Thin-layer electrode arrangement

Electrochemical measurements carried out in solutions simulating LWR coolant conditions or in actual plant conditions are complicated due to the low conductivity of the coolant, especially in typical BWR environments. To obtain useful information of reactions and transport processes occurring on and within oxide films, a thin-layer electrode arrangement for versatile electrochemical measurements was developed [18, 19].

5.1.1 Description of the technique

The principle of the thin layer electrode arrangement is shown in Figure 15. In this arrangement the working electrode and the inert counter electrode (usually made of Ir-metal) are constructed as two parallel surfaces (tips of small diameter rods). The distance between the electrodes can be adjusted with an accuracy of

10-9 m up to about 100 µm by means of a step motor and spring system

developed originally for contact electric resistance (CER) measurements. The reference electrode is situated at the side of the working electrode. When the working electrode is polarised, the current flows between the working and

counter electrodes across the µm-range gap, while the potential is measured

from the side of the specimen thus greatly suppressing the effect of the ohmic drop in the electrolyte.

The bulk solution can be pumped through the counter electrode ensuring that representative solution chemistry prevails between the electrodes during the measurements. Therefore, this test arrangement can also be conveniently used to study the effects of solution flow rate and novel water chemistries on the electrochemical properties of oxide films. A detector electrode, analogous to the ring electrode in a conventional rotating ring-disk electrode set-up, can also be included in the arrangement to identify soluble species released during

Inlet Outlet Adjustable distance 0...100 µm Counter electrode Reference electrode Ring electrode Working electrode CE RE WE Step motor Potentiostat Conventional Thin layer Contact Insulator kam981.dsf

Figure 15. A scheme of the experimental set-up for thin-layer electrochemical measurements

The arrangement shown in Figure 15 can be employed to obtain electronic and electrochemical information of the oxide films on the studied construction materials by using the following measurements:

Thin layer electrochemical measurements

* Thin Layer Electrochemical Impedance (TLEI) measurements to characterise the oxidation and reduction kinetics and mechanisms of metals as well as properties of metal oxide films even in low conductivity aqueous environments. * Thin Layer Walljet (TLW) measurements to detect soluble products released from the working electrode.

* Other controlled potential and controlled current measurements.

Solid contact measurements

* Contact Electric Resistance (CER) measurements to investigate and/or to monitor the electronic properties of surface films

* Contact Electric Impedance (CEI) measurements to measure the solid contact impedance spectra of oxide films.

5.1.2 Experimental verification

The versatility of information which can be obtained when using the thin layer electrode arrangement is demonstrated in Figures 16 and 17. The polarisation

curve of AISI 316L in 0.1 M Na2B4O7 at 200

o

C is shown as a function of potential in Figure 16, together with the potential dependence of the electronic resistance of the oxide film measured with the CER technique. The curves show the potentials region where the material is passive and where transpassive oxidation and secondary passivation take place, as well as the correlations between these phenomena and the electronic resistance of the film.

AISI 316L-Ir / 0.1 M Na2B4O7, 200 °C sweep 1 mV s-1 0 0.5 1 1.5 2 2.5 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 E / V vs. SHE i / mA cm -2 0.0001 0.001 0.01 0.1 1 10 R / Ω cm 2

The conventional impedance diagram measured at 0.22 V comprises a high- frequency time constant and a Warburg line at low frequencies (see Fig. 17a). The values of the impedance magnitude at low frequencies are of the order of

1 kΩcm2. The value of the impedance magnitude at high frequencies can be

attributed to the ohmic resistance of the whole system. This value is of the order of 2 Ωcm2 and thus approaches the value of the resistance of the electrolyte. The dc electronic resistance of the film determined with the CER technique (see Fig. 17c) is of the same order of magnitude (i.e. about 2.5 Ωcm2). Together these results demonstrate that the film is a good electronic conductor. This is further substantiated by the results from the measurement of the contact electric impedance, in which two relaxations are detected (see Fig. 17b). The depressed semicircle at high frequencies probably depicts the distributed geometric capacitance of the film, whereas a finite length Warburg-like response at intermediate frequencies suggests a diffusion-migration mechanism of electron transport. The low frequency limit of the contact electric impedance spectrum is practically identical to the value measured by the dc resistance CER technique.

10k1k 100 10 1 0.1 0 1 2 3 -1 Z' / Ωcm Z'' / Ω cm 2 2 0.1 1 10 100 0 500 1000 -1000 -500 Z' / Ωcm2 Z'' / Ω cm 2 1 2 3 0 1000 2000 3000 t / s a b c R / Ω cm 2 316 L oxidised at 0.22 V vs. SHE in 0.1 m Na2B4O7 at 200 °C

Figure 17. a) A conventional impedance spectrum of AISI 316 L. b) A contact impedance spectrum of the AISI 316 L - Ir couple. c) A dc resistance vs time curve of AISI 316 L by the CER technique.

5.1.3 Concluding remarks on thin-layer electrochemistry

A thin-layer electrode arrangement for impedance, dc resistance (CER) and other electrochemical measurements in low conductivity, high temperature aqueous electrolytes allows for a characterisation of the corrosion performance of construction materials in such environments. The combination of impedance and CER measurements facilitates a comprehensive characterisation of both the electric properties of the oxide films formed on construction materials, the ionic transport through the films and the kinetics of corrosion reactions at the film/electrolyte interface. The proposed arrangement includes also the possibility of contact electric impedance measurements as a tool for in-situ investigation of the mechanism of electronic conductivity of the oxide layers. Identification of the different soluble products is important for more reliable estimation of the corrosion rate and for accurate determination of the processes involved in phenomena such as activity build-up and stress corrosion cracking.

5.2 In-line monitoring of high temperature water