Localized Corrosion of Alloy 22 -Fabrication Effects-

Abstract

EXECUTIVE SUMMARYThis report deals with the impact of fabrication processes on the localized corrosion behavior of Alloy 22 (N06022). The four fabrication processes that were analyzed are: (1) Surface stress mitigation of final closure weld, (2) Manufacturing of the mockup container, (3) Black annealing of the container and (4) Use of different heats A of Alloy 22 for container fabrication.Immersion and Electrochemical tests performed in the laboratory are generally aggressive and do not represent actual repository environments in Yucca Mountain. For example, to determine the intergranular attack in the heat affected zone of a weldment, tests are conducted in boiling acidic and oxidizing solutions according to ASTM standards. These solutions are used to compare the behavior of differently treated metallic coupons. Similarly for electrochemical tests many times pure sodium chloride or calcium chloride solutions are used. Pure chloride solutions are not representative of the repository environment.1. Surface Stress Mitigation: When metallic plates are welded, for example using the Gas Tungsten Arc Welding (GTAW) method, residual tensile stresses may develop in the vicinity of the weld seam. Processes such as Low Plasticity Burnishing (LPB) and Laser Shock Peening (LSP) could be applied locally to eliminate the residual stresses produced by welding. In this study, Alloy 22 plates were welded and then the above-mentioned surface treatments were applied to eliminate the residual tensile stresses. The aim of the current study was to comparatively test the corrosion behavior of as-welded (ASW) plates with the corrosion behavior of plates with stress mitigated surfaces. Immersion and electrochemical tests were performed. Results from both immersion and electrochemical corrosion tests show that the corrosion resistance of the mitigated plates was not affected by the surface treatments applied. Behavior of Specimens from a Mockup container:Alloy 22 has been extensively tested for general and localized corrosion behavior both in the wrought and annealed condition and in the as-welded condition. The specimens for testing were mostly prepared from flat plates of material. It was important to determine if the process of fabricating a full diameter Alloy 22 container will affect the corrosion performance of this alloy. Specimens were prepared directly from a fabricated container and tested for corrosion resistance. Results show that both the anodic corrosion behavior and the localized corrosion resistance of specimens prepared from a welded fabricated container were the same as from flat welded plates. That is, rolling and welding plates using industrial practices do not hinder the corrosion resistant of Alloy 22.3. Effect of Black Annealing Oxide Scale: The resistance of Alloy 22 to localized corrosion, mainly crevice corrosion, has been extensively investigated in the last few years. This was done mostly using freshly polished specimens. At this time it was important to address the effect an oxide film or scale that forms du...

Crevice corrosion testing methods for measuring repassivation potential of alloy 22

“…It is generally assumed that crevice corrosion will only occur when E corr of the alloy exceeds its corresponding E R,crev in a certain environment 5. The determination of E R,crev is a key factor for assessing the likelihood of crevice corrosion 3 – 5. Several methods for determining E R,crev were evaluated in the present work.…”

“…Alloy 22 is one of the candidates for the construction of engineered barriers of deep geological repositories for high level nuclear waste isolation 5. The susceptibility of alloy 22 to crevice corrosion depends on environmental and metallurgical variables, including chloride concentration, temperature, presence of inhibitors and applied potential 3 3,4. The susceptibility to crevice corrosion is measured by the value of the repassivation potential E R,crev in each tested condition.…”

“…If there is a possibility of crevice corrosion attack, the propagation rate is to be studied as a second step. Models may be used to predict if the localised attack will eventually perforate the engineered barrier 3 3,4…”

Characterization of the Corrosion Products of Crevice Corroded Alloy 22

“…The applied potential was Ϫ0.150 V SCE, which was above the repassivation potential of Alloy 22 in 4 M NaCl solution at 100°C. 4,7 After testing, the specimen was rinsed with deionized water and methanol and was examined with a stereomicroscope. Scanning electron microscopy ͑SEM͒ and energy dispersive X-ray spectroscopy ͑EDS͒ analyses were performed on an FEI/Philips XL-30, and Auger electron spectroscopy ͑AES͒ analysis was performed on a Perkin-Elmer PHI680.…”

“…1 Localized corrosion is an important degradation mode to be evaluated for the corrosion performance of Alloy 22 over long exposure periods, and it has been examined by a number of researchers. [3][4][5][6][7][8][9][10][11][12][13] Many factors affecting the localized corrosion behavior of Alloy 22 have been studied, such as the effects of Cl − and oxyanions, 3,4,[14][15][16] the effects of potential and temperature, 5,17 and the propagation and stifling of the crevice corrosion. 5,9,18,19 During the crevice corrosion process, corrosion products are formed.…”