The Effect of Environment on the Stress Corrosion Cracking of Incoloy 800

Alice Dinu

doi:10.37358/rc.20.1.7852

Abstract

This paper studies the stress corrosion cracking (SCC) behaviour of Incoloy 800 in pure 10%NaOH and, respectively, 10%NaOH+0.33g/l NaCl solutions. Notched and un-notched C-ring specimens were tested in a static autoclave at 2600C and 50 bars. To evaluate the electrochemical behaviour of different metal/environment systems, the most important electrochemical parameters were calculated from polarization curves recorded between (�800)mVvsSCE and (+1200)mVvsSCE (scan rate 1mV/s), at 850C. It was emphasised that the presence of Cl- in caustic environment conducts to the decreasing of corrosion rate and corrosion current. Optical microscopy (OM) and scanning electron microscopy (SEM) revealed that the initiation mechanism and penetration depth of SCC cracks depend on the composition of the testing solution: in pure caustic solution, the SCC initiation is transgranular the SCC cracks propagated up to 120m, while in the presence of the Cl-, the SCC cracks started from the pits formed on the samples surface and the SCC depth is shorter than in the pure caustic solution (only up to 20m). X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) analyses were used to identify the composition of the oxide layer build-up on the surface of the specimens exposed in autoclaves. The results indicate that, in the presence of the Cl- anions, the spinel oxide NiCr2O4 is formed in a bigger amount. Due to this compound formation, a greater SCC resistance of Incoloy 800 was observed.

Highlights

This paper studies the stress corrosion cracking (SCC) behaviour of Incoloy 800 in pure 10%NaOH and, respectively, 10%NaOH+0.33g/l NaCl solutions
SCC tests in autoclaves No SCC cracks on the surface of the un-notched specimens, tested in the both environments, were observed
Taking into account that the sodium and oxygen are present in a higher percent in the oxide layer formed at the surface of specimen tested in pure caustic solution, we can associate the larger particles from Figure 6a (SEM) with compounds of Na, Fe and oxygen

Summary

Experimental part Materials and methods

The specimens were cut from the CANDU steam generator tubes, in their as-received state. For the un-notched specimens, the maximum stress was 540MPa 0 This value is localised on the external surface of the C-ring, in the opposite site of the axial cut (Fig. 3a). This means that the maximum values of applied stresses used in our experiments are 430MPa, respectively 490MPa. The SCC susceptibility tests for Incoloy-800 were accomplished in 10% NaOH pure caustic solutions (pH25oC=13) or containing Cl- (0.33g NaCl at 1l 10%NaOH solution, pH25oC=12.5). To simulate the SCC occurrence conditions in the restricted areas of CANDU steam generators tubing, the Incoloy800 stressed C-rings were tested in a Prolabo static autoclave of 1 litter at 2600C and a pressure of 50 bars. To investigate the tested specimens, XRD, SEM, EDS and OM methods were used

Electrochemical behaviour

SPECIMENS EXPOSED IN CAUSTIC ENVIRONMENT

Conclusions