John C. Estill

Corrosion Behavior of Alloy 22 in Oxalic Acid and Sodium Chloride Solutions

Abstract Nickel-based Alloy 22 (UNS N06022) is used extensively in aggressive industrial applications, especially due to its resistance to localized corrosion and stress corrosion cracking in high-chloride environments. The purpose of this work was to characterize the anodic behavior of Alloy 22 in oxalic acid (COOHCOOH) solution and to compare its behavior to sodium chloride (NaCl) solutions. Standard electrochemical tests such as polarization resistance and cyclic polarization were used. Results show that the corrosion rate of Alloy 22 in oxalic acid solutions increased rapidly as the temperature and the acid concentration increased. Extrapolation studies show that even at a concentration of 10−4 M oxalic acid, the corrosion rate of Alloy 22 would be higher in oxalic acid than in 1 M NaCl solution. Alloy 22 was not susceptible to localized corrosion in oxalic acid solutions. Cyclic polarization tests of artificially creviced specimens in 1 M NaCl showed that Alloy 22 was susceptible to crevice corrosion...

Anodic Behavior of Alloy 22 in Calcium Chloride and in Calcium Chloride Plus Calcium Nitrate Brines

Alloy 22 (UNS N60622) is a nickel-based alloy, which is extensively used in aggressive industrial applications, especially due to its resistance to localized corrosion and stress corrosion cracking in high chloride environments. The purpose of this work was to characterize the anodic behavior of Alloy 22 in concentrated calcium chloride (CaCl{sub 2}) brines and to evaluate the inhibitive effect of nitrate, especially to localized corrosion. Standard electrochemical tests such as polarization resistance and cyclic polarization were used. Results show that the corrosion potential of Alloy 22 was approximately -360 mV in the silver-silver chloride (SSC) scale and independent of the tested temperature. Cyclic polarization tests showed that Alloy 22 was mainly susceptible to localized attack in 5 M CaCl{sub 2} at 75 C and higher temperatures. The addition of nitrate in a molar ratio of chloride to nitrate equal to 10 increased the onset of localized corrosion to approximately 105 C. The addition of nitrate to the solution also decreased the uniform corrosion rate and the passive current of the alloy.

Review of Corrosion Modes For Alloy 22 Regarding Lifetime Expectancy of Nuclear Waste Containers

Alloy 22 (UNS N06022) was selected to fabricate the corrosion resistant outer barrier of a two-layer waste package container for nuclear waste at the designated repository site in Yucca Mountain in Nevada (USA). A testing program is underway to characterize and quantify three main modes of corrosion that may occur at the site. Current results show that the containers would perform well under general corrosion, localized corrosion and environmentally assisted cracking (EAC). For example, the general corrosion rate is expected to be below 100 nm/year and the container is predicted to be outside the range of potential for localized corrosion and environmentally assisted cracking.

Characterization of the Corrosion Behavior of Alloy 22 after Five Years Immersion in Multi-ionic Solutions

Alloy 22 (N06022) is the candidate material for the corrosion resistant, outer barrier of the nuclear waste container. Two of the potential corrosion degradation modes of the container are uniform corrosion and localized corrosion. A testing program is under way at the Lawrence Livermore National Laboratory to determine the susceptibility of Alloy 22 to these two forms of corrosion using immersion tests. Metallic coupons are being exposed to several electrolyte solutions simulating concentrated underground water from pH 3 to 10 at 60°C and 90°C. This paper describes the results obtained after more than a five-year exposure of 122 specimens to the testing electrolyte solutions. Results show little general corrosion and the absence of localized corrosion. The maximum general corrosion rate was 23 nm/yr.

The effect of water vapor on the corrosion of carbon steel at 65{degree}C

AISI 1020 carbon steel was exposed to air at various relative humidities at 65{degrees}C. A ``critical relative humidity`` (CRH) of 75--85% was determined. The CRH is the transitional relative humidity where oxidation/corrosion changes from dry oxidation to aqueous film electrochemical corrosion. Short term testing suggests that aqueous film electrochemical corrosion results in the formation of an inner oxide of Fe{sub 3}O{sub 4}, and an outer oxide of a powdery Fe{sub 2}O{sub 3} and/or Fe{sub 2}O{sub 3}{center_dot}xH{sub 2}O.

Characterization of the Resistance of Alloy 22 to Stress Corrosion Cracking

In its current design, the high-level nuclear waste container includes an external layer of Alloy 22 (Ni-22Cr-13Mo-3W-3Fe). Since the containers may be exposed to multi-ionic aqueous environments over their lifetime, a potential degradation mode of the outer layer could be environmentally assisted cracking (EAC). The objective of the current research is to characterize the effect of applied potential and temperature on the susceptibility of Alloy 22 to EAC in simulated concentrated water (SCW) using the slow strain rate test (SSRT). Results show that Alloy 22 may suffer EAC at applied potentials approximately 400 mV more anodic than the corrosion potential (Ecorr ).Copyright

Correlation Between Two Types of Surface Stress Mitigation and the Resistance to Corrosion of Alloy 22

When metallic plates are welded, 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 (N06022) 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 compare 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 show that the corrosion resistance of the mitigated plates was not affected by the surface treatments applied.

Anodic Behavior of Specimens Prepared From a Full-Diameter Alloy 22 Fabricated Container for Nuclear Waste

Alloy 22 (N06022) 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 the 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 was the same as from flat welded plates.

General and Localized Corrosion of Austenitic and Borated Stainless Steels in Simulated Concentrated Ground Waters

Boron containing stainless steels are used in the nuclear industry for applications such as spent fuel storage, control rods and shielding. It was of interest to compare the corrosion resistance of three borated stainless steels with standard austenitic alloy materials such as type 304 and 316 stainless steels. Tests were conducted in three simulated concentrated ground waters at 90 C. Results show that the borated stainless were less resistant to corrosion than the witness austenitic materials. An acidic concentrated ground water was more aggressive than an alkaline concentrated ground water.

Corrosion Characteristics of Titanium Alloys in Multi-Ionic Environments

Yucca Mountain (Nevada) is designated as a high-level nuclear waste repository. The nuclear waste will be isolated by a series of engineered barriers. The metallic engineered barriers will consist of a double-wall container with a detached drip shield. The material for the external wall of the container is Alloy 22, a corrosion-resistant Ni-Cr-Mo alloy. Titanium grade 7 has been proposed for the drip shield. Ti alloys are highly resistant to all forms of corrosion due to the formation of a stable, protective and strongly adherent oxide film. The aim of this research was to characterize the general and localized corrosion behavior of Ti Gr 7, 16 and 12 in simulated concentrated ground waters. Welded and non-welded coupons were exposed for up to 5 years to the vapor and liquid phases of acidic and alkaline multi-ionic solutions at 60°C and 90°C. This paper describes the results obtained after approximately 2-1/2- to 5-1/2-year exposure to the testing electrolyte solutions. In general, the highest corrosion rate was obtained for Ti Gr 12; however, in all of the tested conditions, the corrosion rate was generally lower than 100 nm/yr. For all alloys, the highest corrosion rate was obtained in the concentrated alkaline solution.Copyright