Stress corrosion cracking
Stress corrosion cracking is the transcrystalline (through the grain ) or intergranular (along the grain boundaries of the structure ) crack formation in materials under the simultaneous influence of a purely static tensile stress or with superimposed low-frequency tensile stress and a specific attack agent . Tensile stresses in the form of internal stresses are also effective.
In general, there are no visible corrosion products in stress corrosion cracking (SCC) . The separation is low in deformation.
Certain groups of materials are sensitive to stress corrosion cracking. These include copper-zinc alloys ( brass ), some wrought aluminum alloys , rust- and acid-resistant steels (in some cases ) and martensite-hardenable steels. High-strength steels (e.g. for prestressed concrete ) can under certain circumstances be susceptible to hydrogen-induced stress corrosion cracking, and unalloyed and low-alloy steels to stress corrosion cracking caused by nitrates . Even with low-alloy gold alloys (fineness below 585 / -) with zinc components or alloys with nickel components (here also with fineness above 585 / -), damage up to the complete dissolution of the structure occurs. Stress corrosion cracking can also occur in plastics . The prerequisite for this is the presence of tensile stress and a high level of alkalinity in the environment. This problem is important e.g. B. in fastening technology , when plastic anchors (e.g. made of polyethylene ) are to be used in concrete substrates.
Three conditions must be met for stress corrosion cracking to occur:
- the material must be sensitive to stress corrosion cracking,
- Tensile stresses must exist
- a specific means of attack must be available.
In rust- and acid-resistant austenitic steels, chlorides act as specific attack agents , in copper-zinc alloys ( brass ) as well as in gold alloys with a zinc content, ammonia , amines , ammonium salts, sulfur dioxide , nitrogen oxides , nitrite , nitrate, mercury salts etc., also in aluminum Chlorides (sea water). Unalloyed and low-alloy steels are sensitive to alkali hydroxides.
SPRK-resistant steels play an important role in the petroleum / natural gas industry. There were and are frequent mistakes when choosing a suitable material in media containing H 2 S. Even at very low partial pressures, steels can fail in media containing H 2 S. Alloys like 13% Cr steels are still mistakenly used widely. In many cases, however, you have to resort to duplex , super-duplex or similarly expensive materials. H 2 S can be found in a relatively large number of gas reservoirs or in the associated gas in crude oil reservoirs. Even 600 ppm H 2 S can cause irreparable damage. In some deposits up to 20% (200,000 ppm) H 2 S is extracted.
These attack agents work even in very low concentrations, sometimes in the ppm range.
The crack initiation time and the crack propagation speed depend on the level of tensile stresses, the concentration of the attack agent and the degree of work hardening . The very low tensile stress of 10 MPa is specified as the threshold value for brass .
The time until the component is completely torn through, i.e. until it fails, can range from minutes to several years. In the case of gold jewelery with a fineness of 333, the alloy can, in extreme cases, be attacked after just one wear.
There have been some spectacular accidents due to stress corrosion cracking:
- The Kongresshalle Berlin ("Pregnant Oyster") partially collapsed on May 21, 1980 due to stress corrosion cracking of the prestressed concrete steel wires.
- On May 9, 1985, as a result of stress corrosion cracking from moisture containing chloride, the concrete ceiling of the indoor swimming pool in Uster / CH, which was suspended from anchors made of austenitic steel, fell.
- On February 14, 2004, the Transvaal Park in Moscow collapsed due to stress corrosion cracking on components made of A4 stainless steel (1.4404). There were 28 dead and 198 injured.
- On December 4th, 2005 the Delphin swimming pool in Chusovoy (RU) collapsed due to stress corrosion cracking of components made of stainless steel. 14 dead and 38 injured.
- On November 1, 2011, two loudspeakers fell on a baby and her mother in the Reeshof swimming pool in Tilburg (NL). There was one dead and one injured. The cause was stress corrosion cracking of stainless steel 1.4529 (6% Mo stainless steel).
