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2012-12-03
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.
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
Classifiction of SCC cases by temperature
Classification of SCC cases by type of
austentic stainless steels
The concrete roof had been held up by a set of stainless-steel tie bars
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:
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.
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
lsocorroslon diagram for type 300 series
austenitic SS (caustic)
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.
The following inspection procedures of safety-critical stainless steel components for SCCand loss of section by pitting are recommended:
Specific steps can be taken to prevent the onset of SCC and minimize its consequences when it does occur by:
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;
Stainless Steels are found in the pool water as:
Reason for widespread use is:
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