Embrittlement environmental

Rockwell C 22 is the commonly selected limit above which sulfide embrittlement and resultant sulfide stress cracking become problems. The change, however, is not that abrupt but the critical "gray band" is about C 20 to 25, with the point of change affected by mechanical, physical and chemical environmental factors. 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

Owing to hydrogen embrittlement, the mechanical properties of metallic and nonmetal-lic materials of containment systems may degrade and fail resulting in leaks. Hydrogen embrittlement depends on many factors such as environmental temperature and pressure, purity of metal, concentration and exposure time to hydrogen, stress state, physical and mechanical properties, microstructure, surface conditions, and the nature of the crack front of material [23]. 

The previous sections provided general guidance on materials for hydrogen gas service and emphasized the metallurgical variables that influence hydrogen embrittlement. This section describes additional factors that impact hydrogen embrittlement, primarily environmental and mechanical-loading conditions. 

Low temperature can lead to embrittlement of plastics. This is not seen as a time-dependent effect, but it can be the cause of rapid failure should environmental degradation be followed by a fall in temperature. 

This section deals with chemical aging and related physical phenomena, such as diffusion and embrittlement. Apart from the chemical problem there is mechanical deterioration which is also related to long term environmental effects, but this was covered briefly in the preceding cumulative damage discussion. 

Results of these on-going experiments are helping to clarify the mechanisms acting to produce environmentally induced embrittlement of Fe and Ni and suggest different roles for various segregated impurities. It is expected that understanding of the important mechanisms will help the designing of new alloys which are more durable in particular environments. 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

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