Hydrogen entry promoters

There is strong electrochemical evidence that hydrogen is at least partly responsible for see of a and near-a titanium alloys in aqueous solutions [194-196]. Figure 23 shows the resistance of a near-a titanium alloy to S ee in 3.5% Naei with and without arsenic present [196]. As to be expected for a hydrogen-dependent process, the presence of the hydrogen entry promoter has a marked effect on the SCC resistance of the alloy under the appropriate type of mechanical loading (Mode I). 

G. Zheng, B.N. Popov, R.E. White, The role of thallium as hydrogen entry promoter on cathodicaUy polarized HY-130 steel, J. Electrochem. Soc. 141 (1994) 1526-1532. 

The entry of hydrogen into metals is promoted by various compounds, especially those involving elements from Groups VB (P, As, and Sb) and VIB (S, Se, and Te) of the periodic table. Some anions such as CN , CNS , and 1 and certain carbon compounds such as CS2, CO, and CSN2H4 also promote hydrogen entry. 

The effectiveness of promoters can differ considerably, even within specific groups. However, the order of effectiveness varies between different studies [18]. The experimental evidence indicates that the hydride of the promoter is the species that primarily affects hydrogen entry and that the amount of hydrogen adsorbed is related to the bond strength of the hydride [18,19]. 

The effect of a promoter varies with its concentration. Figure 1 shows the rate of hydrogen permeation through a steel membrane under cathodic polarization in the presence of Group VB and VIB elements [12]. In most cases, the promoters achieve their maximum effect at relatively low concentrations. Because of hydrolysis and other secondary reactions, higher concentrations often lead to the deposition of insoluble products that can inhibit hydrogen entry. 

The amount of the hydrogen that is liberated on or near a metal surface, which then enters the metal, varies according to the environment and condition of the metal. The main factor that promotes the entry of hydrogen into a metal is the presence on the metal of a surface poison such as sulfide or other species, which inhibit the hydrogen recombination reaction. 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

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