Info

Total quantity of hydrogen in each bonding state is estimated by integrating the original concentration versus time data to calculate the area under each peak. Results for the four peaks are shown in Fig. 9.26 (a, b, c, and d), where the amount of hydrogen (expressed in ppmw relative to the specimen weight) is plotted as a function of exposure time in the exfoliation solution. The three strong traps T2, T3, and T4 share common features. Linear increase of the amount of hydrogen with exposure time is initially observed, followed by asymptotic approach to a constant value. This behavior is reminiscent of a saturation process by depletion of available active sites.

State T4 reaches a plateau concentration of 1200 ppmw after ~35 h exposure in the exfoliation solution, while states T2 and T3 saturate at concentrations 40 and 300 ppmw, respectively, after the elapse of ~60 h. The fact that hydrogen desorbs from state T4 at the highest temperature of all identified trapping states indicates that T4 is energetically favored. The fact that this is the first state to become saturated further supports the above result.

The amount of hydrogen in the T1 state (Fig. 9.26a) increases linearly with exposure time, and no saturation is evidenced up to an exposure of 120 h in the exfoliation solution. Since it is the lowest energy state observed, T1 could in principle be associated with adsorbed hydrogen. However, in aluminum alloys the energy of chemisorption is lower than the migration energy and no peak should appear.

The physical origin of the above trapping states is difficult to identify solely from thermal analysis. However, some speculations can be made based on information in the literature. Given the fact that T1 is a low-energy state and—when compared to the other states—relatively unsaturable, it would appear that this trapping state is related to hydrogen at interstitial sites. The continuous increase in the amount of hydrogen with exposure time to the exfoliation solution is attributed to the creation, by the corrosion process, of new penetration paths for hydrogen (intergranular cracks and surfaces), as reported by Haidemenopoulos et al. [60].

FIGURE 9.25 Hydrogen concentration in furnace purge steam as function of furnace temperature, during rapid heating. Curves correspond to different exposure times to exfoliation solution.

TflC)

FIGURE 9.25 Hydrogen concentration in furnace purge steam as function of furnace temperature, during rapid heating. Curves correspond to different exposure times to exfoliation solution.

Trapping state T2 is an intermediate energy state that saturates with relatively little hydrogen. A possible physical origin of this trap is the interface between the Mg2Si precipitate and the matrix lattice. The Mg2Si precipitate is incoherent with the matrix and the interfacial dislocations that exist around it can trap hydrogen. This has been shown by Saitoh et al. [47], using tritium autoradiography.

The critical temperature of 410°C, below which no hydrogen evolves from state T3, compares favorably with the thermal decomposition temperature of MgH2 as reported by Tuck [55] (450°C with a heating rate of 50°C/min, which should bias the peak to higher temperature). Thus, trapping state T3 could tentatively be associated with Mg hydride. It has been noted by Saitoh et al. [47] that Mg is bonded to Si by a strong ionic bond that precludes formation of MgH2. However, Mg content in the 2024 alloy presently tested is in roughly 40% excess over the stoichiometric analogy with Si and—if saturated—results in a 500-ppmw concentration of hydrogen. This estimate compares favorably with the plateau of 300 ppmw shown in Fig. 9.26c.

0 0

Post a comment