Roofconsult Website  Swimming in the Dangerous Waters of Stress Corrosion Cracking by Peter Reilly of SFS intec
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The days of the chilly Victorian swimming baths are long gone; swimming pools are now found in leisure centres or spas with warm, humid environments boasting whirl pools, saunas and steam rooms. This change in use has had a profound affect on the fabric of the structures in which swimming pools are housed.
The air temperature in pool halls is generally held around 1°C above the water temperature. As the building is heated up and cooled down on a daily basis, a condensation cycle tends to occur every 24 hours. Due to the high levels of chlorine used in modern pool environments, water vapour saturated with Chlorine will condense on cool surfaces, including roof and wall cavities, as well as other important construction components that are not protected by an effective vapour barrier. Chloride salts are left behind when the vapour evaporates as temperatures rise again. Left in place, these salts can start to attack the metal surface and cause stress corrosion cracking (SCC).
SCC is a type of localised cracking in the component material which is defined by fine cracks that can spread rapidly, leading to failure. Steel components, under load in such environments, release hydrogen atoms produced as a by-product of the corrosion mechanism. These atoms pass through the crystalline structure of the metal, sometimes causing microscopic cracks to form. Over time, these cracks can combine to cause a sudden tensile failure in the product.
In harsh swimming pool environments, SCC can occur if the correct grade of stainless steel is not specified. In recent years, fasteners made from 304 and 316 grade austenitic stainless materials were used above the vapour barrier. Further research has been conducted to understand the wider effects of chloride salt deposits attacking the protective "skin" of Chromium Oxide layers of under load components. The research has indicated possible component failure, particularly within the pool atmosphere. picture 1
SCC is not visible to the naked eye, and is therefore hard to diagnose, unlike the normal ‘red rusting’ associated with carbon steel. In addition, these vulnerable components are often hidden from view within cavities, where there is no opportunity for regular inspection and cleaning.
Correct specification
The Nickel Development Institute (NDI) publication ‘Stainless steel in swimming pool buildings’ advises that stainless steels which are susceptible to stress corrosion must not be used for components which are classed as ‘safety critical’. Handrails, suspended ceiling hangers, staircases and structural fasteners fall into these categories.
SFS intec has been monitoring stress corrosion cracking and has developed a range of super austenitic stainless steel fasteners, in line with recent published findings by the British Stainless Steel Association and the NDI. These corrosion resistant fasteners are made of 1.4547 grade "super austenitic" stainless steel, specifically for swimming pools and other extreme applications. Picture 2
The fasteners are also designed so that their thermal conductivity is much lower than that of a normal low alloy carbon steel fastener, thus reducing the risk of condensation forming on the fastener.
Managing stress corrosion cracking
Corrosion can be controlled effectively by a combination of good design, correct selection of SCC resistant materials, and effective swimming pool environment management, maintenance and inspection. SFS intec recommend that grade 1.4547 steel fasteners are used on the pool atmosphere side of an effective vapour barrier, to maintain a secure seal over the useful life of the building and protect the roof cavity. Many new pool structures are specified with timber or Glulam beams that are inherently corrosion resistant; where steel sections are used, powder coating or other long life coatings are required to protect exposed surfaces.
Whilst stress corrosion cracking is most common in swimming pool halls, there are a number of other environments where it needs to be taken into consideration. Facilities such as bleaching plants, laundries, water treatment plants and PVC manufacturing factories all use, and can produce, high levels of airborne Chlorine-rich water vapour. Where a condensation problem exists, caused by poor ventilation or inadequate insulation, salts will be left on vulnerable areas. Components used at motorway flyovers, junctions, linings and tunnels can also be affected when saturated by spray from nearby road surfaces that are heavily exposed to winter salt treatments.
Always keep the internal atmosphere in mind
As with any building, due consideration needs to be given to the internal atmosphere and products specified in swimming pool developments. In harsh environments where SCC can occur, materials such as super austenitic grade 1.4547 steel, offer a safe solution.
If you would like further information on stress corrosion cracking and SFS intec’s range of austenitic stainless steel fasteners then please contact Peter Reilly at SFS intec; telephone 0113 208 5500.
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