PSI - Issue 18

Mirco Peron et al. / Procedia Structural Integrity 18 (2019) 538–548 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In the process of deposition, the TDMAZ precursor, H 2 O precursor, and delivery lines were heated to 75 °C, 25 °C, and 160 °C, respectively. During the deposition process, the ZrO 2 layer deposition rate was about 1.08 Å/cycle. 2.3. Potentiodynamic polarization curves Discs with a diameter of 29 mm and a thickness of 2 mm were manufacturing using a lathe from the commercially available bars. The samples were then grounded up to 2000 grit silicon carbide papers and cleaned with acetone and ethanol for five minutes in ultrasonic bath. Some of these samples were then coated as described in Section 2.2. Potentiodynamic polarization curves of bare and ZrO 2 coated samples were carried out in simulated body fluid (SBF) with a pH of 7.4 on the Gamry Reference 600+ potentiostat. The electrochemical tests used three-electrode equipment with the bare or coated samples as a working electrode, a Hg/Hg 2 SO 4 electrode as a reference electrode, and a platinum plate electrode as a counter electrode. The area of the samples exposed to SBF was 1 cm 2 . The potentiodynamic polarization test was conducted at a stable open-circuit potential after a stabilization period of 30 min. The scan rate of the potentiodynamic polarization test was 0.5 mV/s. 2.4. Immersion tests Cubic samples of side 5 mm were obtained from the as-received material and they were grounded up to 2000 grit silicon carbide papers and cleaned with acetone and ethanol for five minutes in ultrasonic bath. Some of these samples were then coated as described in Section 2.2. The immersion tests were carried out in SBF at 37 °C for 7 days individually to monitor the hydrogen evolution. Hydrogen bubbles were collected into a burette from each sample as suggested in (Song, Atrens, and StJohn 2013). 2.5. Slow strain rate tests Cylindrical dog-bone-shaped samples, whose dimensions are reported in Figure 2, were used according to the standard (ASTM International n.d.). The sample were machined from the received bars using a lathe.

Figure 2. Geometry and dimensions of the samples for SSRTs

The samples were then grounded up to 2000 grit silicon carbide papers and cleaned with acetone and ethanol for five minutes in ultrasonic bath. Some of these samples were then coated as described in Section 2.2. The SSRTs were carried out both on bare and coated samples at a strain rate of 3.5ꞏ10 -6 s -1 in SBF solution at body temperature (37 ± 1°C). The strain rate value was chosen in order to render the Mg alloy susceptible to SCC according to (Bobby Kannan et al. 2008). A schematic representation of the experimental set-up is shown in Figure 3. The sample was immersed for the whole duration of the test and the SBF was constantly changed by means of a pumping system. The SBF container was immersed in a water bath, whose temperature was constantly monitored by means of a thermometer. When the temperature was below its set value, a commercial resistance heating element placed inside the water bath automatically turned on until the desired temperature was reached again. In addition, while carrying