Stress corrosion cracking
Stress corrosion cracking is transgranular. ( By the structural grain ) or intergranular (along the grain boundaries of the microstructure) cracking in materials under the simultaneous influence of a purely static tension or with superimposed low-frequency Zugschwellspannung as well as a specific plea Also tensile stresses in the form of residual stresses are effective.
Materials and Mechanism
The stress corrosion cracking ( SCC ) generally occur no visible corrosion products. The separation is little deformation.
To stress corrosion cracking certain groups of materials are sensitive. These include copper - zinc base alloys ( brass ), some aluminum alloys, stainless and acid resistant steels ( in part) and maraging steels. Even high-strength steels (eg for prestressed concrete ) may be susceptible to hydrogen-induced stress corrosion cracking, unalloyed and low-alloy steels to stress corrosion cracking caused by nitrates in certain circumstances. Even with low- gold alloys ( Fineness under 585 / -) with zinc alloys with nickel shares or interests ( here even at fine levels above 585 / -) damage occurs to the complete resolution of the structure. Furthermore, stress corrosion cracking occur in plastics. The prerequisite for this is the presence of tensile stresses and a high alkalinity of the environment. Of importance, this problem is, for example, in fastening technology when plastic anchors (eg, polyethylene ) are to be used in concrete substrates.
For the occurrence of stress corrosion cracking three conditions must be met:
- The material has to be sensitive to stress corrosion cracking,
- Tensile stresses must be present
- A specific plea must be present.
As specific plea also act in rust and acid resistant austenitic steels chlorides of copper -zinc base alloys as well as in gold alloys with zinc content are there ammonia, amines, ammonium salts, sulfur dioxide, nitrogen oxides, nitrite, nitrate, mercury salts, etc., with aluminum chlorides ( seawater ).
A significant role have SCC - resistant steels, among others, in the Erdöl-/Erdgas-Industrie. There were and are common mistakes when choosing a suitable material in H2S - containing media. Even at very low partial pressures can fail steels in H2S - containing media. Alloys such as 13% Cr steels are used incorrectly still common. In many cases, you have access to, however duplex, super duplex or similar expensive materials. H2S is found in relatively many gas reservoirs, or in the associated gas in oil reservoirs. Already 600 ppm H2S can cause irreparable damage. In some deposits ( 200.000ppm ) H2S can be promoted up to 20%. In case of damage, the result could be serious accidents.
These pleas are effective even at very low concentrations, partly in the ppm range.
The crack initiation period and the crack growth rate will depend on the level of tensile stress, the concentration of the plea and the degree of work hardening. As the threshold value, the very low tensile strength of 10 MPa is indicated in brass.
The time to complete tearing of the component, ie, to failure, can range from minutes to several years. With gold jewelry with 333 Fineness can, in extreme cases, even after wearing once an attack of the alloy take place.
Due to stress corrosion cracking, there have been some spectacular accidents:
- The Congress Hall in Berlin ( " pregnant oyster " ) is partially collapsed on May 21, 1980 due to stress corrosion cracking of prestressed steel wires,
Remedy
In order to avoid stress corrosion cracking, at least one of three conditions must be avoided. So you can either keep the plea, avoid the tensions or select an insensitive material.
The keeping of the plea is often not possible. For copper -zinc alloys is often enough to the overall air pollution, a farm nearby ( ammonia from the manure pile ) or the retention of an ammonia-based household cleaner in the vicinity of the component. Even in the case "Indoor pool type ' chloride contamination was difficult to avoid. The tensile stresses can not be avoided many times as the suspension of the indoor swimming pool ceiling. In such cases remains only the choice of a material that is resistant to stress corrosion cracking.
Study opportunities
A direct investigation of which does not seem possible. Existing cracks or breaks in prestressing steel due to stress corrosion cracking can locate the remanent magnetism method.
- Prestressing steel fracture location