Data Collection

Chapter 2: Methods

2.2 Data Collection

2.2.1 Synthesizing the 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ Metallic Glass Samples

𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ samples are synthesized by a Rapid Solidification Process (RSP) which is the melt spinning as mentioned earlier. The ingots of the alloy with nominal compositions were produced by arc melting the mixtures of zirconium, copper, hafnium, and silver (> 99.9 𝑤𝑡%) in a titanium-guttered argon gas atmosphere. The ingots were melted several times in order to get the desired homogeneous composition.

The amorphous ribbons were produced from the alloys mentioned above by single roller melt spinning on a copper wheel surface with a velocity of 20 m/s in the argon

atmosphere. The solidification rate must be very high to get the non-crystalline structure

23 of the molten metals, where the rapid cooling prevents the crystal nuclei from forming.

The minimum rate that is required to obtain the glassy nature is about 105 to 106 (^𝐾

𝑠) in many cases. As a result, the ribbons of 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ with the cross section

(thickness × width) of 0.04 × 2.5 mm were produced when the heat is suddenly removed from the molten metals which helps achieving the high solidification rates.

2.2.2 Verifying the Amorphous Nature of the 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ Metallic Glass Samples Using X-Ray diffraction (XRD)

X-ray diffraction (XRD) was used to characterize the glassy alloys obtained by the rapid solidification process and to determine the present phases. The XRD was performed using PANalytical X’pert-Pro diffractometer with Cu-K𝛼 (𝜆 = 0.154 nm) were carefully placed on double sided tape attached to glassware for the XRD analysis.

X-pert High Score Plus software was used to carry out the phase analysis. Table 4 below mentions the experimental conditions that were maintained during the XRD

measurements.

Table 4: XRD experimental conditions Mode Reflection mode

Angular range 20 - 100⁰ diffraction angle (2𝜃) Scan Rate 0.00067⁰

Beam Size 12 mm wide × 5 mm long

Voltage 40 kV

Current 40 A

2.2.3 Preparation of Various Solutions

In order to perform the Potentiodynamic polarization tests, different solutions were prepared in the laboratory. The same solutions were prepared for PDP and EIS tests. Firstly, the HCl solution of 0.01 M concentration was prepared. Knowing the percentage of the purity (37%), the specific gravity 1.19 ( ^𝑔

𝑚𝑜𝑙), and the molecular weight

24 36.5( ^𝑔

𝑚𝑜𝑙), the molarity in ^𝑚𝑜𝑙

𝐿 of the available concentrated HCl solution was calculated using the following relation:

𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦(𝑀) =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑆𝑜𝑙𝑢𝑡𝑒

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 (𝐿) (6) 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦(𝑀)

=𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 × 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑝𝑢𝑟𝑖𝑡𝑦 × 1000 𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑊𝑒𝑖𝑔ℎ𝑡

(7)

𝐻𝐶𝑙 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦(𝑀) = 1.19 × 0.37 × 1000

36.5 = 12.06 (𝑚𝑜𝑙 𝐿 )

The following relation was used to find the volume of HCl required in 1000 mL of distilled water to give the 0.01 M concentration:

𝑀₁𝑉₁ = 𝑀₂𝑉₂ (8) Where 𝑀₁is the required molarity of the solution, 𝑉₁ is the volume of the required solution, 𝑀₂ is the molarity of the HCl, and 𝑉₂ is the volume of HCl.

For 0.01 M concentration of HCl, the following volume of HCl is required in 1000 mL of distilled water:

0.01 𝑀 × 1000 𝑚𝐿 = 12.06 𝑀 × 𝑉₂ 𝑉₂= 0.83 𝑚𝐿

Similarly, for preparing 0.1 M and 1 M concentrations of HCl solution, 8.3 mL and 83 mL of the concentrated solution were required respectively in 1000 mL of distilled water.

In the case of NaCl, which has a molar mass of 58.44( ^𝑔

𝑚𝑜𝑙), 0.584 g, 5.84 g and 58.4 g of NaCl were needed to prepare 1000mL of 0.01 M, 0.1 M and 1 M concentration NaCl solution respectively.

Different pH levels of NaOH solution were prepared based on the available literature of characterizing the electrochemical behavior of copper-zirconium based

25 metallic glasses. For preparing pH 8 of NaOH solution, the following relation was used to convert the pH to molarity:

𝑝𝑂𝐻 = −log [𝑂𝐻⁻] (9)

𝑝𝐻 + 𝑝𝑂𝐻 = 14 (10)

Where [𝑂𝐻⁻] is the molarity of [𝑂𝐻⁻] ions.

