a) b) Fig. 2 Mutual location of working electrode (specimen) and auxiliary (counter) electrode (a) and general view of corrosion cell (b) The specimens were coated by special dielectric isolation, excepting the small area at the notch (Fig. 3). The size of auxiliary (counter) electrode was 20x40 mm. In this case its square is significantly larger than the working electrode area, which to be polarised. During testing the specimens were immersed into the cell with NS4 solution (Fig. 2) and exposed under constant potential of cathodic polarisation const Ecath . For this experimental procedure the Potentiostat VMP has been used. During all test the level of pH was controlled and regulated between 6.6 and 6.7 by bubbling of 2 CO gas. In given tests, the specimens were hydrogen charged at some constant potential of polarisation, which is slightly negative than free corrosion potential for the given steels. Taking into account that for studying steels the free corrosion potential is about ) ( 800mV SCE Ecorr , it has been chosen. Moreover, for simulating of the service conditions, the specimens were loaded during the hydrogen- charging process. The level of load was defined as the gross hoop stress gross =194 and 195.7 MPa respectively which corresponds of the internal pressure in pipe under exploitation bar p 70 exp The hydrogen-charging process was controlled by registration of the cathodic polarisation current cath I . The total quantity of evaluated hydrogen on metal surface during time of exposition exp τ can be found as: . exp 0 const I d under E Q cath cath ev H (1) Hydrogen concentration in metal has been determined on the base of hydrogen discharging process under anodic polarisation with using of hydrogen electrochemical oxidation method proposed in work [10]. For this aim, the standard three-electrode electrochemical cell has been used. The hydrogen discharging of specimen was carried out in 0.2 M NaOH (pH=12.4) solution under anodic polarisation ) ( 168mV SCE Eanodic during some defined time dis The total quantity of absorbed hydrogen by metal can be defined as: . 0 const I d under E I Q anodic ref H abs H dis (2) h= 20mm Working Electrode Auxiliary (Counter) Electrode Open Metal Surface Dielectric Isolation 20mm 3..4mm Notch
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