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57 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 decreases sharply in the plastic deforma- tion range. In the elastic range, after the applied stress is increased above the YS of Ni, the thin layer of Ni coating is plastically deformed and damaged, resulting in an unstable background CD in the permeation test. After reaching the specimen's plastic range, numerous dislocations are gener- ated that are able to capture and trap a large amount of hydrogen atoms, which is accompanied by a sharp decrease of the passivation CD. Hydrogen Permeation Current Density During SSRT Figure 4 shows the background current and apparent hydrogen permeation CD (including background current) during the entire SSRT process. The CD increases with e l a s t i c d e f o r m a t i o n , a n d s u d d e n l y decreases after the plastic range is reached. By subtracting the background CD from the hydrogen permeation CD, Figure 5 was obtained. Figure 5 shows the hydrogen perme- ation behavior throughout the SSRT. In Fig- ures 4 and 5, th e background CD and hydrogen permeation CD began to increase after ~2.5 h, but the hydrogen permeation CD increased more quickly than the back- ground current. This indicates that the lat- tice expansion allows hydrogen atom per- meation deeper into the steel in the elastic deformation range after the reversible dis- location traps are saturated. During elastic deformation, the perme- ation current increases with increasing elastic deformation and reversible disloca- tions and lattice expansion occur. After the reversible dislocation traps are saturated, only lattice expansion influences hydrogen permeation. At a steady state, the perme- ation current can be written as shown in Equation (1): P t = D L C 0 /L (1) where P t is the steady state permeation cur- rent, D L is the lattice diffusivity, C 0 is the hydrogen concentration just below the charging surface, and L is the specimen thickness. The dif fusivity was found to decrease with the deformation increase, so the increase in P t must be due to either an increase of C 0 or a decrease in specimen FIGURE 5 The hydrogen permeation behavior during the SSRT process. FIGURE 6 The relationship between measured hydrogen permeation and calculated hydrogen trapping current by dislocations. thickness. 4 Since the maximum deforma- tion was not more than 0.2%, the contribu- tion of thickness decrease is minimal; therefore, the increase of C 0 is the main rea- son for the increase in P t . Surface hydrogen concentration is related to the surface cov- erage of hydrogen and the corrosion rate. Active sites are generated on the metal sur- face (e.g., by lattice expansion or oxide film cracking ); and these active sites act to enhance both the reduction of hydrogen ions and the entry of hydrogen into the lat- tice. Th e enhanced entr y of hydrogen results in an increase of the surface hydro- gen concentration and, thus, the perme- ation current. Plastic Deformation Effect on Hydrogen Permeation During plastic deformation, numerous di slo cations are gen erat ed , and many hydrogen atoms gathered in these disloca- tions cannot dif fuse through the steel . Hence, the measured hydrogen permeation Effect of Stress on Hydrogen Permeation for X70 Carbon Steel in Seawater