Oxidation Products in Inconel Alloys 600 and 690 Under Hydrogenated Steam Environments and Their Role in Stress Corrosio
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Oxidation Products in Inconel Alloys 600 and 690 Under Hydrogenated Steam Environments and Their Role in Stress Corrosion Cracking. Hugo F. Lopez Materials Department, University of Wisconsin-Milwaukee 3200 N. Cramer St. Milwaukee WI 53209, ABSTRACT Thermodynamic considerations for the stability of Ni and Cr compounds developed under PWR environments (PH2O and PH2) are experimentally tested. In particular, the experimental outcome indicates that Ni(OH)2 and CrOOH are thermodynamically stable products under actual PWR conditions (T > 360oC and Pressures of up to 20 MPa). Accordingly, a mechanism is proposed to explain crack initiation and growth in inconel alloy 600 along the gbs. The mechanism is based on the existing thermodynamic potential for the transformation of a protective NiO surface layer into an amorphous non-protective Ni(OH)2 gel. This gel is also expected to form along the gbs by exposing the gb Ni-rich regions to H2 supersaturated water steam. Crack initiation is then favored by tensile stressing of the gb regions which can easily rupture the brittle gel film. Repeating the sequence of reactions as fresh Ni is exposed to the environment is expected to also account for crack growth in Inconel alloy 600. The proposed crack initiation mechanism is not expected to occur in alloy 690 where a protective Cr2O3 film covers the metal surface. Yet, if a pre-existing crack is present in alloy 690, crack propagation would occur in the same manner as in alloy 600. INTRODUCTION Inconel Alloys 600 used in pressurized water reactor (PWR) environments are often found to undergo stress corrosion cracking (SCC) [1-3]. Under these conditions, hydrogen supersaturated steam at temperatures above 300oC in combination with a stressed susceptible microstructure can lead to crack development along the grain boundaries (gbs). It has been observed that cracks always initiate at grain boundaries, but they will not initiate in single crystals [4]. The susceptibility of alloy 600 to undergo SCC has lead to costly repairs and to seek other alloy alternatives such as Inconel alloy 690. The main difference between these two alloys is in the Cr content (15 wt% in alloy 600 vs. 30 wt% in alloy 690). From the published literature, it is apparent that alloy 690 is not susceptible of undergoing crack initiation [5,6]. Yet, when a crack has been initiated by some other means such as fatigue cracking, it will be able to propagate at rates similar to those exhibited in alloy 600 [7,8]. In turn, this strongly suggests that the Cr content might be able to inhibit crack initiation, yet it has little effect on crack propagation. In addition, current research indicates that alloy 600 will not crack in pure steam or in dry H2 environments [9, 10]. Hence, any proposed cracking mechanism must account for the role of H2 in combination with water steam. In particular, it has been reported that the highest SCC susceptibility occurs when the H2 partial pressures are near the Ni/NiO stability lines [11]. In this work, thermodynamic calculations are carried
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