Stainless Steel Fasteners in the atmosphere of indoor swimming pools!
publicatiedatum juli 2018
Dutch Law ‘Bouwbesluit art. 5.12.’ on this subject
Jan Heselmans, Ironhaven BV, Hoofddorp (NL).
On November 1st 2011 two speaker boxes landed on a mother and her child in swimming pool ‘de Reeshof’ in Tilburg NL. The baby sadly died and her mother was injured. The cause of this accident, is probably stress corrosion cracking of stainless steel bolts. This accident has never been investigated; it’s unconfirmed what material the bolts were made of. The rumor goes they were made of the exotic stainless steel 1.4529 (a 6% Molybdenum stainless steel). The Dutch parliament demanded for a Law on this subject, the Bouwbesluit art. 5.12. Since July 1st 2016 stainless steel loaded elements in the atmosphere of swimming pools are forbidden in the Netherlands.
The cracking occurs on loaded austenitic stainless steel elements in the atmosphere of indoor swimming pools, being fasteners and hanging constructions. Such elements are often highly cold formed, making them susceptible to stress corrosion cracking.
What is the corrosion mechanism? This is hydrochloric acid stress corrosion cracking. Urine and other nitrogen bearing (a.o. ammonia and ureum) human waste products in the water react with free chlorine (Cl2), thus forming the gas di-, tri-chloramine. Especially during peak crowdedness, this gas escapes into the atmosphere of the indoor swimming pool. Combined with a humidity of 60-95% this chloramine forms a microscopic thin hydrochloric acid layer on solid parts like stainless steel. All austenitic stainless steels are susceptible to hydrochloric acid stress corrosion cracking at temperatures as low as 20 °C and higher. Galvanized steel is not susceptible to stress corrosion cracking because of it’s ferritic structure. General corrosion of the zinc layer occurs at a rate of 0,3 μm per year. According to ISO 4042 the layer is >5 μm thick, meaning the lifetime of the zinc layer is in average 15 years. After this...