Measurement and Calculation of Electrochemical Potentials in Hydrogenated High Temperature Water, including an Evaluation of the Yttria-Stabilized Zirconia/Iron-Iron Oxide (Fe/Fe3O4) Probe as a Reference Electrode.
The importance of knowing the electrochemical corrosion potential (ECP, also referred to as E(sub con)) of nickel-base alloys in hydrogenated water is related to the need to understand the effects of dissolved (i.e., aqueous) hydrogen concentration ((H(sub 2))) on primary water stress corrosion cracking (PWSCC). Also, the use of a reference electrode (RE) can improve test quality by heightening the ability to detect instances of out-of-specification or unexpected chemistry. Three methods are used to measure and calculate the ECP of nickel-based alloys in hydrogenated water containing(approx) 1 to 150 scc/kg H(sub 2) (0.1 to 13.6 ppm H(sub 2)) at 260 to 360 C. The three methods are referred to as the specimen/component method, the platinum (Pt) method, and the yttria-stabilized zirconia/iron-iron oxide (YSZ/Fe-Fe(sub 3)O(sub 4)) RE method. The specimen/component method relies upon the assumption that the specimen or component behaves as a hydrogen electrode, and its E(sub corr) is calculated using the Nernst equation. The present work shows that this method is valid for aqueous H(sub 2) levels(ge)(approx) 5 to 10 scc/kg H(sub 2). The Pt method uses a voltage measurement between the specimen or component and a Pt electrode, with the Pt assumed to behave as a hydrogen electrode; this method is valid as long as the aqueous H(sub 2)level is known. The YSZ/Fe-Fe(sub 3)O(sub 4), which represents a relatively new approach for measuring E(sub corr) in this environment, can be used even if the aqueous H(sub 2) level is unknown.
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