Stress-sorption cracking

Stress-sorption cracking is the basic mechanism applying to stress cracking of plastics by specific organic solvents [41, 42] and to liquid-metal embrittlement—the cracking of solid metals by specific liquid metals. It is also the mechanism proposed earlier by Fetch and Stables [43] to account for stress cracking of steel induced by interstitial hydrogen (see Section 8.4). 

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... 

Stress Sorption. This mechanism supposes that the reaction between a species in the environment and the metal atoms at the crack tip can cause a redistribution of electrons in the orbits of the atoms so that the bond between them is weakened (19). It is not possible to cite experimental data at would support this concept for the fracture of metals although the absence may merely reflect the difficulties of obtaining such data. 

A systematic investigation of the sorption of H2S as a component of gas mixtures resulted in the consideration of COS formation as one of the aspects of the complex process [25,26], This phenomenon cannot be neglected, although COS itself is not known to cause stress corrosion cracking, especially, because COS is a very volatile compoimd that is able to leave the gas purification plant together with the purified product. However, COS may be as toxic as H2S. A gas purified in a way that neglects the possible occurrence of COS may not be used as town gas. 

Sorption is permeation by liquid or gas from the environment, resulting in swelling of the material. Swelling may be followed by dissolution into the solvent. This is generally not observed with cross-linked polymers. Crazing appears on the surface of polymer extrusions, caused mainly by internal stresses. Crazing may extend to cracking. 

It is known that the concentration of tie chains within lamellae is one of important factors controlling crack initiation. However, it is also important to understand the effect of the environment on the ESCR of materials. Crack initiation and slow crack growth is normally regarded as a semi-brittle or brittle phenomenon. In the case of environmental stress cracking a certain amount of ductile deformation occurs which is governed by the nature of the test media. Plasticization of HDPE due to the diffusion and sorption of IGEPAL is an important component of the ESC mechanism. 

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