Lietai Yang

An In-Situ Galvanically Coupled Multielectrode Array Sensor for Localized Corrosion

Abstract A localized corrosion sensor consisting of multiple, corrodible, miniature electrodes was tested in different chemical environments. The miniature electrodes were coupled together by connecting each of them to a common joint through independent resistors, with each electrode simulating an area of a corroding metal. In a localized corrosion environment, anodic currents flow into the more corroding electrode, and cathodic currents flow out of the less or noncorroding electrodes. These currents are measured from the voltages across the resistors. The variation among the galvanic currents measured from the miniature electrodes responded well to changes in the environment with respect to localized corrosion. It was demonstrated that statistical parameters derived from the currents flowing through the miniature electrodes, such as the standard deviation or the 90th percentile anodic value, can be used as effective indictors for localized corrosion. Measurement showed the following order of corrosivenes...

Localized Corrosion Susceptibility of Alloy 22 in Chloride Solutions: Part 1—Mill-Annealed Condition

Abstract Alloy 22 (UNS N06022) is a Ni-Cr-Mo-W alloy developed for corrosion resistance in a variety of aggressive environments. Because of its excellent corrosion resistance, Alloy 22 was selected as a candidate container material for the disposal of high-level nuclear waste at the proposed repository at Yucca Mountain, Nevada. The localized corrosion susceptibility of mill-annealed Alloy 22 was assessed in chloride-containing solutions at temperatures ranging from 60°C to 150°C by comparing the crevice corrosion repassivation potentials with corrosion potentials measured in separate tests. Crevice corrosion repassivation potentials were found to be strongly dependent on temperature, chloride concentration, and nitrate-to-chloride molar concentration ratio. Corrosion potentials were dependent on solution pH but independent of chloride concentration.

Localized Corrosion Susceptibility of Alloy 22 in Chloride Solutions: Part 2—Effect of Fabrication Processes

Abstract The effect of fabrication processes on the localized corrosion susceptibility of Alloy 22 ([UNS N06022] 56Ni-22Cr-13.5Mo-3W-4Fe), a candidate container material for the disposal of high-level nuclear waste in the proposed repository at Yucca Mountain, Nevada, was assessed in chloride-containing solutions at temperatures ranging from 60°C to 95°C. The results of tests covering a range of metallurgical conditions, including as-welded, welded plus solution-annealed, and thermally aged materials, were compared with those for the as-received mill-annealed alloy. The localized corrosion susceptibility of Alloy 22 was determined by comparing the crevice corrosion repassivation potentials with corrosion potentials measured in separate tests. The crevice corrosion susceptibility was found to be dependent on temperature, chloride concentration, and the nitrate-to-chloride molar concentration ratio. Welding and short-term thermal aging decreased the localized corrosion resistance of Alloy 22 compared with t...

Corrosion Behavior of Carbon Steel and Stainless Steel Materials under Salt Deposits in Simulated Dry Repository Environments

In-situ coupled multielectrode array sensors were used to measure the non-uniform corrosion of carbon steel and stainless steel materials under KCl salt deposit in simulated dry repository environments. It was found that the initiation of non-uniform corrosion occurs at a relative humidity that is 14% lower than the deliquescence relative humidity of the chloride salt. It was found also that once significant corrosion had occurred, the non-uniform corrosion process for the carbon steel material under the salt deposit continues at relative humidities as low as 27%.

Experimental Determination of the Deliquescence Relative Humidity and Conductivity of Multicomponent Salt Mixtures

The conductivity of hygroscopic salt deposits containing Na + , K + , NO 3 – and Cl – ions was measured in air as a function of relative humidity at constant temperatures. The deliquescence relative humidity (DRH) of multicomponent salts containing Na + , K + , NO 3 – and Cl – was also determined experimentally. The results of the conductivity experiments show that the conductivity of initially dry salt deposits start to increase after reaching a relative humidity value that is 15 to 20% lower than the DRH of the salt. When the DRH is reached, the conductivity increases dramatically as the salt dissolves and transforms into a saturated aqueous phase. The increase in conductivity at humidities below the DRH is attributed to the adsorption of water on the surface of the salt particles. Because of the increase in conductivity, the initiation of aqueous corrosion of metals in contact with hygroscopic salts may occur at a relative humidity much lower than the DRH of the salt. Thus, the onset of aqueous corrosion of metallic nuclear waste package and the drip shield may be earlier, the duration may be longer, and the temperature at which it occurs may be higher than assumed based on the DRH of the salt. The results of the DRH experiments show that the DRH of a salt mixture is usually significantly lower than that of any of its component pure salt.

