November 2001

Frequent cleaning has maintained the condition of this 25-year-old stainless steel rail and gutter system.

It’s possible, however, for problems to occur regarding stainless alloys in natatorium environments. But aquatics professionals can generally avoid potential problems by paying proper attention to pool operation, maintenance and material selection.

Aggressive environments
The chloramines produced by pools disinfected with chlorine-based chemicals can pass into the atmosphere, where they can accumulate as condensate on above-water surfaces. When the chloramines decompose, they can form a solution that initiates corrosion of underlying metals.

The aggressiveness of a natatorium environment depends on many variables, including water and air temperature, relative humidity, bather load and ventilation strategy. The presence of waterfeatures such as slides, fountains, jets and wave machines increases the evaporation rate and tends to make the environment more aggressive. Water-chemistry practices that minimize formation and accumulation of chloramines will minimize the associated aggressiveness in the pool environment.

Note that the same conditions which create an environment aggressive to stainless steel also increase the risk of deterioration of other materials commonly used in natatoriums, including painted galvanized steel, brass, aluminum, nylon, masonry and timber.

Occasionally, operators report problems with stainless steel. These problems tend to fall into one of three categories: staining caused by the pool environment, staining from contact with undesirable contaminants and, in rare cases, premature fracture of load-bearing components due to stress corrosion cracking (SCC).

Preventive measures
Excessive airborne chloramines can result in brown stains on stainless steel components. In addition to the previously mentioned variables, the production of brown stains are influenced by the type of stainless steel used and each facility’s maintenance schedule.

Brown staining typically does not impair the structural integrity of stainless steel. Stain removal may require aggressive cleaning or surface reconditioning with acid-based products such as stainless steel cookware cleansers or Naval Jelly.

Operators can help prevent staining by rinsing all stainless steel items in the immediate pool area each day. Stainless items in remote locations — ventilation ducts or rails on viewing platforms — should be wiped down once a week. Components located in areas that are difficult to clean, such as near ceilings, are most prone to problems.

Brown staining also occurs from contact with inappropriate contaminants. For example, operators who use muriatic acid to clean concrete or tile, or bleach solutions for disinfection, should immediately and thoroughly rinse stainless surfaces in the vicinity. Operators should never use brushes with carbon steel bristles or steel wool on stainless surfaces.

Although incidents are rare, SCC has been extensively investigated because of the potentially severe consequences. Highly stressed stainless steel components with grades such as 201/201L, 304/304L, 316/316L and 321 can become susceptible to fracture in natatoriums when there is excessive accumulation of chlorine-containing compounds on the surface of the alloy. Load-bearing components in hard-to-reach locations are most susceptible. Components that are immersed or frequently splashed with water are not affected.

Facility designers and operators can avoid SCC by selecting an appropriate grade of stainless steel for load-bearing components such as hangers, fasteners, brackets and supports. The 317LMN, 904L and 6-percent Mo grades resist this mode of attack — no SCC failures of these alloys in natatorium environments have ever been reported.