generator (ECG), where it can remain undetected until the problem is well advanced. It also hampers the effectiveness and lifespan of the ECG cell, which can be costly both to pool owners, who may need to prematurely replace a cell, and to service companies, which will have to spend extra time on unplanned service calls to combat the issue.

As a natural byproduct of the electrolytic process that produces chlorine from salt molecules, scale will inevitably occur in every saltwater pool to some degree. However, it can be controlled—or, conversely, made worse—depending on the kind of salt and treatment products used in the pool and whether proper care and maintenance principles are followed.

The problem with scale

Scale is created when minerals precipitate out of solution and form mineral deposits. In saltwater pools, the most common minerals that create scale include calcium, phosphate, sulfate and silicate, all of which exist naturally in pool water and salt.

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Pool water, in particular, contains high levels of calcium, which serves to protect pool finishes. When a pool is properly balanced, pH is approximately 7.2 to 7.8 and calcium is less than 400 parts per million (ppm), it generally remains dissolved in solution. As pH rises above 7.8 or the Langelier Saturation Index (LSI) exceeds +0.5, dissolved calcium is more likely to form insoluble compounds such as calcium carbonate, calcium sulfate, calcium phosphate and calcium silicate. These compounds collect on pool surfaces to comprise scale.

In the ECG, where salt molecules are converted to chlorine, the cathode plates produce a strong alkaline (high pH) substance called sodium hydroxide. Since the temperature on these plates is extremely high, as is the pH (almost 14), scale will naturally accumulate as the ECG operates.[3]

As scale forms, it can cause significant heat buildup within the ECG. First, scale is a poor conductor of electricity, so a coating of scale on the cell plates makes the ECG work harder to push electric current through, generating more heat. Second, scale inhibits the natural cooling process within the ECG by reducing direct contact between pool water and the cell plates. As pool water passes through the plates, heat is transferred from the plates to the water; if scale is present, it prevents water from effectively cooling the plates. Excess heat causes damage to the plates’ protective coating, thereby shortening cell life.

Scale also reduces chlorine production, which, in turn, quickly results in chlorine demand, cloudy water and even algae growth. The ECG must also work harder to produce the required amount of chlorine.

The bottom line is that scale prevents the ECG from doing what it is designed to do— produce chlorine that maintains pool water quality. It can also shorten ECG cell life or result in failure, requiring premature replacement.

Early warning signs

As scale forms on the ECG’s cell plates, pool owners may observe small white flakes coming out of the pool returns or accumulating on the pool floor. They may also notice an unusual reduction in chlorine levels or difficulty maintaining chlorine, without other factors such as heavy rain or high bather load.

The following is a typical scenario. When scale accumulates on the ECG cell plates, chlorine production is hampered, and pool water begins to appear hazy. Testing confirms low chlorine levels, prompting most pool owners to turn up the ECG setting or pump time to increase chlorine production. When the new settings fail to solve the problem, pool owners inspect the cell and find scale. Some kinds of scale are visible, such as calcium carbonate; other kinds, such as calcium phosphate, are more difficult or even impossible to see.

Fortunately, once scale is confirmed by a visual inspection of the cell, it can be removed by washing the cell plates. The first step is to remove the ECG cell from the system, which is a relatively simple process. With the cell now accessible, try using a low-pressure hose to rinse off the plates. This will remove any large chunks of scale and may actually clear up the majority of the problem.

If more washing is needed, diluted muriatic acid (hydrochloric) can be used. Soak the cell for 15 to 20 minutes in a solution of four parts water to one part muriatic acid. This will remove most scale from cell plates; however, it also wears away the protective plate coating over time. Preventing scale—and minimizing the need for muriatic acid washings—will extend the ECG’s life.

Preventing scale

Most ECGs have an average lifespan of 10,000 hours, or between five and seven years.  The lifespan of an ECG is determined by the amount of ruthenium oxide coating that is applied to the titanium cell plates. To prevent scale from shortening this lifespan, most ECGs incorporate reverse polarity. Since scale only forms on the cathode, the ECG will automatically reverse its current every two to three hours, so the anode becomes the cathode and vice versa. This allows scale to form uniformly on each terminal. Also, each time the polarity reverses, a bit of scale is dislodged.

While it helps combat the problem, reverse polarity is not a perfect scale prevention technique. It takes a toll on the ECG cell plates, because each time polarity reverses, a small amount of the ruthenium coating is lost. As such, proper care and maintenance is needed to further prevent scale. The following are some basic recommendations for scale prevention:

Use high-quality salt low in scale-forming minerals

Depending on how and where it was made, salt may contain naturally occurring minerals that can contribute to scale. Mined rock salt, which should never be used in saltwater pools, has the highest degree of inorganic contaminants and scale-forming minerals, including calcium, phosphate, sulfate and silicate. Solar salt, which is also not recommended for saltwater pools, may contain scale-forming minerals and large amounts of organic contaminants that can cause cloudy water and chlorine demand in the pool.

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Mechanically evaporated varieties are best for saltwater pools, since the production process usually results in the purest kind of salt, with minimum amounts of both scale-causing minerals and other contaminants. However, the quality of mechanically evaporated salt can vary depending on the geographic location of the salt mine; despite the production method, some mechanically evaporated salts may still contain high levels of contaminants.

Ask suppliers to verify their manufacturing method and level of contaminants in the salt used. They should be able to guarantee the salt’s quality or provide a product specification sheet showing the level of minerals present.

Use treatment products designed for saltwater pools

Anti-scale products designed for traditional pools commonly contain ingredients such as sulfates (dry acid) and phosphates, two substances that contribute to difficult-to-remove scale. For instance, as phosphonate-based anti-scale products pass through the harsh environment within the ECG—with its extreme pH ranges, high temperatures and increased chlorine levels—they break down or denature into elements like orthophosphates, which actually contribute to scale formation. Treatment products specifically designed for saltwater pools should maintain their integrity and effectively do the jobs for which they were designed.

Proper pool maintenance

Using high-quality salt and saltwater-specific treatment products are the two most important ways to minimize scale. However, proper pool maintenance can also help extend the life of the ECG.

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Regular testing to ensure proper water balance is recommended. For residential pools, pH and chlorine levels should be tested weekly, and total alkalinity and calcium hardness should be tested monthly. High levels of pH and calcium can lead to scale formation, so keep these in check.

Finally, periodic inspection and hosing off of cell plates will help prevent the level of scale buildup that requires excessive washing with muriatic acid. Follow ECG manufacturer instructions for recommended cell maintenance.

A manageable challenge

While scale is an inevitable challenge with saltwater pools, proper maintenance and using the right products (including salt) can certainly help. Minimizing scale will pay off in the long run, with properly sanitized water, longer-lasting equipment and happier customers.

 

Bob Harper is general manager of pool products at North American Salt Company, a subsidiary of Compass Minerals. He has more than 20 years of experience in the pool and spa industry and is an expert in saltwater pool systems. He can be reached via e-mail at harperb@compassminerals.com[6].

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