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    These images show the damaging effects of corrosion: steel piping in filter room (right); corroded steel filter with patches and active leak (inset).

Corrosion is only one of the destructive forces acting in and around waterparks and aquatics facilities. But it can be very frustrating for pool operators, and perhaps a little confusing for pool designers and building architects. Selecting appropriate materials and finishes is critical during the design process. Operators must understand their pool and building systems, particularly as they relate to corrosion protection of materials and equipment. 

We’ve all experienced the dreaded appearance of rust spots on our stainless steel ladders and gutters, but corrosion around indoor pools goes much deeper.  With the advent of water slides, multiple sprays, greater surface areas, and warmer water, corrosion potential is greater than ever. 

All the attractive play features and the shallow, zero-depth pools mean that attendance is greater and the moisture loading on dehumidification systems is greater than ever. Perhaps the most helpful design criterion is the ventilation system capacity for fresh air. ASHRAE says to design a natatorium system so that up to 30 percent of the recirculation rate can be brought in as fresh air. We recommend greater fresh air capacity, 50- to 100 percent where feasible.

Stainless steel often is a disappointing material because the expectation is that stainless means no corrosion or staining.  Unfortunately, stainless steel is subject to staining, corrosion or even failure under the right conditions. Dozens of stainless steel alloys exist, so choosing the right alloy during the design process can be very helpful. 

Perhaps the most catastrophic and hidden corrosion danger is the use of stainless steel cables or rods to support a ceiling over a pool area. Several structural failures over pools have led to sudden collapse of ceilings, along with deaths and injuries. The cause was chloride-induced stress corrosion cracking (SCC).

Common stainless steel alloys in tension are subject to SCC due to water vapor from a pool. The structural designer must choose the appropriate stainless steel and related materials for the pool environment.

All of the materials used to build an indoor pool and the building enclosure are potential corrosion targets as well, with a few exceptions. Plastic, glass, tile, stone, masonry and concrete typically resist corrosion quite well. Carbon steel, cast iron, ductile iron, copper, bronze and aluminum are all subject to varying degrees of corrosion, depending on how they are used.

Carbon steel, cast iron and ductile iron can be coated with paint, but this is typically a short-lived protection. Copper will show green corrosion resulting from chlorine in the air. Red brass and bronze typically resist pool corrosion well. Common aluminum alloys are poorly suited for submergence in pool water, but they perform quite well above water as door and window framing.

Pool chemicals can quickly deteriorate the filter room structure and the water treatment equipment. Acid, in particular, is a challenge regarding corrosion. An uncoated steel bar joist roof structure will corrode quickly, creating a maintenance headache. Other metal materials in the chemical room also will quickly corrode. The answer is to ventilate the chemical and filter room air to the outside. Fiberglass and plastic materials also must be considered.

Metal buildings are occasionally considered for pool enclosures. The corrosion potential with an indoor pool environment will definitely challenge a metal building, requiring extensive coating protection and ongoing coating maintenance. 

Galvanizing is one method of trying to protect steel in the pool air environment, with typically poor long-term results. A thin zinc coating is not adequate. Even a hot-dipped galvanized coating will have a limited life in the filter room for a pool. Stainless steel or aluminum air ducting is acceptable. Fiberglass doors and PVC piping are appropriate for most pool area applications. PVC electrical conduit is corrosion resistant and can be painted for better aesthetics.

Something new to some designers and operators may be the growing use of “salt systems” for disinfection. The pool is dosed with 4,000 ppm of salt and an electrical unit is used to create chlorine within the piping system. This disinfection method is gaining acceptance, particularly for smaller pools, but can cause new corrosion problems. Items at risk include some heater cores that are not nickel aluminum bronze, and underwater lights with the wrong bronze alloy.  Adequate electrical bonding is a must with salt pools.

Sacrificial anodes are recommended as well. Most designers have an understanding of the above-mentioned materials. As with many things in life, the challenge is in applying them effectively. As pool systems and equipment continue to evolve, the potential for pool corrosion will continue to challenge our ability to use the right materials to resist the damaging effects of corrosion.