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This paper was originally published in The Structural Engineer, (Volume 82 Issue 9 4th May 2004), and reviews structural failures in swimming pool buildings, attributed to stress corrosion cracking of stainless steel and the progress made since it was identified where the risks of failure can occur. Pool building environments are reviewed in terms of water temperature, disinfection systems, (chlorine), humidity and condensation. Prevention of SCC, (stress corrosion cracking), failure is the joint responsibility of the designer, structural engineer and pool operator. Grade selection to avoid SCC involves considering grades such as 1.4547, 1.4529, 1.4565 or 1.4507 rather than the 1.4301, (304), and 1.4401 / 1.4404, (316), types, which are more suited to non stressed, non safety critical or load bearing applications.
Guidelines for the grade selection, design, fabrication, maintenance, cleaning and inspection of stainless steels items for use in swimming pool buildings. Avoidance of stress corrosion cracking, (SCC), of safety-critical, load-bearing components. (57)
Grades 1.4301, (304), 1.4401, (316), and 430, (1.4016), are compared for food contact applications. A list of typical applications for martensitic, 1.4028, (420), and 1.4116, ferritic, 1.4016, (430), austenitic, 1.4301, (304), 1.4401, (316) and 1.4539, (904L), austenitic 1.4362 and 1.4462, (2205), duplex and 1.4547, (254SMO), superaustenitic types is presented. Corrosion hazards in food applications, i.e. pitting crevice and stress corrosion cracking are discussed. Suitable cleaning and disinfection systems should avoid the use of hypochlorite or chloride solutions.
Localised corrosion mechanisms pitting, crevice and stress corrosion cracking are mentioned, but normally stainless steels are considered “inert” in supply, (or town’s), waters. The affect of chloride levels, temperature, oxygen levels, flow rates and bacterial oxidants, i.e. chlorine on the resistance of stainless steels in waters is discussed. Crevice corrosion should be rare at chloride levels below 200 and 1000 ppm, (mg/lt), respectively, for 304, (1.4301 / 1.4307), and 316, (1.4401 / 1.4404) types. Water chloride ranges for duplex 1.4462, (2205), super austenitic, 254SMO, (1.4547), and super duplex types, SAF2507, (1.4410) and Zeron 100, (1.4501) are also shown.
This Health and Safety Executive Sector Information Minute SIM 5/2002/18 outlines the background to stress corrosion cracking failure hazards to stainless steel items used in indoor swimming poll buildings. This follows on from roof failures in Switzerland in 1985 and more recently in the Netherlands and notes that the atmospheres of indoor swimming pool buildings are amongst the most aggressive ones found in building interiors, where stainless steels are used. The minute discusses the background, noting the effects of chlorine based water disinfection systems and the chloramines generated by body fluid excretions on stainless steel components under applied or residual tensile stress. The stress corrosion process is outlined and the susceptibility of grades 1.4301, (304), and 1.4401, (316), to SCC in swimming pool environments noted. Preventative measures, inspection procedures and recommended actions for HSE inspectors are also covered.
The resistance to sulphur containing gasses is related to chromium content, in the same way as oxidation. High sulphur level fuel oils are not normally considered hazardous to stainless steels. Sulphur dioxide, hydrogen and hydrogen sulphide liquid sulphur and sulphur vapour environments are discussed.
Welding austenitic stainless steels to carbon and low alloy steels are established methods in the process and construction industries. Over-alloyed fillers are used to avoid dilution of the parent stainless steel in the fusion zone. Filler type 308 can be used for joining a 304 type ‘parent’ to a carbon steel but more highly alloyed fillers, such as the 309 type are preferable. There should be no risk of post weld bimetallic, (galvanic), corrosion, if the joint is repainted.
The corrosion resistance of stainless steels is derived from the alloying element chromium. A chromium-rich oxide film forms naturally on the surface of the steel. If damaged, the film will normally repair itself. In this condition the steel is in the passive state. If the film is destroyed the surface is in the active state.
The magazine “Nature” published two articles in the February 14th 2002 which reported research work done at Imperial College of Science, Technology and Medicine, by Mary P. Ryan and co-workers. This BSSA publication sets out to clarify some of the issues raised on the pitting corrosion resistance of stainless steels. The chromium distribution around sulphide type inclusions in a machinability enhanced, (free machining), 316 type is discussed. Reduced chromium in the metal next to the inclusions is suggested as the reason for reduced pitting resistance of stainless steels in general. The sulphur level of the steel investigated is not typical of most of the stainless steels made and so the conclusions on the causes of corrosion may be misleading.