Materials Performance

AUG 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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56 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 loading stage, the background current again showed almost no change, while the stress is lower. When the amount of stress increased to a higher load than that in the previous loading stage (>300 MPa), the background CD increased sharply. When the loading stopped, the background cur- rent again decreased to the previous level. Two factors can affect the background current: one is a damaged Ni coating and the other is lattice expansion. When a Ni- coated steel sheet is subjected to externally applied stress above the YS of Ni (<250 MPa), the thin layer of Ni coating is plasti- cally deformed and damaged, resulting in an unstable background CD in the perme- ation test. 12 With an increase in the applied elastic load, the value of the apparent dif- fusivity (D app ) decreased slightly, and the valu e of th e apparent solubi lity (C app ) increased slig htly, indicatin g that th e amount of hydrogen in the lattice and reversible traps increased with the increase in applied load within the elastic range of the steel. The elastic deformation is not expected to increase the hydrogen trap concentra- tion, which explains the phenomenon as lattice expansion caused by the elastic loading. This suggests that the steel can accommodate more hydrogen atoms inter- stitially under tensile stress. Accordingly, the increasing background current is due to the damaged Ni coating. This is also demonstrated by the second loading pro- cess, when the background CD did not change until the stress was increased to a load higher than that of the previous load- ing process and the thin Ni coating was damaged and more fractures were gener- ated, which exposed more base metal to th e s o luti on . Thi s i n di c at e s th at th e increasing background CD was related only to the damage of the Ni coating. When the stress was stopped, the fresh metal exposed to solution was passivated again and th e background C D rapidly decreased. If the strain rate is low enough, the background CD change can be ignored. Background Passivation Current Density During SSRT Figure 3 shows the background passiv- ation CD during SSRT, which increases in the elastic deformation range and then large amount of hydrogen trapping by dis- locations might be expected. To avoid this dominant effect, a relatively low strain rate was selected. Results and Discussion Background Passivation Current Density in Elastic Deformation To clarify the inf luence of the applied tensile stress on the background passive current, a stress-loading experiment was carried out in the elastic deformation range in the following sequence: 1. Loading 2. Stopping 3. Static loading 4. Unloading 5. Loading again 6. Stopping (while the stress is greater than previous stopping) 7. Static loading again Figure 2 presents the typical variation in background current of the specimen with var ying stress loads in the elastic deformation range. At first, the background CD remained steady with increasing stress. Once the stress was raised above 210 MPa, the back- ground current begins to increase. When the loading stopped, the background CD began to quickly decrease. In the second MATERIALS SELECTION & DESIGN FIGURE 3 Background current of the specimen during SSRT. FIGURE 4 The background current and apparent hydrogen permeation CD (including background current) during the SSRT process.

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