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Swimming pool roof collapse

in Uster,Switzerland

2012-12-03

Appearance

of the corroded parts

Description of the Case

Failure caused by pool chemistry

Jingrong Zhao

Yilin Chu

Yongming Jing

In 1985, 12 people were killed in Switzerland when the concrete roof of a swimming pool collapsed only after 13 years of use, which was supported by stainless steel rods in tension and the result of collapse is chloride-induced stress corrosion cracking.

Stress Corrosion Cracking

intergranular SCC of an Inconel heat

exchanger tube with the crack

following the grain boundaries

Tree-like stress corrosion cracks

SCC in a 316 stainless steel chemical

processing piping system

The swimming pool at Uster,

the day after the accident.

Corrosion accident happened in Uster, Switzerland in 1985

  • Atmospheric moisture in pool buildings comes from evaporation of pool water and as droplets from the turbulent water, mostly repeated cycle.
  • Higher water temperatures combined with an increase in the number of bathers, has led to higher levels of chemical disinfection.
  • Chlorine is added at quite high levels to swimming-pool water supplies in order to control bacterial contamination from swimmers which is a very powerful oxidising agent
  • Recirculation of pool air (a common method of reducing energy costs) can increase humidity, as well as adding to the build up of contaminants in the atmosphere.

Stress Corrosion Cracking

WHY???

Chroride

Classifiction of SCC cases by temperature

Classification of SCC cases by type of

austentic stainless steels

Environment

The concrete roof had been held up by a set of stainless-steel tie bars

  • Humid
  • High temperature
  • High Chloride concentration
  • Recirculation of pool air

Materials

Traces of chlorine gas in the general atmosphere of the building were found to be the cause.

Chloride

The chloride was either already present in the concrete or came from the pool via water vapor.

Chloride can overcome the passivity of the natural oxide film on the surface of the steel. The steel, lacking its passive film, readily releases iron atoms into solution (in this case, moisture is present in the concrete due to the humid environment).

Collapse was the result of chloride-induced stress corrosion cracking (SCC). The steel rods had been pitted, causing the roof to cave in.

In the

concrete

Water vapor

from pool

SCC is a type of localized corrosion, which appears only under the specific combination of the following 3 conditions:

  • The use of susceptible grades of stainless steel
  • Tensile stress, either from structural loading or present as residual stresses from forming or welding operations during manufacture and installation
  • The presence of a specific aggressive environment.

It causes microscopic cracks in steel, which can propagate quickly and cause immediate failure. When chloramines reach a certain level of concentration due to poor water maintenance, they evaporate into the air above the pool and then condense along with water vapor on structural steel surfaces, forming a corrosive chloride-rich mixture. This liquid will eat away at almost any steel component of the surrounding structure. When stress cracking corrosion occurs, the steel loses its elasticity and becomes very brittle, it is more prone in stainless steel alloys that are subject to tensile stress in corrosive conditions. Failure can occur without notice and often with tragic consequences.

Two steps:

Methods used to control&prevent

Personal solutions

of the problem

The maximum estimated temperature above a swimming pool ceiling is 40 °C, lower than the 50–60 °C which general corrosion literature states as the minimum temperature at which stress corrosion cracking can occur. However atmosphere in swimming pools, containing the strong oxidator hypochlorite, can cause stress corrosion cracking at much lower temperatures, such as 25–30 °C. Near the ceiling, chlorine containing chemical species in vapors from the pool water can condense onto the stainless steel components and dry out. As this can be a repeated cycle, very aggressive concentrations of chlorine-containing species may build up. The situation is aggravated by the fact that components may not be easily accessible for regular cleaning.

Apparent stoichiometry as a function of solution pH.

Model predictions for monochloramine reduction rates

Inspection Procedures:

Preventive Measures:

lsocorroslon diagram for type 300 series

austenitic SS (caustic)

Coating

Proper Design

The design process is the first and most important step in corrosion control.

Coatings are the most commonly used method for combating corrosion.

Fight with corrosion, abrasion, electrical.

Improve appearance, impact resistance.

