The effect of strain on the trapping of hydrogen at grain-boundary carbides in Ni-Cr-Fe alloys
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I. INTRODUCTION
Ni-Cr-Fe alloys are widely used in pressurized-water nuclear reactors.[1,2,3] The occurrence of intergranular stress corrosion cracking (IGSCC) of alloy 600 reactor components has generated interest in obtaining an improved understanding of IGSCC.[4–8] There is extensive experimental evidence that supports the development of a high hydrogen concentration in Ni-based alloys in a primary water environment[9,10] with the belief that the hydrogen affects the material behavior.[11,12,13]. In deaerated water, the predominant cathodic reaction is the reduction of water to form atomic hydrogen.[8] The result of this corrosion reaction is to develop very high local fugacities of hydrogen, especially at the metal surface of a crack tip. The hydrogen content has been measured in the alloy 600 and alloy X-750 test specimens after constant-extensionrate tests (CERTs) in hydrogenated water with hydrogen overpressures of 0.005 to 0.1 MPa. In the region of the CERT specimen where SCC was occurring, the measured total hydrogen concentration by thermal emission methods within the metal was between 20 and 80 ppm by weight for alloy 600[9] and between 10 and 45 ppm for alloy X-750.[10] These hydrogen concentrations are much higher than would be calculated using Sievert’s law for the solubility of hydrogen in the metal at the hydrogen overpressure in these tests. In fact, for the 20 to 80 ppm hydrogen measured in the specimens, a Sievert’s-law hydrogen pressure of 5 to 75 MPa is required for alloy 600 and of 20 MPa for the 45 ppm in alloy X-750.[14] To achieve local hydrogen fugacities this high, the hydrogen could only have come from the local cathodic reactions (e.g., water reduction) that accompany the local corrosion reactions that occur transiently at the crack tip as the crack advances. These high hydrogen concentrations measured from the CERTs are consistent with the levels that could be calculated either by assuming thermodynamic equilibrium between the D.M. SYMONS, Manager, is with the Bettis Atomic Power Laboratory, West Mifflin, PA 15122-0079. G.A. YOUNG, formerly Graduate Student with the Department of Materials Science and Engineering, The University of Virginia, is Engineer with Knolls Atomic Power Laboratory, Schenectady, NY 12301. J.R. SCULLY, Professor, is with the Center for Electrochemical Science and Engineering, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted June 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
water and the metal or by calculating the fugacity from repassivation testing. Using the first method, the maximum hydrogen fugacity that can occur at the crack tip is calculated from the corrosion reactions of the bare metal. The theoretical fugacities at 360 ⬚C for equilibrium between the pure metals and oxides are shown subsequently. They were previously calculated by Rummery and Macdonald for Ni and Fe[15] and are calculated for Cr using the data from Reference 16. Ni ⫹ H2O ⇔ NiO ⫹ H2;
fH2 ⫽ 0.02 MPa
3Fe
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