BS EN ISO 7539-11:2014 pdf free

BS EN ISO 7539-11:2014 pdf free.Corrosion of metals and alloys – Stress corrosion cracking
Hydrogen atoms are mobile and can diffuse to sites of potential susceptibility, which may be some distance from the primary source. A fundamental question is how long should a laboratory test be to ensure that hydrogen uptake is sufficient in reflecting behaviour in service, for which exposure times are of the order of years. The critical issue is the location of cracking with respect to the primary source of hydrogen. If the latter is remote, then test times need to reflect this. Hence, hydrogen diffusivity and test time are important. In delayed hydrogen cracking, for example, analysis of the hydrogen distribution with time in response to concentration and stress gradients may be necessary to assess the likelihood of cracking in service.
The location of cracking will be system-dependent. It may be associated with mid-thickness of a low-alloy carbon steel pipeline with centre-line segregation. If using a pre-cracked specimen, it is self-evidently local to the crack tip. In a weld, it could be sub-surface.
The primary source of hydrogen is most likely at a locally strained region if testing corrosion-resistant alloys in the passive state because film rupture sites provide the main points of entry. In this case, test times may be relatively modest unless testing under conditions of pitting corrosion (or crevice corrosion). The local aggressive chemistry associated with pitting and crevice corrosion, together with the dissolution of protective films, will encourage hydrogen uptake. If the crack initiates from a pit, pit size may be a factor and, thus, there may be an effect of exposure time specific to that aspect. Failure may not be expected unless above the critical pitting/ crevice temperature, though there could be an effect of stress on the value.
For systems with no protective film, the primary source of hydrogen is a complex function of the solution chemistry and applied potential. If there is a species in the bulk solution that enhances hydrogen generation and absorption but is depleted in a crack, then the primary source is the external surface exposed to the bulk solution. Examples are acid solutions and solutions containing hydrogen sulphide.
However, in H2S environments, the formation with exposure time of an iron sulphide film on the exposed external surface will progressively lead to a reduction in hydrogen entry and may change the locality of the primary source to that of the crack tip.BS EN ISO 7539-11 pdf download.