Effects of NiB, PbO, and TiO[sub 2] on SCC of Sensitized Inconel Alloy 600 in RT-Tetrathionate Solution

Abstract

The effects of NiB, PbO, and TiO 2 additives ͑impurities͒ on stress corrosion cracking ͑SCC͒ of sensitized alloy 600 were investigated in 0.1 M Na 2 S 4 O 6 solution using potentiodynamic polarization experiment, electrochemical impedance spectroscopy, and electrochemical noise measurement. From the analysis of the electrochemical noises based upon the shot-noise theory, the induction time for stress corrosion ͑SC͒ crack initiation was quantitatively determined. Furthermore, SC crack propagation was satisfactorily distinguished from uniform corrosion in terms of the frequency of events in an engineering sense by employing the stochastic theory based upon the Weibull distribution function. The results revealed that electrochemical noises contain valuable information on SC crack propagation as well as SC crack initiation; SC crack propagation rate increased with increasing conditional event generation rate. The effects of NiB, PbO, and TiO 2 additives have been discussed in terms of induction time for SC crack initiation and the conditional event generation rate. The susceptibility to SCC decreased in the following order: PbO Ͼ absence of additives Ϸ NiB Ͼ TiO 2 .

In-situ Monitoring of SCC of Alloy 600 SG Tubing in PWR using EN Analysis

“…On the other hand, Pyun et al reported that the value of m increases from 0.7 up to unity with a decreasing applied potential (Pyun et al, 1993) for the pit generation at the surface of Al alloy at room temperature, and the value of m decreases from 0.4 to 0.1 with the addition of SO 4 2-and MoO 4 2-ions in a 0.1 M NaCl solution (Na et al, 2007a) and with the addition of SO 4 2-, NO 3-and PO 4 3-ions in a 1M HCl solution (Na & Pyun, 2007) for the pit initiation at the surface of pure Al at room temperature. Interestingly, they also reported that the value of m is 0.33 for the SCC propagation of Alloy 600 in a neutral solution containing PbO at room temperature (Na et al, 2007b). In our previous works (Kim & Kim, 2009a;Na et al, 2007a;Na et al, 2007b), it was reported that the shape parameter m of the Weibull distribution can be regarded as an indicator of types of corrosion.…”

“…Interestingly, they also reported that the value of m is 0.33 for the SCC propagation of Alloy 600 in a neutral solution containing PbO at room temperature (Na et al, 2007b). In our previous works (Kim & Kim, 2009a;Na et al, 2007a;Na et al, 2007b), it was reported that the shape parameter m of the Weibull distribution can be regarded as an indicator of types of corrosion. In the present work, the value of m determined from Fig.…”

“…(1), the cumulative probability F(f n ) at each f n i s d e t e r m i n e d n u m e r i c a l l y b y a m e a n r a n k approximation (Cottis, 2001). In this work, the probability of f n for the Pb-assisted SCC was analyzed using the Weibull distribution function, the most commonly used cumulative probability function for predicting a life and failure rate (Na & Pyun, 2007;Na et al, 2007a;Na et al, 2007b). Using the Weibull distribution function, the probability of the mean timeto-failure, t n , which corresponds to 1/f n , is expressed as,…”

“…In previous works (Na & Pyun, 2007;Na et al, 2007a;Na et al, 2007b;Park & Pyun, 2004;Pyun et al, 1993;Pyun et al, 1994), the conditional event generation rate r(t) of a localized corrosion was introduced as a failure rate using the values of m and n as,…”

“…Depending on the corrosion mechanism, the value of r(t) was regarded as the rate of localized corrosion events such as the pit generation (Pyun et al, 1994), the pit growth (Park & Pyun, 2004) and the propagation of SCC (Na et al, 2007b). There are few reports on the stochastic approach for the localized corrosions of Alloy 600 materials at high temperature in the literature.…”