Temperature sensitivity and transition kinetics of uniform corrosion of zirconium alloys in superheated steam

Abstract

Corrosion transition during uniform corrosion of zirconium alloys receives much attention since it is the major degradation procedure. However, predicting the time and oxide thickness at transition has been hindered by the lack of knowledge about transition kinetics and how it responds to varied temperatures. Current study investigated the temperature-sensitivity of corrosion kinetics, transition behavior and microstructures of various zirconium alloys corroded in superheated steam ranging from 390 °C/10.3 MPa to 455 °C/10.3 MPa by autoclave experiment and microscopy analyses. Transition time was found to follow Arrhenius-type relationship with temperature for the first time. Both the transition oxide thickness and metastable oxide thickness increased with temperature, which was theoretically deduced and experimentally confirmed. In Zr-4 oxides, a transition thickness varying from 3.3 μm at 390 °C to 4.2 μm at 455 °C was observed. Microstructure results presented rather large HCP-ZrO particles (200∼400 nm) at O/M interface and they were even larger at the protruded positions. An intense sub-stoichiometric atmosphere was identified at O/M interface, promoting the growth of metastable oxides. The activation energy of transition kinetics was 86∼114 kJ/mol, which is close to diffusion activation energy of oxygen in tetragonal zirconia. A new model based on parabolic-law empirical relationship was thus proposed to predict transition kinetics. Predictions regarding the time to oxidation breakaway at 900-1000 °C were reported, and the results were in good agreement with the experimental data.

Keywords: Activation energy; Metastable suboxides; Superheated steam corrosion; Transition kinetics; Zirconium alloys.

Fig. 1

Weight gain profiles of N36, N2, N3 and Zr-4 claddings corroded in superheated steam at (a) 390 °C/10.3 MPa, (b) 416 °C/10.3 MPa, (c) 427 °C/10.3 MPa and (d) 455 °C/10.3 MPa.

Fig. 2

(a) Results and fitting curves of pre-exponential factor k versus 1/T and (b) time exponent of different alloys from 390 °C/10.3 MPa to 500 °C/10.3 MPa.

Fig. 3

(a) Results and fitting curves of transition time versus 1/T, and (b) results and prediction curves of transition weight gain ($\Delta W_T$) versus 1/T.

Fig. 4

SEM observations for cross-section of corroded specimens in back-scattered mode: (a)∼(d) N36 alloys corroded in 390, 416, 427 and 455 °C for 120, 39, 34 and 26 days respectively; (e)∼(h) Zr-4 corroded in 390, 416, 427 and 455 °C for 120, 39, 34 and 26 days respectively.

Fig. 5

TEM results of N2 alloy corroded at 455 °C for 21 days which was right at the transition time, (a) BF image of entire thin-foil sample, (b) STEM-DF image of selected region b, (c) magnified STEM-DF image of selected region c, (d) HRTEM image of region d, (e) EDS result of line test marked in (b), (f) diffraction pattern of region f, (g) and (h) the FFT images of region g and h.

Fig. 6

TEM results of Zr-4 alloy corroded at 455 °C for 26 days, (a) BF image of entire thin-foil sample, (b) STEM-DF image of selected region b, (c) magnified BF image of region c, (d) EDS result of line test marked in (b), and (e) diffraction pattern of region e.

Fig. 7

STEM-DF imaging on O/M interface of Zr-4 alloy corroded at 390 °C for 120 days.

Fig. 8

TEM results on isolated lateral cracks (path in red) and the surrounding oxide morphology, (a) N2 alloy corroded at 455 °C for 21 days by STEM-DF imaging, and (b) f Zr-4 alloy corroded at 455 °C for 26 days by BF imaging.

Fig. 9

Schematical view of diffusion and growth kinetics of different zirconia phases.