Effect of Alloy Composition on the Localized Corrosion Resistance of Nickel Alloys
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Effect of Alloy Composition on the Localized Corrosion Resistance of Nickel Alloys Santiago Sosa Haudet1, Martín A. Rodríguez1,2,3 and Ricardo M. Carranza1,2 Departamento Materiales, Comisión Nacional de Energía Atómica, Argentina. 2 Instituto Sabato, UNSAM / CNEA, Argentina. 3 CONICET, Argentina.
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ABSTRACT Nickel base alloys are considered among candidate materials for engineered barriers of nuclear repositories. The localized corrosion resistance is a determining factor in the materials selection for this application. This work compares the crevice corrosion resistance of selected nickel base alloys, namely 625, G-30, G-35, C-22, C-22HS and HYBRID-BC1. The crevice corrosion repassivation potential (ER,CREV) of the tested alloys was determined by the Potentiodynamic-Galvanostatic-Potentiodynamic (PD-GS-PD) method. The testing temperature was 60ºC and the chloride concentrations used were 0.1 M, 1 M and 10 M. A linear relationship between ER,CREV and the logarithm of chloride concentration was found. ER,CREV increased linearly with PREN (Pitting Resistance Equivalent Number) in concentrated chloride solutions. ER,CREV is the sum of three contributions: ECORR ȘDQGǻĭ ECORR DQGȘLQFUHDVHGOLQHDUO\ZLWK35(1ZKLOHǻĭLQFUHDVHGOLQHDUO\ZLWK35(N for concentrated chloride solutions, not showing a definite trend with PREN for the less concentrated solutions. INTRODUCTION Nickel is able to retain large amounts of different alloying elements leading to the development of several nickel base families of alloys. [1] Nickel base alloys are considered among candidate materials for engineered barriers of nuclear repositories. The main engineering barrier is the waste container, which must comply with an excellent performance against corrosion, good mechanical strength and considerable stability to heat and radiation. Alloys 625 and C-22 have been proposed as corrosion resistant barriers of high level nuclear waste containers. Expected degradation modes for these alloys include general corrosion, localized corrosion (pitting and crevice corrosion) and stress corrosion cracking. [2,3] Pitting and crevice corrosion are considered the same process from an electrochemical viewpoint. Crevice corrosion is more likely since it is stabilized in occluded regions at lower potentials than pitting corrosion. [4] Some nickel base alloys belonging to the Ni-Cr-Mo family are practically immune to pitting corrosion. [3,5] It is assumed that localized corrosion will only occur when the corrosion potential (ECORR) is equal or higher than a critical potential (ECRIT). [2] The crevice corrosion repassivation potential (ER,CREV) has been proposed as the critical potential. [2,3] The resistance to localized corrosion of stainless steels and chromium containing nickel base alloys has been correlated with PREN (Pitting Resistance Equivalent Number). [4] The PREN is defined in equation 1 as a function of the weight content of chromium, molybdenum and tungsten. PREN = %Cr + 3.3 Â(%Mo + 0.5 %W)
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(1)
Chromium is the key element for enha