  • Avoid Sharp Corner
  • Add Drip Skirt
  • Avoid high localized stresses whenever possible
  • The introduction of compressive stresses can sometimes minimize SCC (do not use when H2 is involve)
  • Improve construction and fabrication practices
  • Improve surface finish(avoid rough surface, residual stresses, microcracks and deleterious metallurgical changes)

The following inspection procedures of safety-critical stainless steel components for SCCand loss of section by pitting are recommended:

  • Compile an inventory of all stainless steel components within the pool building, identifying their grade, location and function;
  • Specifically inspect all stainless steel components at least twice a year for any evidence of staining or corrosion.
  • Reliance should not be placed on visual means alone.
  • The corrosion products should be removed and the loss of cross-section and integrity assessed where staining or corrosion is found;
  • The services of a qualified engineer should be employed to undertake a full risk assessment of the affected components and to recommend a suitable course of action where tests reveal the presence of SCC;
  • If necessary, components should be replaced with a suitable more corrosion resistant stainless steel grade.

Paint

Specific steps can be taken to prevent the onset of SCC and minimize its consequences when it does occur by:

  • Careful consideration of the potential for SCC during the design and fabrication of the swimming pool building and components;
  • Careful selection of appropriate stainless steel grades - some are much more suitable for safety critical use than others;
  • Carefully maintaining the chemical balance of the pool water by regular monitoring and dosing;
  • Ensuring that the bathing load of the pool is not exceeded and that the potential for organic contamination is minimized by the provision of good pre-shower facilities, toilets, and instructions to bathers;
  • Maintaining air quality by the correct operation of ventilation and heating plants;
  • Regular inspection by a competent third party of safety-critical components for signs of corrosion and SCC.

Paints

Paints are the liquid-applied organic coating and lining materials used for corrosion protection in atmospheric or immersion service.

Acylic (Water based)

  • Good light and color stability
  • Moisture and chemical resistance.
  • Ease of application and cleanup.
  • Good compatibility as topcoat over most other generic types.

Metallic Coating

Metallic Coatings

Metals, and in some cases their alloys, can be applied to almost all other metals and alloys as protective coatings.

Electroplatng is the electrode position of an adherent metallic coating upon an electrode. It can confer more than one function

Zinc and cadmium plating of fasteners and other hardware items

AVOIDE Stainless steels and 2xxx, 5xxx and 7xxx alloy

- Stress-corrosion cracking are particularly susceptible to SCC in chloride environments.

- Only alloys that contain appreciable amounts of soluble alloying elements (Cu, Mg, Si, Zn) are susceptible (2xxx, 5xxx and 7xxx);

Resistance, which is measured by magnitude of tensile stress required to cause cracking, is highest when the stress is applied in the longitudinal direction, lowest in the short-transverse direction, and intermediate in other directions.

SCC:

- Directional grain structure introduce sanisotropy;

Material Selection

Applicaiton

Stainless Steels are found in the pool water as:

  • ladders, stairs
  • components of wave machines,
  • diving boards
  • air conditioning systems
  • doors and windows

Reason for widespread use is:

  • Good corrosion resistance
  • Attractive appearance
  • Good workability
  • Acceptable price

Stainless-steel like Type 304 and the 2% molybdenum containing. Type 316 perform well in swimming pools below water level, above water level, not suitable for safety-critical application.

Only the highly corrosion resistant 6% molybdenum-type stainless steel can resist stress corrosion cracking in the aggressive environment.

[1] M.G. Fontana and N.D.Grene, Corrosion Engineering, 2nd Edition, McGraw-Hill, Boston,1978.

[2] Preventing stress corrosion cracking of austenitic stainless steels in chemical plants, ,Masao Nakahara, Asahi Chemical Industries, Ltd.

[3] M. Nakahara, K. Takahashi: Corrosion Prevention'86,213(1986).

[4] T. Nishino,M. Fujisaku: Petroleum Society Journal,13,555(1970).

[5] Lewis, Peter Rhys, Reynolds, K, and Gagg, C, Forensic Materials Engineering: Case studies, CRC Press (2004).

[6] M. Faller and P. Richner: Material selection of safety-relevant components in indoor swimming pools, Materials and Corrosion 54 (2003) S. 331 - 338.

[7] Stainless Steel in Swimming Pool Buildings (1995), Nickel Development Institute (NIDI).

[8] Arup, H and Parkins, RN, Stress Corrosion Research, NATO, 1979.

[9] Newman, RC and Procter, RPM, Stress Corrosion Cracking: 1965-1990, British Corrosion Journal, vol. 25, no. 4, pp. 259-269, 1990

[10] Gangloff, RP (editor) Embrittlement by the Localized Crack Environment, TMS–AIME, 1984.

[11] Staehle, RW and others (editors) Stress–Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, NACE, 1977

References

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