Sub-surface corrosion research on rock bolt system, perforated SS sheets and steel sets for the Yucca Mountain Repository â•fl Quarterly technical report No. 8

Alloy 22 (Ni-22Cr-13Mo-3Fe-3W) is a candidate alloy for nuclear materials storage containers in the High LevelNuclear Waste Repository as well as for other applications. In this study, we present the results of our investigation onthe corrosion behavior of Alloy 22 as a function of temperature and concentration in a complex multi-ionic electrolyte.This electrolyte used in this study is a simulation of ground water which has significance the environment of therepository in which the alloys are used. Electrochemical potentiodynamic and potentiostatic tests have been used todetermine the passivation behavior of Alloy 22 in simulated electrolyte. Corrosion rate was calculated by usingpolarization resistance method using deaerated (nitrogenated) and aerated (oxygenated) electrolyte. It was interesting tonote that the corrosion rate was higher using deaerated electrolyte as compared to the aerated; suggesting aeration isconducive to formation of passive films that inhibit the corrosion process. The activation energies were calculated for theprocess. The corrosion rates for Alloy 22 in this study were compared to the rates reported in literature. We notedtranspassive behavior in higher concentration electrolyte and at higher temperatures. Results from Potentiodynamic,Potentiostatic, Optical and Scanning Electron Microscopy (SEM) coupled with Energy Dispersive spectroscopy (EDS)are presented. IntroductionThe use of nuclear materials for production of electricity as well as for defense purposes over the past decades hasproduced in a large stockpile of used radioactive materials. There is an urgent need for safe storage and/or disposal ofthese spent nuclear materials to safeguard the environment from any inadvertent exposure. It has been proposed that adeep underground repository is suitable for storage of these nuclear materials. Selection of a unique site with appropriateground water conditions is necessary; a site has been selected in Nevada from which electrolyte concentrations havebeen determined [1]. The spent nuclear fuel containers are generally referred to as waste packages. These wastepackages consist of two concentric metal containers; the outer container is made of Alloy 22 (a highly corrosion resistantNi-Cr-Mo alloy) and a thick inner container made of type 316 nuclear-grade stainless steel. The purpose of the innercontainer is to provide shield for radiation and mechanical integrity. The waste packages will be approximately 18 feetin length and 5 feet in diameter. Once the radioactive waste is placed in the waste packages, the lid will be closed bymultipass gas tungsten arc welding with ERNiCrMo-10 filler metal, which is close to the composition of Alloy 22(N06022). Completely sealed waste packages will be stored in the repository tunnels. Alloy 22 (N06022) is a nickel based superalloy containing 22% chromium, 13% molybdenum, 3% tungsten by weight.This is used extensively for industrial applications because of its high corrosion resistance under various aggressiveaqueous environments. The high corrosion resistance of Alloy 22 under both oxidizing and reducing conditions has ledto the selection of this superalloy for use in outer shell of the radioactive disposal container in the Yucca mountainnuclear repository [2,3]. The corrosion resistance of this Ni-based superalloy is attributed to the alloying elementchromium that increases the passive region and also reduces the passive current [4,5]. The alloying elements Mo and Wlower the current for hydrogen discharge under reducing conditions [6]. The corrosion behavior of Alloy 22 has been thesubject of intensive research since it has been selected for the waste package container for YM repository. Rebak et al [7] have extensively studied the electrolytic behavior of nickel based alloys and reported that Ni-based alloysare susceptible to stress corrosion cracking. They [7] also observed increased corrosion rates with increases inelectrolyte temperature and concentration. Cragnolino et al. [8] studied the passive corrosion behavior of Alloy 22 underdifferent concentration of chloride ions. They determined that the passive current density that was independent oftemperature and the passivity was almost independent of potential, Cl-concentration and the pH. The susceptibility ofAlloy 22 to localized corrosion in lead-containing solution was reported by Pan et al [9]. They observed that the presence of lead species in a deaerated, super-saturated PbCl2 solution with a pH of 0.5 promotes a pronounced anodicpeak that increases the passive current density and simultaneously enhances the dissolution of Alloy 22. Evans et al [10]studied the anodic behavior of Alloy 22 in calcium chloride and calcium nitrate brines. The observed corrosion rateswere lower in calcium nitrate containing solutions than in pure calcium chloride solutions. The addition of nitrate alsodecreased the anodic passive current density of Alloy 22. Day et al studied the corrosion behavior of Alloy 22 in oxalicacid and sodium chloride solutions and found that the corrosion rate of alloy 22 in 0.1M oxalic acid at 60°C was200μm/year [11]. They also determined that Alloy 22 was not susceptible to localized corrosion in oxalic acid solutionalthough, they found an increased corrosion rate as the temperature