Hydrogen attack Mechanism

M. Prager, Hydrogen Attack Susceptibility Of Chrome-Moly Steels And Weldments, PVP Vol. 239/MPC Vol. 33, Serviceability of Petroleum, Process, and Power Equipment, American Society of Mechanical Engineers, New York, 1992. 

The slope is indicative of the type of release mechanism. A slope of 0.5 indicates a diffusion-controlled release a slope of 1.0 indicates that a corrosion-related mechanism is operable.The diffusion release mechanism is characterized by surface adsorption, ion exchange, and migration. Chemical corrosion, or alteration of the silicate lattice, is characterized by hydroxyl attack on silicon or by hydrogen attack on bridging oxygens. 

Since enantioselectivity in this reaction is a result of the energy difference between the diastereomeric transition states after H2 is added, Landis modeled the addition of Hj to the diastereomers of the CHIRAPHOS and DIPAMP complexes with MAC as the substrate. Landis posed a simple question Is there a significant barrier to hydrogen attack at the Rh center that can be modeled by molecular mechanics In the first study Landis found that all possible attack trajectories allowed almost strain-free attack of dihydrogen (molecular mechanics barriers were less than 3 kcal/mol) (32). In a subsequent study, a better picture of the reaction coordinate was generated using DFT and quantum mechanical models, which are outside the scope of this chapter. 

Mention was made earlier in this chapter of hydrogen attack being a potential life-limiting degradation mechanism. As experience indicates that this can manifest itself in reactors, pipework and heat exchangers, it is expedient to deal with the available assessment procedures in a common section. 

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