For pH 8, 𝑝𝑂𝐻 = 14 − 8 = 6

Therefore, from Equation 9, 6 = −log [𝑂𝐻⁻], which gives the molarity [𝑂𝐻⁻] = 1 × 10⁻⁶ 𝑀 for pH 8 NaOH solution. Knowing that 1 M of NaOH has 40 (^𝑔

𝐿) of NaOH in a Liter of distilled water, we can calculate how many grams per Liter are needed to produce 1 × 10⁻⁶ 𝑀 of NaOH solution, which is 4 × 10⁻⁵(^𝑔

𝐿). Similarly, for pH 10 and pH 12 of NaOH solution, 4 × 10⁻³(^𝑔

𝐿) and 0.4 (^𝑔

𝐿) of NaOH in a Liter of distilled water respectively.

2.2.4 Corrosion Behavior Evaluation Using the Potentiodynamic Polarization (PDP) The samples of the 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ metallic glass were tested using a Metrohm Autolab 204 Potentiostat that is shown in Figure 7 below. Before performing the

Potentiodynamic polarization tests, the samples were cleaned and washed in distilled water. After the samples were dried using air, and while having the solutions prepared, the tests were performed at room temperature. Figure 7 below sows the setup of the three electrodes, where the 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ sample is the Working Electrode (WE), the Counter Electrode (CE) is platinum wire, and the Saturated Calomel Electrode (SCE) is the counter electrode. For testing the samples in the acidic environment, HCl solution was prepared in three concentrations, 0.01 M, 0.1 M and 1 M. As mentioned earlier, the process is repeated for the three concentrations of the neutral NaCl solution and the basic NaOH solution with pH 8, pH 10 and pH 12 levels (Nair et al., 2019).

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Figure 7: Three electrode experimental setup for PDP and EIS tests.

The Potentiodynamic polarization tests are done using 0.000167 (^𝑉

𝑠) scanning rate in 100 ml of the prepared solutions. The exposed area of the samples is approximately 1 𝑐𝑚². Before starting the tests, the samples are immersed in the electrolyte solution for 20 minutes to get a stable potential. The PDP tests are repeated more than 3 times to make sure that the obtained results are reliable and reproducible (Sudha et al., 2021).

2.2.5 Corrosion Behavior Evaluation Using the Electrochemical Impedance Spectroscopy (EIS)

For EIS analysis, the same experimental setup and solutions is used, where the three-electrode system is connected to VersaSTAT 3F Potentiostat shown in Figure 8 below. In the EIS tests, the AC frequency varies from 10 kHz to 10 Hz. The data

obtained from the VersaSTAT is then exported using Nova software and fitted using EC- Lab Software and OriginPro 8.5 software. The analysis of the impedance is done with the help of EC-Lab software which converts the impedance data to the equivalent circuit using the following steps.

27 Figure 8: VersaSTAT 3F Potentiostat used for EIS analysis.

Firstly, after running the EIS tests at the mentioned frequencies, the data from the Excel file are taken to OriginPro 8.5 and exported in Ascii format to be prepared for use in EC-Lab software. The data is then imported in EC-Lab software, columns are

labelled, Nyquist and Bode plots are plotted and fitted using the Z-fit option in the electrochemical impedance spectroscopy analysis, and the equivalent circuit is chosen.

The software has several built-in common circuits which are commonly used in the electrochemical analysis of materials. The equivalent circuit is chosen so that it has the best fit with the curves obtained from the EIS tests. In addition, the Circuit

Description Code is obtained which is abbreviated by (CDC).

The Nyquist and Bode plots are used to represent the data, where in the Nyquist plot, the negative of the imaginary part of the impedance is plotted versus the real part of the impedance as discussed in the literature chapter earlier. On the other hand, the bode plots have once the phase shift plotted versus the log frequency, and once with absolute value of the impedance versus the log frequency. The Nyquist plot is to be converted to an equivalent circuit and analyzed using Chi squared values (𝜒²). Further description is discussed in the literature review of the first chapter of this thesis.

2.2.6 Surface Morphology Analysis Using SEM Coupled with EDS.

The Thermo Scientific Prisma E Scanning Electron microscopy that is coupled with energy dispersive X-ray spectroscopy as shown in Figure 9 is used to examine the surface of the 𝐶𝑢₅₁𝑍𝑟₃₀𝐻𝑓₁₄𝐴𝑔₅ metallic glass samples after performing the

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Potentiodynamic polarization and EIS tests on them. The tested area of each sample is put under the SEM to examine the chemical composition and the surface morphology.

All SEM images to be taken in 2000× magnification with a focus on the areas of expected corrosion.

Figure 9: Thermo Fisher Prisma E Scanning Electron Microscopy (“Environmental Scanning Electron Microscope | Prisma E SEM – AE”, n.d.)

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