Primary Water SCC Understanding and Characterization Through Fundamental Testing in the Vicinity of the Nickel/Nickel Oxide Phase Transition Page: 2 of 11
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Primary Water SCC Understanding and Characterization Through
Fundamental Testing in the Vicinity of the Nickel/Nickel Oxide Phase Transition
D.S. Morton, S.A. Attanasio and G.A. Young
P.O. Box 1072
Schenectady, NY 12301
This paper quantifies the nickel alloy stress corrosion crack growth rate (SCCGR) dissolved hydrogen level
functionality. SCCGR has been observed to exhibit a maximum in proximity to the nickel/nickel oxide phase
transition. The dissolved hydrogen level SCCGR dependency has been quantified in a phenomenological model in
terms of the stability of nickel oxide not the dissolved hydrogen level. The observed SCCGR dependency has been
extended to lower temperatures through the developed model and Contact Electrical Resistance (CER)
measurements of the nickel/nickel oxide phase transition. Understanding obtained from this hydrogen level SCC
functionality and complementary SCC subprocesses test results is discussed. Specifically, the possible SCC
fundamental subprocesses of corrosion kinetics, hydrogen permeation and pickup have also been measured for nickel
alloys. Secondary Ion Mass Spectroscopy (SIMS) analysis has been performed on SCCGR specimens tested in
heavy water (D20).
Hydrogen is often added to high temperature water to maintain low levels of dissolved oxygen and thereby minimize
corrosion of structural metals. Numerous studies17 have shown that hydrogen dissolved in high temperature water
(288 to 360*C) or hydrogen gas in steam (400*C) affects the stress corrosion cracking (SCC) performance of nickel
alloys (e.g., alloys 600 and X-750). Test results have demonstrated that a crack growth rate maximum, with respect
to coolant hydrogen variation, is observed in proximity to a key phase transition, the nickel (Ni) to nickel oxide
(NiO) phase transition, see Figure 1.
Reference (1) identified that the SCC hydrogen dependency is fundamentally described by the extent that the alloy's
corrosion potential deviates from the potential of the Ni/NiO phase transition. This potential difference represents
the relative stability of the SCC controlling oxide films (e.g., crack tip oxides are often of a NiO structures).
This paper reports results from numerous tightly controlled SCC crack growth rate (SCCGR) tests as a function of
dissolved hydrogen level for three nickel alloys (X-750 AH, X- 750 HTH and alloy 600). Except for new tests of
alloy 600 at a stress intensity factor of 65.9 MPaIm, these tests are a direct continuation of the Reference (1) effort.
A phenomenological SCCGR correlation has been developed to describe the observed nickel alloy SCC dependency
on dissolved hydrogen. This hydrogen SCC model, in association with Contact Electrical Resistance (CER)
measurements9 of the Ni/NiO phase transition from 288 to 360 C permits the ability to extrapolate the SCCGR
hydrogen functionality to lower temperatures.
The nickel alloys investigated in this study were mill-annealed alloy 600, alloy X-750 condition HTH, and alloy X-
750 condition AH. The compositions of these alloys have been provided in Reference (1). Alloy 600 was fabricated
into standard 25.4 mm compact tension (CT) specimens with 10% side grooves and alloy X-750 was fabricated into
standard 10.2 mm CT specimens. All specimens were fabricated in a longitudinal-transverse (LT) orientation. Test
specimens were air fatigue precracked, to a nominal a/W of 0.5 for alloys 600 and X-750 AH and 0.45 for alloy X-
750 HTH, using the fatigue precracking procedure from ASTM E399 Annex 2. The alloy X-750 tests were
conducted under pure constant load test conditions whereas a 10% daily unload was performed in the alloy 600 tests.
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Morton, D.S.; Attanasio, S.A. & Young, G.A. Primary Water SCC Understanding and Characterization Through Fundamental Testing in the Vicinity of the Nickel/Nickel Oxide Phase Transition, report, May 8, 2001; Schenectady, New York. (digital.library.unt.edu/ark:/67531/metadc739268/m1/2/: accessed January 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.