By Dr. Peter Dierschke, Materials Consultant to the Hempel Special Metals Group, Oberhausen, Germany
In 1985 the suspended ceiling of a swimming pool in Uster, Switzerland crashed down into the pool and killed 12 bathers. Several years later, in 2001, a similar accident occurred in The Netherlands – fortunately in the second case during the night without casualties. The accidents were found to be caused by stress corrosion cracking of molybdenumfree Type 304 stainless steel fasteners and hangers that carried the weight of the suspended ceiling. While the standard grade stainless steels like Type 304 and the 2% molybdenum containing Type 316 perform well in many applications in and around swimming pools, they should not be used for safety-critical, load-bearing applications. Only highly corrosion resistant 6% molybdenum-type stainless steel can resist stress corrosion cracking in the aggressive environment that can build in spaces where maintenance cleaning is difficult or impossible.
Stainless steels are well established as corrosion resistant, low maintenance, construction materials in and around swimming pools. They are found in the pool water as ladders, stairs, and components of wave machines, around the pool, for example as diving boards, and as parts of the building like air conditioning systems, doors and windows. The widespread use is due to the good corrosion resistance, the attractive appearance, the good workability and an acceptable price. The formation of a thin but extremely dense oxide layer on the surface of stainless steels, the passive layer, protects the steel from corrosion. Because of their corrosion resistance stainless steels are also used in structural applications in swimming pool buildings such as for hangers and fasteners of components such as suspended ceilings, wall panels or water piping and air ducts.