To avoid stress corrosion cracking, at least one of the three conditions must be avoided. So you can either keep away the attack agent, avoid tensile stresses or choose an insensitive material.
Keeping the attack at bay is often not possible. With copper-zinc alloys, general air pollution, a farm nearby ( ammonia from the dung heap) or keeping an ammonia-containing household cleaner near the component is often sufficient . In the case of the “Uster indoor swimming pool”, chloride pollution could hardly be avoided. The tensile stresses can often not be avoided, as the suspension of the indoor pool ceiling shows. In such cases, the only option is to choose a material that is insensitive to stress corrosion cracking.
A direct investigation does not currently appear possible. Existing cracks or breaks in prestressing steels as a result of stress corrosion cracking can be located using the magnetic stray field method.
- Corrosive medium
- Vibration corrosion cracking
- Corrosion element
- Stress crack
- High Frequency Impact Treatment : Weld seam treatment process to extend the service life by rounding, smoothing and strengthening the notch at the seam transition and introducing residual compressive stresses.
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- J. Rückert: Stress crack corrosion on copper alloys Materials and Corrosion 47 (1996) pp. 71-77.
- "Technical and scientific principles of goldsmithing: Materials science of precious metal processing: Part 2", pp. 88–93; BVA Bielefeld, 1999 ( ISBN 3-87073-270-9 ).
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- M. Faller and P. Richner: Safety-relevant components in indoor swimming pools , Switzerland. Ing. Arch. 2000 (16), pp. 364-370. ( Online (3.7MB) ).
- the editors: RVS in zwembaden is as een kanarie in een kolenmijn. Ed .: AluRVS. Leiden 2017.
- Johan van den Hout: Technisch onderzoek ongeval zwembad de Reeshof dd November 1, 2011 . Ed .: Provincie Noord Brabant. 's Hertogenbosch 2012.
- Stress corrosion cracking of PMMA and PS Youtube Video: Crazes at PS
- DIN 50922, edition 1985-10: Corrosion of metals; Investigation of the resistance of metallic materials to stress corrosion cracking; General
- DIN EN 14977, edition 2004-07: Copper and copper alloys - Finding tensile stresses - 5% ammonia test; German version prEN 14977: 2004
- DIN 50908, 1993-04 edition: Testing the resistance of wrought aluminum materials to stress corrosion cracking
- DIN 50915, Edition 1993-09: Testing of unalloyed and low-alloy steels for resistance to intergranular stress corrosion cracking in attack agents containing nitrates; Welded and unwelded materials
- DIN 50916-1, edition 1976-08: Testing of copper alloys; Stress corrosion cracking test with ammonia, testing of tubes, bars and profiles
- DIN 50916-2, edition 1985-09: Testing of copper alloys; Stress corrosion cracking test with ammonia; Testing of components
- DIN EN 14101, Edition 2002-10: Aerospace - Criteria for the choice of material to avoid stress corrosion cracking; German and English version EN 14101: 2001
- DIN EN 12502-2, Edition 2005-03: Corrosion protection of metallic materials - Instructions for estimating the probability of corrosion in water distribution and storage systems - Part 2: Influential factors for copper and copper alloys; German version EN 12502-2: 2004
- DIN EN ISO 196, edition 1995-08: Copper and wrought copper alloys - Finding residual stresses - Mercury (I) nitrate test (ISO 196: 1978); German version EN ISO 196: 1995
- DIN EN ISO 7539-1, edition 1995-08: Corrosion of metals and alloys - Testing of stress corrosion cracking - Part 1: General guidelines for test methods (ISO 7539-1: 1987); German version EN ISO 7539-1: 1995
- DIN EN ISO 7539-2, edition 1995-08: Corrosion of metals and alloys - Testing of stress corrosion cracking - Part 2: Preparation and application of bending specimens (ISO 7539-2: 1989); German version EN ISO 7539-2: 1995
- NACE TG 498. Former task group for a DIN-EN-ISO standard for better safety in relation to stress corrosion cracking of stainless steel in swimming pools.