increased. Estill [12] el al studied the corrosion rateof Alloy 22 as a function of immersion time in six different mixtures of NaCl and KNO3 solution at 100°C. Theyreported that as the immersion time increases the corrosion potential increase and the corrosion rate decreases. They after8 months observed a maximum corrosion rate of 50nm/year in concentrated brine solutions at 100°C. In this study, we have assessed the corrosion behavior of Alloy 22 in simulated YM environment using electrochemicalmethods. The effect of various experimental parameters such as temperature of the electrolyte, aeration/deaeration on thecorrosion behavior has also studied. Yilmaz et al have conducted a similar study of the corrosion behavior of carbon steelrock bolt in this simulated YM water [13]. The effect of temperature (from 25°C to 90°C) and the environment (aeratedor deaerated) on the corrosion behavior of Alloy 22 has also been studied in 100x YM water. The composition of thesimulated YM water is given in Table 1. The film formed on Alloy 22 at different potential at room temperature has beencharacterized by using SEM and EDX under deaerated conditions. Since the passive film formed on Alloy 22 is fewnanometers thick, it is very hard to characterize the film by using SEM and EDX. So the X-ray FluorescenceSpectroscopy XPS and Auger Electron Spectroscopy (AES) are the best techniques for this kind of analysis. Experimental Procedure Alloy 22 specimens for electrochemical tests were prepared from commercially available mill annealed Alloy 22. TheAlloy 22 from Haynes International was reanalyzed for chemical analysis at LTI Corporation. The nominal compositionof the as-received alloy as well as analyzed composition is given in Table 1. Table 1. Chemical Composition (wt%) of Alloy 22Element (weight percent)Ni Co Cr Mo W Fe Si Mn C P S VHaynes 56.60 1.05 21.38 13.55 3.07 3.88 0.028 0.24 0.005 0.006 0.0057 0.14LTI 56.86 1.00 21.20 13.50 3.00 3.90 0.050 0.31 0.002 0.010 0.0100 0.16 Disc shaped test specimens were cut out from the plate sample. The test specimen was mounted in epoxy with ~1 cmexposed surface area. After the molding, the electrical connection between the sample and the wire checked with avoltmeter. Before the experiment the specimens were polished down to 600 grit SiC emery papers. These preparedspecimens were degreased with acetone and ultrasonically washed it with de-ionized water for 5 minutes before usingthem for electrochemical testing. All the electrochemical experiments were conducted in a typical 1 liter Pyrex glass flask covered with apolytetrafluoroethylene lid. The lid had many ports containing working electrode, counter electrode (platinum), gasspurge, thermo couple, inlet and outlet for the gas, luggin probe connected to silver/silver chloride (Ag/AgCl) referenceelectrode through salt bridge (agar-agar solution) and a gas trap (ASTM G5) [14]. A schematic of the cell can be foundin the work by Yilmaz et al. [13] on carbon steel. A large (~10 cm) platinum sheet sealed to a glass capillary was used asa counter electrode. The reference electrode was saturated silver/silver chloride (Ag/AgCl) electrode, which has apotential of 199mV more positive than the standard hydrogen potential. The Luggin probe tip and Ag/AgCl referenceelectrode were connected via the electrolyte (YM water) in the bridge. Continuously purged gas (nitrogen or oxygen) inthe sealed cell maintained constant pressure above the solution, and provided a slight positive pressure. A sealed glasscapillary was used for the thermocouple for controlling the temperature of the electrolyte. A fritted glass capillary wasused for continuous aeration/deaeration of the solution throughout the experiment at the rate of 100mL/min. An electricmantle heater surrounded the test cell and a PID (Proportional, Integral, Derivative) type temperature controllermaintained the temperature of the solution. For high temperature experiments a condenser has been used to avoidelectrolyte evaporation during the test. The electrolyte was purged continuously with nitrogen/oxygen for de-aeration/aeration for 1⁄2 hour before immersing the test specimen. The specimen was then inserted into the cell and maintained a constant distance (2-3mm) between the specimen and Luggin tip. After immersing the sample in theelectrolyte for one hour the potentiodynamic tests were conducted at a scan rate of 0.2mV/sec using a commerciallyavailable Potentiostat. The corroded surface was characterized by using optical microscope for different temperaturesand concentrations. The composition of the simulated YM water is established by Yilmaz et al [13] and is given in Table 2 for 1Xconcentration. The electrolyte was prepared by mixing different kind of salts in distilled, de-ionized water. Table 3shows the different salts that were used accordingly to make 1X YM water. After adding the salts to the de-ionized waterthe solution was stirred in a hot plate (50°C-60°C) for half an hour and then it is cooled down to room temperature. Theexcess or undissolved salts precipitated from the solution at the bottom of the flask. The clear solution was filtered beforeusing as an electrolyte. In this study we used 1X, 10X and 100X YM solutions and the measured pH of the solution was7.8, 8.2 and 8.4 accordingly. Table 2. Chemical Composition of simulated YM water in mg/L Ions Na+SiO2-Ca2+K+Mg2+HCO3-Cl-SO42-F-NO3-pH1XYM 61.3 70.5 101 8.0 17.0 200 117 116 0.86 7.8100XYM 6130 705