Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes delamination b) c) Figure 11 Macroscopic morphologies of the rupture on tube B:(a) outside wall, ( b)inside wall reduced into the hydrogen atoms, seen in Equation(8), and then these hydrogen atoms will be easily absorbed into the titanium matrix material at temperatures above 77C[14]. Then, another doubt will be brought about here that the service temperature of deformation erosion the titanium tubes in this event was ranging from 35 to 40C, much lower than the threshold value 77C. so how could the hydrogen atoms be absorbed? Actually, this critical te of 77C is only valid in sole Ti-H system. Once alien substances like ferrous ions in this event were introduced, this system would Figure 12.Cross-sections of the rupture on tube B: (a)delamination of be disturbed, and thus the temperature would be changed. matrix material, (b) descaling of matrix material, (c) thinned by erosion Detailedly speaking, with progress of the galvanic corrosion, the content of ferrous ions kept increasing, hence more and hydrogen ions would be continuously produced due to hydrolysis, and accordingly the concentration of hydrogen atoms would As a matter of fact, this absorption process of hydrogen each a relatively high value. As a result, the proneness of atoms into titanium is always substantially retarded by the passive hydrogen atoms to be absorbed into titanium was facilitated on film TiOz on titanium Quantificationally, the hydrogen diffusion the basis of Le Chatelier's principle. To sum up, it can be briefly coefficient in pure titanium is 1.07 x 10cm/s, however that put forward that the threshold temperature for hydrogen atoms to value in TiO2 is only 7.5 x 10-20cm/s[27]. Consequently,the be absorbed into titanium will be decreased with the catalysis hydrogen atoms will first be absorbed by the Tioz,even effect of ferrous ions, at least around 35-40C. composing metastable compounds TiO(OH)[Equation (9), [28) if their contents continuously in hen, Zeng et al. [28] (8) thought that the hydrogen atoms in TiO(OH) will serve as the www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheimreduced into the hydrogen atoms, seen in Equation (8), and then these hydrogen atoms will be easily absorbed into the titanium matrix material at temperatures above 77 8C [14]. Then, another doubt will be brought about here that the service temperature of the titanium tubes in this event was ranging from 35 to 40 8C, much lower than the threshold value 77 8C, so how could the hydrogen atoms be absorbed? Actually, this critical temperature of 77 8C is only valid in sole Ti–H system. Once alien substances like ferrous ions in this event were introduced, this system would be disturbed, and thus the temperature would be changed. Detailedly speaking, with progress of the galvanic corrosion, the content of ferrous ions kept increasing, hence more and more hydrogen ions would be continuously produced due to hydrolysis, and accordingly the concentration of hydrogen atoms would reach a relatively high value. As a result, the proneness of hydrogen atoms to be absorbed into titanium was facilitated on the basis of Le Chatelier’s principle. To sum up, it can be briefly put forward that the threshold temperature for hydrogen atoms to be absorbed into titanium will be decreased with the catalysis effect of ferrous ions, at least around 35–40 8C. Hþ þ e ! H (8) As a matter of fact, this absorption process of hydrogen atoms into titanium is always substantially retarded by the passive film TiO2 on titanium. Quantificationally, the hydrogen diffusion coefficient in pure titanium is 1.07
1012 cm2 /s, however that value in TiO2 is only 7.5
1020 cm2 /s [27]. Consequently, the hydrogen atoms will first be absorbed by the TiO2, even composing metastable compounds TiO(OH) [Equation (9), [28]] if their contents continuously increase. Then, Zeng et al. [28] thought that the hydrogen atoms in TiO(OH) will serve as the Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 15 Figure 11. Macroscopic morphologies of the rupture on tube B: (a) outside wall, (b) inside wall Figure 12. Cross-sections of the rupture on tube B: (a) delamination of matrix material, (b) descaling of matrix material, (c) thinned by erosion www.matcorr.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim