E corr and i corr Analysis

Corrosion potential (Ecorr) and corrosion current density (icorr) were calculated using Tafel extrapolation

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Ecorr. Ecorr and icorr are dependent on the autocatalytic reduction of nitric acid and increased availability of

HNO2, as described above or the greater availability of H2. Both Ecorr and icorr are known to increase with

higher autocatalytic contribution via reactions 1.22 and 1.29 (which increases with increasing HNO3

concentration), which together describe the global reduction of HNO3 [157]:

𝐻𝑁𝑂 + 2𝑁𝑂 + 𝐻 𝑂 ⇄ 3𝐻𝑁𝑂( ) (1.29)

Figure 3-3a shows calculated Ecorr values obtained at 5 - 15% wt. HNO3 concentrations. From which it

can be seen that Ecorr increases near linearly with HNO3 concentration.

Figure 3-3 - a) Corrosion potential, Ecorr, values vs. HNO3 concentration b) Corrosion current density, icorr, values vs. HNO3 concentration and associated error bars calculated from Figure 3-2a for 316L SS in 5 - 15% wt. nitric acid. NOTE: Higher concentrations of HNO3 have been left on the x-axis and will

be filled in in Chapter 4.

Figure 3-3b shows the icorr values for 5 – 15% wt. HNO3. At the HNO3 concentrations seen here, icorr

values decrease. The decrease in icorr suggests the formation of a stable passivating oxide film which then

reduces the availability of surface sites for reaction (1.22). This is consistent with the observed increase in Ecorr, into the passive potential range of Figure 3-2b. This, in turn, suggests that system behaviour is

governed by the coupling of the steel surface oxidation and nitrous acid catalysed nitric acid reduction half reactions. As HNO3 concentration increases, H+ or NO3- reduction also increases leading to an

increase in Ecorr, a greater extent of stainless steel oxidation and the formation of a thicker passive layer,

leading to a lower icorr at equilibrium.

5 10 15 20 25 30 35 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 E co rr ( V )

Nitric Acid Concentration

5 10 15 20 25 30 35 0 5 10 15 20 Ic or r (  A /c m 2)

Nitric Acid Concentration

b) a)

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Figure 3-4 - icorr vs. Ecorr (data points minus error bars for clarity) calculated from LSV results in Figure 3-2a for 316L SS in 5 - 15% wt. (1-3 respectively) nitric acid. NOTE: Higher concentrations of

HNO3 have been left on the x-axis and will be filled in in Chapter 4.

Figure 3-4 shows a plot of icorr vs. Ecorr values calculated from LSV results of Figure 3-2a and initially

shown in Figure 3-3. As discussed above, icorr decreases as Ecorr increases into the passive potential region.

This shift into the passive region leads to a greater extent of stainless steel oxidation and the formation a thicker passive layer which, in turn, leads to a reduced icorr. Thus, overall, corrosion rate counter-

intuitively decreases with increase in HNO3 concentration in the range of 5-15% wt. This behaviour will

be further examined below using other electrochemical techniques such as; Electrochemical Impedance Spectroscopy and Electrochemical Quartz Crystal Microgravimetry.

In summary, icorr decreases with an increase in HNO3 concentration as Ecorr increases with an increase in

HNO3 concentration. This indicates that the increase in HNO3 is allowing for higher cathodic currents to

be accessed, either via H+ _{or NO}

3- reduction which, through galvanic coupling, allows for higher anodic

current to be supported. This leads to an Ecorr that resides deeper into the passive range and, in turn, thicker

passive layers and lower icorr values at equilibrium. The increase in HNO3 is leading to an increase in Ecorr,

which moves the system further into the steel’s passive region.

Importantly, the LSV results indicate that in order to artificially ‘grow’ oxide layers on 316L SS surfaces in HNO3 concentrations of ≤15% wt. the applied potential needs to be <1 V to avoid transpassive

dissolution of the Fe-Cr oxide film, and >-0 V to allow passivation to occur. Thus, differences in oxide layer growth modes in the passive region as a function of HNO3 concentrations are now investigated in

further detail using Electrochemical Impedance Spectroscopy.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0 2 4 6 8 10 12 14 16    Icorr (  A /c m 2 ) E_corr (V)

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Electrochemical Impedance Spectroscopy (EI S) Studies on 316L

SS in Nitric Acid

EIS is a very useful technique when assessing small changes at the metal solution interface which may indicate passive film growth. 316L SS was analysed in the same potential region (-0.5 to 1.5 V vs. SCE) and HNO3 concentrations (5-15% wt.) as the LSV studies in the previous section to allow for direct

comparison with these results. Results detailed in this section are first presented using the raw experimental data, in the form of nyquist plots and E vs. Z’ plots, at low frequencies. The data is then modelled using physically relevant equivalent circuits in Z-View2 impedance software, with the results presented alongside LSV results from the previous section. Z-View2 is based on the method of nonlinear least squares, which allows non-ideal electrochemical behaviour (elements that exhibit a combination resistive, capacitive or inductive behaviours) to be modelled [170], [171].