Development of Crevice-Free Electrodes for Multielectrode Array Sensors for Applications at High Temperatures

Abstract Coupled multielectrode array sensors (CMAS) have been used as online and real-time monitors for localized corrosion in both laboratory and field systems. In a CMAS probe, multiple miniature electrodes made of materials identical to the engineering component are used as sensing electrodes. The application of CMAS at high temperatures, up to 150°C in corrosive environments, however, is limited by degradation of the insulating mounting material, such as epoxy. Degradation of the mounting material can lead to crevice corrosion and causes an increase in the exposed surface area of the electrodes. In this study, the effectiveness of a diamond-like carbon film to protect the Alloy 22 (UNS N06022) electrodes in an acidic sodium chloride-sodium nitrate-potassium nitrate (NaCl-NaNO3-KNO3) salt mixture was demonstrated at 150°C. The average localized corrosion rate measured from the probe with coated electrodes in the NaCl-NaNO3-KNO3 salt mixture at 150°C was approximately 0.43 μm/y. In contrast, the averag...

Internal Current Effects on Localized Corrosion Rate Measurements Using Coupled Multielectrode Array Sensors

Abstract Coupled multielectrode array sensors (CMAS) have been used for real-time monitoring of corrosion, particularly localized corrosion. The internal anodic current on the most anodic electrode...

Deliquescence Behavior of Multicomponent Salts: Effects on the Drip Shield and Waste Package Chemical Environment of the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada

Thermodynamic calculations were conducted to determine the deliquescence behavior of salt mixtures and to simulate the evaporation of Yucca Mountain groundwaters. The results are consistent with published experimental data that show the deliquescence points of salt mixtures are lower than that of individual salts. For mixtures of NaCl and KCl salts, the deliquescence point of pure NaNO3 salt is an appropriate lower bound. However, mixtures containing magnesium and calcium salts have much lower deliquescence points than pure NaNO3. If magnesium and calcium salts are deposited on waste package and drip shield surfaces, it could lead to earlier initiation of aqueous corrosion than assumed by the DOE in its performance assessment abstractions. Such salt mixtures can be formed by evaporation of waters with compositions similar to some Yucca Mountain porewaters and would be characterized by low deliquescence relative humidity, high chloride concentration, and low concentrations of anions such as free (uncomplexed) nitrate and sulfate that could mitigate against the chloride-enhanced corrosion of the waste package. Evaporation of Yucca Mountain groundwaters also could lead to fluoride concentrations that are above the threshold for accelerated corrosion of the titanium drip shield.

High-Temperature Electrochemical Sensor For Online Corrosion Monitoring

Abstract Electrochemical sensors incorporating diamond-like carbon (DLC)-coated electrodes are effective tools for online, real-time corrosion monitoring at high temperatures. A vacuum-based chemical vapor deposition process was used to deposit a DLC thinfilm coating on the surface of the sensing electrodes. The chemical inertness of the DLC coating on the sensing electrodes produced a crevice-free electrode that can be used at temperatures above 100°C. In this paper, we present the coating deposition process for sensor electrodes and fabrication of coupled multielectrode array sensors (CMAS) for high-temperature, high-pressure applications. Probes were fabricated using uncoated and DLC-coated Alloy 22 (Ni-22Cr-13Mo-3Fe-3W [UNS N06022]) and titanium Grade 7 (Ti-0.2Pd [UNS R52400]) electrodes. The Alloy 22 probes were tested in high-temperature, high-pressure conditions in a pH 10 caustic solution. The measured nonuniform corrosion rate using the Alloy 22 probe with the DLC-coated electrodes was approximat...

Estimated Effects of Temperature-Relative Humidity Variations on the Composition of In-Drift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada

We define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances.