In situ detection of SCC crack initiation and growth

Laboratory studies were recently started on the use of electrochemical noise (EN) to detect crack initiation under PWR conditions. The first probe design involved a pressurised tube surrounded by a platinum foil electrode (see Fig. 40, left), but this proved to have too high a capacitance. A modified design (see Fig. 40, right) was successfully used under simulated PWR conditions and examples of the initial results obtained are shown in Fig. 41. The ECP of stainless steel is not stress dependent below the yield strength, but plastic deformation can be detected both through a shift in DC potential and a reduction in the high-frequency component of the noise. Following development of a PWR-compatible electrical feedthrough, work is ongoing to test the technique in the Belgian BR2 research reactor.

FIG. 40. First and second designs of the SCK-CEN electrochemical noise (EN) sensor for detecting crack initiation.

FIG. 41. Examples of initial EN results obtained under simulated PWR conditions.

3.1.2. Laboratory work at the China Institute of Atomic Energy

In the context of an Agency Research Contract, preliminary work has been carried out at the China Institute of Atomic Energy in Beijing to assess the use of EN techniques. Studies of pitting corrosion in room temperature chloride solutions and of SCC in boiling magnesium chloride at 143 °C were used to build up monitoring expertise.

3.1.3. Electrochemical studies at EdF in France

The electrochemical behaviour of Alloy 600 & 690 at 320–360 °C has been studied in laboratory autoclaves [46]. The first study aimed to progress on stress corrosion cracking (SCC) mechanisms understanding. The strategy was to assess if the electrochemical behaviour and repassivation kinetic of Alloy 600 & 690, in primary water at 320–360°C, were compatible with the predictions of different SCC models. The mechanisms assessed were Slip Step Dissolution model, Hydrogen Embrittlement, Internal Oxidation model and Corrosion Enhanced Plasticity Model. EdF studied the variation of electrochemical response as a function of H2 overpressure and chromium content, using ECP measurements, I-E curves with ohmic drop correction (see Fig. 42) and measure of current responses during jumps of potential and Slow Strain Rate tests at various controlled potentials. The reference electrodes were external pressure-balanced electrodes.

0.1 1 10 100 1000

E (V) / ref. EDF

-0.9 -0.8 -0.7 0 0.2

20 ba r H 2

4 ba r H 2 1 ba r H 2

Deaerated

4 ba r O 2

0.1

FIG. 42. I-E curves obtained at 360 °C in primary water on alloy 600, with various hydrogen or oxygen overpressures.

0 200 400 600 800 1000

0 10 20 30

H

V

E

0.1 µ m/h

E

- 80 mV 0.9 µµµµ m/h

E

- 320 mV 1 µ m/h

Argon

FIG. 43. Effect of cathodic potential on SCC of alloy 600 in 360^oC primary water.

The major results were:

• a strong enhancement of SCC at cathodic potentials (see Fig. 43),

• very little effect of hydrogen overpressure on film stability or repassivation kinetics,

• no clear correlation between repassivation kinetics and SCC behaviour when the H overpressure or chromium content vary.

These results and others SCC tests and metallurgical examinations led to the conclusion that the dissolution model, the global hydrogen embrittlement model or the internal oxidation model were not able alone to explain the crack propagation. The corrosion enhanced plasticity model, a quite new model which is based on local interactions between corrosion, hydrogen and deformation, is more suitable [47,48].

This work was the first one at EDF with such a wide application of HT electrochemical monitoring. Such data acquisition and control were found to be very useful techniques. They were the only way to get the right information needed in this study. It would have been very difficult to advance in alloy 600 SCC mechanisms understanding without it. However, they need highly qualified staff for quality control and interpretation and are time consuming.

These techniques are now currently applied, when needed, in some of EdF's laboratory studies.

3.1.4. Use of electrochemical noise to detect SCC initiation in simulated BWR environments at CML (UK) and GE CRD (USA)

The objective of this research [49,50] was to investigate the use of electrochemical noise (EN) for detecting stress corrosion crack (SCC) initiation in boiling water reactor (BWR) environments. Initial experiments examined the response of thermally sensitised AISI Type 304 stainless steel (SS) in slow strain-rate tensile (SSRT) tests in oxygenated, 288°C/10.4 MPa water, a laboratory simulation of the normal BWR environment. This combination of specimen condition and geometry assured abundant nucleation of intergranular cracks, with controllable propagation and arrest via changes in either loading or environment. In the latter case, addition of gaseous hydrogen was used to simulate BWR hydrogen water chemistry (HWC) and lower the specimen potential into a non-cracking region.

The SSRT tests provided an ideal platform for optimising the electrochemical cell configuration, while establishing the nature of electrochemical potential and current noise (EPN and ECN) responses to crack initiation, propagation and arrest. The standard deviation of electrochemical potential, a measure of EPN amplitude, proved to be the best indicator of SCC initiation, as shown by the example in Fig. 44. However, the degree of correlation depended upon both the periodicity of the calculation and the electrode configuration. Further development work is expected to lead to a useful, in-plant sensor for real-time detection of SCC activity.

3.1.5. Development and qualification of electrochemical noise methods at PSU in the USA Electrochemical noise corrosion sensors have been developed at Penn State University (PSU) and tested both in laboratory autoclaves and in various non-nuclear installations [51–53]. An example of the data obtained is shown in Fig. 45. They are considered to be effective in situ monitors, which provide a continuous measure of corrosion activity, but not corrosion rate.

0 00

5-min. Std. Dev. of Potential (mV)

0

FIG. 44. Notched SSRT specimen, NWC: effect of periodic unloading on 5-minute standard deviation of corrosion potential.

FIG. 45. Evolution of electrochemical noise induced by corrosion of carbon steel during a heating-cooling cycle.