Darrell S. Dunn

An Electrochemical Approach to Predicting Long-Term Localized Corrosion of Corrosion-Resistant High-Level Waste Container Materials

Abstract Lifetime predictions of containers used for the disposal of high-level radioactive waste (HLW) are necessary to determine the overall performance of a HLW repository. Failure of the contai...

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...

Effects of Water and Gas Compositions on the Internal Corrosion of Gas Pipelines Modeling and Experimental Studies

Abstract Results of a laboratory study of internal corrosion in wet gas pipelines under conditions involving a stagnant or slowly flowing aqueous phase indicate that the corrosion rate is not affected by calcium and magnesium scale-forming tendency of the test solutions, provided that they are buffered. It is possible that the scales formed are sufficiently porous to permit electrolyte contact with steel. However, an increase in pH and decrease in oxygen resulted in significant corrosion rate reduction. Thermodynamic analyses indicate that the formation of metastable iron sulfide (FeS) precipitates is promoted by higher pH and higher dissolved iron concentration, and hindered by the presence of carbon dioxide (CO2). The presence of oxygen on corrosion may lead to the transformation of a metastable FeS phase, mackinawite (Fe1+xS), to another metastable phase, greigite (Fe3S4). Electrochemical polarization data indicate that the steel behaves in an active manner symptomatic of a nonprotective corrosion prod...

Effect of applied potential on changes in solution chemistry inside crevices on type 304L stainless steel and alloy 825

Abstract Changes in pH, chloride (Cl−) concentration, and potential inside a rectangular crevice of metal against polymethyl methacrylate (PMMA) were monitored using microelectrodes as a function o...

Corrosion of Iron Under Alternating Wet and Dry Conditions

Abstract In-situ alternating current (AC) and direct current (DC) electrochemical techniques were used to determine the corrosion rate and corrosion potential of high-purity iron under alternate wet and dry conditions. Comparisons between DC electrochemical measurements and weight loss were conducted to verify the validity of the corrosion rate measurements. Identification of the corrosion products was performed using Raman spectroscopy. Corrosion products contained layers of iron oxides and oxyhydroxides. At low Cl− concentrations, corrosion products consisted of lepidocrocite (γ-FeOOH) and magnetite (Fe3O4). At higher Cl− concentrations, the formation of akaganeite (β-FeOOH) was observed. Corrosion rates and corrosion potentials fluctuated throughout the wet cycles depending on electrolyte layer thickness, ionic strength, and wetting cycle. Accelerated corrosion rates of high-purity iron, up to three times those observed under continuously immersed conditions, were the result of Fe(III) reduction in the...

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.

Predicting Localized Corrosion in Seawater

Abstract A number of alloys, including stainless steels, aluminum, and nickel-based alloys, are used in seawater for various applications. The localized corrosion of these materials is affected, among other factors, by temperature, microbial activity, chlorination, and flow rate. A predictive model, based on the calculation of repassivation and corrosion potentials, is presented and compared to field experiences of these alloys in seawater systems. An empirical model is used for calculating the repassivation potential of these alloys as a function of seawater composition. A mechanistic model is used for calculating the corrosion potential as a function of oxygen and chlorine concentrations. The parameters for the corrosion potential are derived from tests in flowing natural seawater or synthetic seawater. The model calculations agree with the relative ranking of these alloys in seawater. Limitations of the current model and improvements are suggested.

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...

Application of a General Reactive Transport Model to Predict Environment Under Disbonded Coatings

Abstract Understanding the evolution of the chemical environment and potential inside a disbonded region is essential to a quantitative risk assessment of corrosion and stress corrosion cracking of...

Long-term prediction of localized corrosion of alloy 825 in high-level nuclear waste repository environments

Abstract Long-term prediction of localized corrosion of high-level nuclear waste container materials is a necessary step in the performance assessment of the engineered barrier system. Localized corrosion of corrosion-resistant materials may occur if the containers are exposed to chloride (Cl−)-containing ground water at elevated temperatures. Potentiostatic tests conducted on alloy 825 (UNS N08825), a candidate container material, have shown that the potential at which localized corrosion can be initiated is a function of surface conditions and exposure time. The initiation potentials for localized corrosion measured in short-term tests with polished specimens overestimated the long-term initiation potentials by as much as 300 mV. On the other hand, the repassivation potential obtained from short-term tests on deep pits can be used to predict the initiation of localized corrosion in long-term tests.