PSI - Issue 13

Evy De Bruycker et al. / Procedia Structural Integrity 13 (2018) 226–231 Evy De Bruycker/ Structural Integrity Procedia 00 (2018) 000 – 000

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After charging, the sample was loaded in the TDS chamber. After one hour (the duration needed to achieve a sufficient vacuum for the mass spectrometer measurement), the sample was heated with a constant rate of 200K/h up to the maximum achievable temperature (about 700°C). 2.5. Apparent coefficient of diffusion On selected materials (T12 BM, T24 BM and T24 HAZ HH) the apparent coefficient of diffusion of hydrogen was determined at room temperature by permeation measurements. The Stachurski and Devanathan (1962) permeation set-up is composed of two half cells separated by the material to be tested. Hydrogen is cathodically charged at the cathodic (entry) side. After diffusion through the material, the hydrogen is oxidised at the detection side. The permeated hydrogen is thus measured as an oxidation current. The permeation curve depicts the evolution of this current as a function of time. There are several options to draw a diffusion coefficient from a permeation curve. The choice of the relevant procedure is still a subject for debate. Based on experience, the breakthrough time was used as the most representative parameter. It is defined as the time needed for the first hydrogen atoms to permeate and reach the other side of the steel membrane. Once the breakthrough time (t b ) is determined, the coefficient of diffusion is given by the following formula: = 2 15.3 where L is the thickness of the material. 3. Results and discussion 3.1. Total and diffusible hydrogen content The results are shown in Fig. 1. This figure shows the maximum and minimum measured total and diffusible hydrogen content (over at least 3 measurements). The difference between the total hydrogen content and diffusible hydrogen content represents the amount of hydrogen that is strongly trapped inside the material. Concerning Hydrogen Induced Cracking (HIC), diffusible hydrogen is deleterious to the ductility - as opposed to strongly trapped hydrogen - since it can move through the material towards areas of high stress concentrations, where it can cause hydrogen embrittlement (Louthan Jr. 2008). The following charging conditions were used:  solution : NaOH (0.1M) + Thiourea (10 mg/l)  charging current density: 2 mA/cm².

Fig. 1. Overview of the maximum and minimum values of total and diffusible hydrogen in the different materials..