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Stainless steel fasteners applied in indoor swimming pools

We are often confronted with questions about the use of stainless steel fasteners in swimming pools, as people are warned for the hazards and accidents that have occurred in the past. The following examples show the importance: 
  • In the sports and recreational centre ‘De Scheg’ in Deventer (the Netherlands), an air duct dropped from the ceiling (no injuries because the pool was empty at the time).
  • In the indoor swimming pool ‘De Waterwijck’ in Steenwijk (also in the Netherlands) the entire ceiling collapsed (no injuries because the pool was empty at the time).
  • In Uster, Switzerland, also the ceiling of a swimming pool collapsed, killing 12 people.
  • In the swimming pool ‘de Reeshof’ in Tilburg (again in the Netherlands) 2 sound speakers dropped from the ceiling, killing a 5 months old baby girl.
In all cases, after examination, corrosion failure of austenitic stainless steel fasteners was the cause of the accidents.

The issue

The environment in indoor swimming pools (a combination of high humidity, chloride vapors and a relatively high temperature) is aggressive for certain metal alloys. In the past common austenitic stainless steel fasteners were assumed to be resistant to this atmosphere and were therefore applied on a large scale in swimming pools. These types of stainless steel turned out to be sensitive to chloride induced stress corrosion cracking. Especially directly above the basin exists a chloride saturated atmosphere, that can freely interact with the materials and when there is no frequent contact with water from the pool, the impact is huge.  
 
This form of corrosion is very hard to detect, and applications may suddenly fail without warning. Standard austenitic stainless steel fasteners in A2 and A4 are therefore definitely unsuited for load bearing applications above the basin of indoor swimming pools. 
 
These safety issues created an urgent need for clarity about the already applied materials and raised questions about what materials to use in the future for these applications.
The information in this newsletter is intended for the entire construction chain from client, designer to contractor, producer and installer, owners and administrators of the swimming pool. 
In addition to applicable materials in different areas and applications in swimming pools, local directives and standards deal extensively with the inspection of swimming pools. These directives and standards are commonly made available by governments and standardization institutes. This document contains a resume of the most important issues. 

Influenced areas 

Attention to chloride induced stress corrosion cracking must be paid to all parts of load bearing constructions and fasteners in indoor swimming pools. This relates to all areas that are in open contact with the basin area including all areas connected by air ducts. This also includes e.g. indoor playground areas, containing chlorinated or salt water. 
 
This information is not applicable for:
  • Plastic construction parts or fasteners.
  • The main construction and/or roof construction. 

Area layout 

In order to determine which materials can be used in what spaces, three area types are distinguished.

Area A

  • The basin itself and its immediate surroundings (the so-called splash zone, within 1 meter vertically and 1 meter horizontally from the basin).

Area B

  • The space outside area A, but within the basin area. 
  • All other areas in direct open contact with the swimming pool atmosphere. 

Area C

  • All areas outside the pool area that are in no way connected to the atmosphere of the basin area.

Choice of materials

In the choice of material a distinction is made based on the area layout mentioned above. Table 1 shows the suitable materials for these different areas. 
 
Table 1
 
Conditions:
  1. Restrict application to easily accessible locations, in order to allow direct visual inspection.
  2. Recognizability by using unique visible markings on every part.
  3. Traceability by recording the origin of the product, the location where it is used (photos and layout schematics) and material certificates.
  4. Apply plain parts only (no coating/paint).
  5. Only apply when no risk for physical injuries exist in case of failure.

Additional information

Several (constructive) parts in swimming pools may have been put up by means of stainless steel elements. This may include ceilings, air ducts, lights, speakers and so on. Stainless steel is a family of alloys of which some members are susceptible to chloride induced stress corrosion cracking, that may cause sudden, unexpected fracture. The swimming pool atmosphere, due to the use of chlorine-based disinfectants, can be aggressive for the most popular types of stainless steel, like A2 (304, 1.4301) or A4 (316, 1.4401), that are therefore referred to as non-resistant. Such material can already show visible corrosion before chloride induced stress corrosion occurs, but this isn’t always the case. Other, high alloyed types of stainless steel are insusceptible to chloride induced stress corrosion in swimming pool atmospheres. These are the so-called resistant stainless steels 1.4529, 1.4547 and 1.4565.
 
The difference between the two groups of stainless steels is made clear in the recent and current directives and standards: only resistant stainless steel may be used in load bearing constructions in the swimming pool atmosphere. This is mandatory for new load bearing elements in swimming pools (see EN 1993-1-4:2006 + A1:2015, Annex A). The directives and standards further clarify that regular inspection is necessary; they recommend a three-year interval for these inspections.
 
Stainless steel parts in other areas of a swimming pool, in direct contact with the swimming pool water, regularly wetted by splashing water or regularly cleaned, are not susceptible to stress corrosion. Therefore standard types of stainless steel, A2 and A4, may be applied in these areas.

EN 13451-1:2011+A1:2016 standard annex F also provides information

Stainless steel is used for many constructions in swimming pools. "Stainless steel" is thereby a collective
name for a multitude of different materials with different alloy-composition.
Beside appropriate constructive solutions (e.g. avoidance of gaps) and a surface as smooth as possible,
the selection of the right material is the most decisive criterion to avoid corrosion problems. Another
decisive criterion is the accessibility for cleaning and inspection.
 
The stainless steel grades indicated in this ”Annex F" shall be used for structural parts in chloride
environment; they represent the current knowledge at the time of development of this European
Standard. As technical development is going on, additional stainless steel grades may be used provided
there is a suitable and complete documentation and experience about their resistance to stress
corrosion cracking.
 
