Stainless Steel in Swimming Pool Buildings
- Stress Corrosion Cracking (SCC) – The Problem
- Stress Corrosion Cracking (SCC) - The Solution
- SCC Education
- Stainless Steel in Swimming Pools – Non-Critical Applications
This section provides a Resource Centre for anyone involved in designing and using stainless steel internally in Swimming Pool Buildings.
Stainless steel has been used successfully in many swimming pools for applications such as:
- Handrails and balustrading
- Pool Ladders
- Changing room equipment, lockers etc
- Movable pool booms
- Structural applications – lighting support structures, heating duct supports, window frames
- Ancillary equipment such as catering in areas immediately adjacent to the pool
- Pool Linings
Its corrosion resistance, aesthetic appearance and ease of maintenance make it an excellent choice for these applications.
However, there are factors which are very specific to swimming pool buildings which need to be taken into account when designing new swimming pools or refurbishing existing ones.
The most important of these factors is the potential for stress corrosion cracking in some grades of stainless steel.
Stress Corrosion Cracking (SCC) – The Problem
Stress corrosion cracking is a relatively rare phenomenon in stainless steels. It requires 4 factors to occur:
- Tensile stress. Either from an externally applied load or as a residual stress from the fabrication process
- Sufficiently high temperature
- Sufficiently corrosive environment
- A susceptible material
Until 1985 all the scientific evidence was that SCC in standard stainless steels did not occur below about 50 deg C. Up until then, SCC was usually found in process engineering for either chemical or food and drink. However, a fatal accident in a swimming pool at Uster Switzerland led to research which completely changed the understanding of the phenomenon.
The design of the pool roof was a concrete slab supported by a large number of 1.4301 (304) stainless steel rods. When a sufficient number of rods had failed by SCC, the remaining rods were insufficient to support the roof, resulting in 12 fatalities and more injuries. Although pools have become warmer over the years, they do not reach the “traditional” 50 deg C mark. When SCC was diagnosed, this came as somewhat of a shock to the corrosion science community.
The subsequent research showed that SCC could occur at 25 deg C in conditions of:
• High chloride content plus high acidity
• Very high chloride in neutral conditions
The cause of these conditions in swimming pool buildings is the production of organic chemicals called chloramines. These are formed by the interaction of chlorine-based pool disinfection chemicals and organic compounds from bathers, notably sweat and urine. These chemicals evaporate and condense on surfaces and can be highly concentrated depending on the combination of temperature and humidity in the pool hall. Areas in the roof space are particularly vulnerable as there is no opportunity for natural washing or easy maintenance of such components. Immersed components are not at all susceptible to SCC.
Since the initial disaster at Uster, there have been a number of further occurrences of SCC in swimming pools and this illustrates the need for an understanding of the issues to be further disseminated to the designers, builders and operators of swimming pools.
Stress Corrosion Cracking (SCC) - The Solution
The Uster accident led to much research and a comprehensive programme of laboratory and field testing was carried out. SCC resistant grades of stainless steel had already been developed for other applications so it was natural that these grades should be tested to see if they could do the same job in a swimming pool environment.
It has been found that many of these grades give a significantly improved resistance to SCC in this environment. These are now recommended for any load bearing safety critical component in the pool hall. Alongside the use of SCC resistant grades, current good design practice avoids the kind of highly stressed suspended ceiling at Uster.
Overall, the number of incidents compared to the number of swimming pools is very low. However, it is still important to ensure that the whole supply chain is educated to a sufficiently high level to ensure that no further incidents occur.
For some years the BSSA has participated with other bodies to ensure the widespread dissemination of useful information. These include:
• Nickel Institute
• Steel Construction Institute
• Euro Inox
• International Stainless Steel Forum
• Pool Water Treatment Advisory Group
• Health and Safety Executive
• Institute of Sport and Recreation Management
Most recently a European standard EN 13451-1:2011 has been introduced to provide guidelines on the corriect materials of construction for swimming pools. The specific guidelines for stainless steels have been summarised by EuroInox and can be downloaded from here.
Information on stainless steel in swimming pools is viewed from the BSSA website about 400 times per month but we are always looking for new ways to get the information to where it is needed.
The BSSA and Nickel Institute have jointly developed a series of CPD Modules for architects. Module 4 specifically covers the use of stainless steels in swimming pools. Architects can book a free seminar by contacting the BSSA on firstname.lastname@example.org at the BSSA office or calling 0114 292 2636.
Health and Safety
Another initiative was taken in discussion with the Health and Safety Executive. This resulted in the issuing of a Sector Information Minute SIM 5/2002/18. This was issued in 2002. This document was circulated to everyone at local authority level having responsibility for the health and safety of public swimming pools. This can also be found on the HSE website. This important document suggests that a full “inventory” of stainless steel components should be made at each pool and inspection and maintenance programmes be set up.
Individual swimming pool projects using stainless steel can be discussed with the BSSA Technical Advisor either by using the Technical Enquiry Form or by contacting him directly on 0114 292 2637.
Stainless Steel in Swimming Pools – Non-Critical Applications
By far the bulk of stainless steel in swimming pools is used for its decorative properties and durability. For these applications, the key issue is not SCC but rather ensuring that the material retains its initial finish throughout its life. The standard stainless steels like 1.4301 (304) and 1.4401 (316) have proven effective in many pools. However, examples of badly stained stainless steel are reported from time to time and it is worth examining the factors which can cause this and the remedial action which might be taken.
The surface finish is just as critical in determining the corrosion resistance of stainless steel as the grade. As with coastal applications, poor quality polished finishes can lead to disappointing performance of stainless steel. A full explanation of this phenomenon can be found at:
Importance of Surface Finish in the Design of Stainless Steel
Swimming pools have the additional complication of the large variations in conditions which can exist in a pool. The corrosivity is influenced by:
- Ventilation and air recirculation
- Disinfection regime
- Cleaning and maintenance regime
- Temperature control
- Bather loading and behaviour
- Local “mini-climates” within the pool hall
These factors can lead to variations in performance in components in different places within the pool hall made from the same grade and surface finish.
A combination of grade 1.4401 with a bright polished surface will give the best performance in any specific environment.
Stainless steel is a suitable choice of material for swimming pool applications provided that:
- The possibility of failure by Stress Corrosion Cracking is properly assessed for load bearing components
- The grade and surface finish is chosen to give optimum corrosion resistance