5 Inhibitory effect of carbon monoxide on hydrogen-assisted degradation of fracture toughness
5.4 Discussion
130
131
5.4.2 Comparison of inhibitory effects of carbon monoxide and oxygen on hydrogen-assisted
degradation of fracture toughness
Based on the fracture toughness test results, it was found that the addition of carbon monoxide inhibited the hydrogen-assisted degradation of the fracture toughness similarly to oxygen.
In this section, the inhibitory effects of carbon monoxide and oxygen will be compared. Figure 5.6 shows the inhibitory effects of carbon monoxide and oxygen on hydrogen-assisted degradation of the fracture toughness test at the same testing conditions. Although the results shown in Fig. 5.6 have been already shown in Figs. 4.14 and 5.2, the results are shown again to compare them. In the case of carbon monoxide, 1000 vol. ppm carbon monoxide was needed to completely inhibit the hydrogen-assisted degradation of the fracture toughness at these test conditions. On the other hand, in the case of oxygen, 100 vol. ppm oxygen completely inhibited the hydrogen-assisted degradation.
In addition, when comparing the test result with 10 vol. ppm addition, the degree of the inhibitory effect was much greater in oxygen than in carbon monoxide.
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
In air
In high-purity H2
In 10 vol. ppm CO-H2 mixture In 100 vol. ppm CO-H2 mixture In 1000 vol. ppm CO-H2 mixture In 10000 vol. ppm CO-H2 mixture A333 grade 6
Gas pressure: 0.6 MPa gauge Temperature: 293 K O2 content < 0.1 vol. ppm V = 2.0×10-3mm/s
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
In air
In high-purity H2
In 10 vol. ppm O2-H2 mixture In 100 vol. ppm O2-H2 mixture A333 grade 6
Gas pressure: 0.6 MPa gauge Temperature: 293 K V = 2.0×10-3mm/s
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
In air
In high-purity H2
In 10 vol. ppm O2-H2 mixture In 100 vol. ppm O2-H2 mixture A333 grade 6
Gas pressure: 0.6 MPa gauge Temperature: 293 K V = 2.0×10-3mm/s
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
In air
In high-purity H2
In 10 vol. ppm CO-H2 mixture In 100 vol. ppm CO-H2 mixture In 1000 vol. ppm CO-H2 mixture In 10000 vol. ppm CO-H2 mixture A333 grade 6
Gas pressure: 0.6 MPa gauge Temperature: 293 K O2 content < 0.1 vol. ppm V = 2.0×10-3mm/s
Fig. 5.6 Comparison of inhibitory effects of carbon monoxide and oxygen on hydrogen-assisted degradation (a) Effect of carbon monoxide (b) Effect of oxygen
132
Staykov et al. reported the potential energy of oxygen, carbon monoxide and hydrogen molecules for iron surface by DFT simulation [17]. Figure 5.7 shows their results. The vertical axis is the potential energy of molecules for the iron surface, the horizontal axis is the distance from the iron surface. In the figure, a decrease in the potential energy when the molecule approaches the iron surface indicates that the attractive interaction between the molecule and the iron surface occurs, and an increase in the potential energy indicates the repulsive interaction. The slope of the potential energy indicates the magnitude of the interaction force.
Figure 5.7 (a) shows the interaction of oxygen and carbon monoxide molecules with iron surface. The distance which the molecule-iron surface interaction occurs is farther in oxygen than in carbon monoxide. In addition, the slope of the decrease in the potential energy is greater in oxygen than in carbon monoxide. Therefore, oxygen can adsorb on the iron surface faster and more efficiently than carbon monoxide.
Figure 5.7 (b) shows the interaction of hydrogen molecules with iron surface with preadsorbed oxygen and carbon monoxide. There is an activation barrier of hydrogen adsorption on the iron surface. The activation barrier is increased by preadsorbed oxygen and carbon monoxide.
Therefore, the preadsorbed oxygen and carbon monoxide inhibit the hydrogen adsorption on the iron surface. The increase in the activation barrier is larger with preadsorbed oxygen. It means that the inhibitory effect of preadsorbed oxygen on hydrogen adsorption is greater than that of preadsorbed carbon monoxide.
These calculation results indicate that the inhibitory effect of oxygen is greater than that of carbon monoxide, and it corresponds to the experimental results shown in Fig. 5.6. Therefore, the difference in the inhibitory effect between oxygen and carbon monoxide on hydrogen-assisted degradation is theoretically explained by the potential energy behavior of molecules with iron surface.
133
Fig. 5.7 Mechanism of prevention of hydrogen adsorption by oxygen and carbon monoxide [17]
0 1 2 3 4 5
-8 -6 -4 -2 0 2
Potential Energy (eV)
Distance (Å)
Carbon monoxide Oxygen
(a) Interactions of molecules with iron surface
(b) Increase in activation barrier for hydrogen adsorption by preadsorbed oxygen and carbon monoxide
0 1 2 3 4 5
-1 -0.5 0 0.5 1
With preadsorbed O2 With preadsorbed CO Free surafce
Potential Energy (eV)
Distance (Å)
134 5.4.3 Future task
As described above, the inhibitory effects of carbon monoxide on hydrogen-assisted degradation of the fracture toughness of two pipe steels were characterized. However, in this study, the crosshead speed which is one of the influence factors on the inhibitory effect as described in Chapter 4 was fixed. In addition, the hydrogen gas pressure and temperature which are considered as the influence factors on the inhibitory effect were also fixed. It is considered that the inhibitory effect of carbon monoxide is changed depending on these test conditions. Furthermore, although the effect of material on the inhibitory effect of carbon monoxide was partially discussed in this study, the two pipe steel used in this study had only slight difference in terms of strength and chemical composition.
It is considered that the inhibitory effect is influenced by the material strength, chemical composition, microstructure, and so on. Therefore, to further characterize the inhibitory effect of carbon monoxide on hydrogen-assisted degradation, it is necessary to evaluate these influence factors in the future.
135