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58 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 current decreases sharply. Kurkela, et al. 9 indicated that the binding energy of hydro- gen atoms to dislocations in body-centered cubic alloys (contrar y to face-centered cubic alloys) is several times higher than the bulk activation energy for diffusion. In ferritic steels, this means that dislocations generated by plastic deformation enhance the trapping of hydrogen atoms rather than their transport, leading to a decrease in the apparent diffusivity of hydrogen. Eventually, the dislocation trap will become saturated, and hydrogen atoms dif- fuse through the steel under the concentra- tion gradient. Thus, during increasing plas- tic deformation, the permeation current also eventually increases as well. When a bal ance i s reach ed b etween hydrogen atoms trapped by dislocations and hydro- gen atoms carried by the moving disloca- tions, th e m easured hydrogen current reaches a steady state. This process may be modeled using the following assumptions: • Dislocations are assumed to be cre- ated at the surfaces and can sweep through the grain boundar y, then travel completely through the speci- men. • Dislocations immediately obtain an atmosphere of hydrogen that is in equilibrium with the local lattice concentration at each surface. • The dislocations carr y this initial atmosphere of hydrogen through the specimen without interacting with the lattice. The measured current change can be written as shown in Equation (2): P = DC 1 /L – DC 0 /L = D(C 1 – C 0 )/L = DC/L (2) where P is the permeation current, D is the lattice diffusion constant, C 1 and C 0 are the lattice hydrogen concentration after and before straining of the specimen, and L is the specimen thickness. For simplification, a lattice diffusion constant for pure iron of 8.25 × 10 –5 cm 2 s –1 was taken. Assuming the increased hydrogen permeation is due to hydrogen escaping from dislocations, the concentration of hydrogen captured by dis- locations can be calculated by subtracting C 0 from C 1 . Figure 6 shows the calculated values and the measured hydrogen perme- ation current. Considering the experimental error, the slope of calculated values fits the slope of the earlier measured current change quite well. Therefore, the initial decrease in per- meation current was a result of hydrogen being trapped by dislocations. Conclusions • Background CD increases with defor- mation increases in the elastic range; the main reason is the Ni coating damage. • Hydrogen permeation current in- creases with increasing elastic defor- mation due to lattice expansion. • The hydrogen permeation current initially decreased sharply due to dis- lo cation s (an d th eir trappin g of hydrogen), then increased sharply. • After the hydrogen traps were filled and the stress increased sufficiently to move the dislocations, the hydro- gen permeation current increased to reach a steady level. Acknowledgments This work was financially supported by the Natural Science Foundation of Jiangsu Province, China (No. BK20141292), the Nat- ural Science Foundation of China (No. 51051055), and the International Coopera- tion Project of the Natural Science Founda- tion of China (No. 51310105001). References 1 G. Schmitt, L. Sobbe, W. Bruckhoff, "Corro- sion and Hydrogen-Induced Cracking of Pipeline Steel in Moist Triethylene Glycol D i lut ed w ith Li qui d Hydrogen Sulf i d e," Corros. Sci. 27, 10-11 (1987): p. 1,071. 2 R .W. Bosch, "Electrochemical Impedance Spectroscopy for the Detection of Stress Cor- rosion Cracks in Aqueous Corrosion Systems at Ambient and High Temperature," Corros. Sci. 47, 1 (2005): p. 125. 3 C. Zheng, B. Yan, K. Zhang, "Electrochemical Investigation on the Hydrogen Permeation Behavior of 7075-T6 Al Alloy and its Inf lu- ence on Stress Corrosion Cracking," Int. J. Miner. Metall. and Mater. 22, 7 (2015): p. 729. 4 C.B. Zheng, H.K. Jiang, Y.L. Huang, "Hydrogen Permeation Behavior of X56 Steel in Simu- l at ed Atm o sph eri c Env ironm ent Und er Loading," Corros. Eng. Sci . Technol . 446, 4 (2011): p. 365. 5 M.A.V. Devanathan, Z. Stachurski, "The Ad- sorption and Diffusion of Electrolytic Hydro- gen in Palladium," Proceedings of the Royal Society of London Series A, Mathematical , Physical and Engineering Sciences 270, 1,340 (1962): p. 90. 6 P. Manolatos, M. Jerome, J. Galland, "Neces- sity of a Palladium Coating to Ensure Hydro- gen Oxidation During Electrochemical Per- meation Measurements on Iron," Electro- chim. Acta 40, 7 (1995): p. 867. 7 C. Zheng, G. Yi, "Temperature Effect on Hy- drogen Permeation of X56 Steel," MP 50, 4 (2011): p. 72. 8 Y. Huang, et al., "Effect of Mechanical Defor- mation on Permeation of Hydrogen in Iron," ISIJ International 43, 4 (2003): p. 548. 9 M. Kurkela, et al., "Influence of Plastic Defor- mation on Hydrogen Transport in 2 1/4 Cr- 1Mo Steel," Scripta Mater. 16, 4 (1982): p. 455. 10 M.A.V. De vanathan , Z. Stachurski , " Th e Mechanism of Hydrogen Evolution on Iron in Acid Solutions by Determination of Perme- ation Rates," J. Electrochem. Soc. 111, 5 (1964): p. 619. 11 C. Gabrielli, et al., "Transfer Function Analy- sis of Hydrogen Permeation Through a Me- tallic Membrane in a Devanathan Cell: Part II: Experimental Investigation on Iron Mem- brane," J. Electroanal . Chem. 590, 1 (2006): p. 15. 12 S.J. Kim, K.Y. Kim, "Electrochemical Hydro- gen Perm eation Measurem ent Throu g h High-Strength Steel Under Uniaxial Tensile Stress in Plastic Range," Scripta Mater. 66, 12 (2012): p. 1,069. 13 A.M. Brass, J. Chêne, "Inf luence of Tensile Straining on the Permeation of Hydrogen in Low Alloy Cr-Mo Steels," Corros. Sci . 48, 2 (2006): p. 481. 14 W.C. Luu , H.S. Kuo, J.K. Wu , "Hydrogen Permeation Through Nickel-Plated Steels," Corros. Sci. 39, 6 (1997): p. 1,051. CHAUNBO ZHENG is an associate profes- sor at the Jiangsu University of Science and Technology, Zhenjiang, Jiangsu Province, China, email: 15952802516@139. com. He works in the area of hydrogen embrittlement and stress corrosion crack- ing of Al alloys and duplex stainless steel. 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