Corrosion can appear visible (e.g. corrosive pitting) or invisible and spontaneous without
announcement (e.g. stress crack corrosion) .
 
The notation shall conform to EN 10088–1 and EN 10088–2. Beside a number, every steel has a short
notation (e.g. steel no. 1.4301 has the short notation X5CrNi18-10). 

Indoor swimming pools with disinfection with chlorine

General

In indoor swimming pools, the possibility of a highly corrosive environment with enrichment of
chlorides caused by drying and evaporation effects shall be taken into account.

Components without the possibility of regular cleaning

For structural parts in chloride environments, the appearance of chloride inducted intercrystalline
stress crack corrosion shall be taken into account. Therefore, in indoor swimming pool equipment and
their structural parts made of stainless steel without regular cleaning only the following materials shall
be used:
  • 1.4565 (X2CrNiMnMoNb25-18-5-4)
  • 1.4529 (X1NiCrMoCuN25-20-7)
  • 1.4547 (X1CrNiMoCuN20-18-7)
NOTE In environments where water has a chloride concentration of less than 250 mg/l (drinking water) also the material 1.4539 (X1NiCrMoCu25-20-5) could be used.

Components with the possibility of regular cleaning

General

Considering the real corrosiveness and further relevant conditions, e.g. temperature, humidity, etc., and
only in case of regular cleaning for easy accessible components and parts of the swimming pool
equipment beside the above mentioned materials one of the following materials shall be used:
  • 1.4401 (X5CrNiMo17-12-2)
  • 1.4404 (X2CrNiMo17-12-2)
  • 1.4578 (X3CrNiCuMo17-11-3-2)
  • 1.4571 (X6CrNiMoTi17-12-2)
  • 1.4439 (X2CrNiMoN17-13-5)
  • 1.4462 (X2CrNiMoN22-5-3)

Inspection and cleaning concept

Conforming to the result of a risk assessment a concept for regular inspection and cleaning has to be
established by the manufacturer. The cleaning concept has to minimize the enrichment of chlorides on the surface of the material and shall require appropriate records.

Outdoor swimming pools with disinfection with chlorine

Outdoor pools with disinfection with chlorine are in general a less corrosive environment, although
locally, e.g. above water surfaces, a higher corrosiveness can occur. The danger of enrichment of
chlorides is smaller, because rainfall will wash the electrolytes away.
 
The selection of the proper material for swimming pools equipment and their structural parts shall be
performed carefully, considering the corrosiveness of the environment and the foreseen cleaning of the
surfaces. In less corrosive environments or where regular cleaning for easy accessible parts and
components is foreseen, one of the following materials shall also be used:
  • 1.4301 (X5CrNi18-10)
  • 1.4307 (X2CrNi18-9)
  • 1.4567 (X3CrNiCu18-9-4)
  • 1.4541 (X6CrNiTi18-10)
  • 1.4318 (X2CrNiN18-7)

Coatings and paintings

The coating of stainless steel surfaces is not sufficient protection against corrosion and never justifies the selection of a less corrosion-proof material.

All this implicates that in new constructions of load-bearing structures above swimming pools only stainless steel fasteners in types 1.4529, 1.4547and 1.4565 are to be used. 

Fasteners in resistant stainless steel type 1.4529 that are readily available

  • DIN 125-1A (≈ISO 7089) Plain washers (M3 up to and including M36)
  • DIN 127B Spring washers (M6 up to and including M30)
  • DIN 931 Hex head bolts (M4 up to and including M52)
  • DIN 933  Hex head screws (M3 up to and including M36)
  • DIN 934 (≈ISO 4032) Hex nuts (M3 up to and including M48)
  • All metal hex prevailing torque nuts ≈DIN 980 (M6 up to and including M24)
  • Hex coupling nuts H=3xD (M8 up to and including M16)
  • DIN 1587 Hexagon domed cap nuts, high type (M5 up to and including M20)
  • DIN 976-1 Threaded rods in different lengths (M6 up to and including M48)
  • DIN 9021 (≈ISO 7093) Plain washers with large OD (M3 up to and including M33)
  • ISO 10642 (≈DIN 7991) Hex socket countersunk head screws (M3 up to and including M30)
  • fischer  anchors

Fasteners in resistant stainless steel type 1.4547 (254SMO) that are readily available

  • Nord-Lock wedge lock washers, (M3 up to and including M39)
  • Nord-Lock wedge lock washers, (¼” up to and including 1”)
  • Nord-Lock wedge lock washers with large OD, (M3,5 up to and including M27)
  • Nord-Lock wedge lock washers with large OD, (¼” up to and including 1”)
Furthermore many other products and/or materials are available on request.

Other situations in which chloride induces stress corrosion cracking is a hazard

Apart from indoor swimming pools other situations exist in which chloride induces stress corrosion cracking plays a role. Places with a chlorinated atmosphere due to e.g. the presence or the use of chlorinated disinfectants, road salt or salt water. Examples of such places are traffic tunnels, chemical plants, offshore, petrochemical industry, pipelines, storage tanks, refineries, drinking water purification plants and salt extraction facilities.

Conclusion

When using fasteners in a situation where chloride induced stress corrosion may be a risk, only use suitable materials and correctly record everything (markings, locations, material certificates, and inspection periods). In case of doubt always seek information from experts.
 
(Sources: ISO 3506-1:2009, EN 1993-1-4:2006/A1:2015, EN 13451-1:2011+A1:2016, Nederlandse praktijkrichtlijn NPR 9200:2015, DIBt Allgemeine bauaufsichtliche Zulassung Nr. Z-30.3-6:2017, SCI Design Manual for Structural Stainless Steel 4th edition:2017)