David G. Kolman

Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

Abstract In an effort to examine the combined effect of HNO 3 , NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO 3 concentrations promoted active dissolution, with continuous passivity noted at HNO 3 concentration extremes. During active corrosion, increased HNO 3 concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO 3 concentrations. Susceptibility to pitting was also greatest at intermediate HNO 3 concentrations: the pit initiation and repassivation potentials decreased with increasing HNO 3 concentration until the HNO 3 concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.

Assessment of Corrosion-Based Failure in Stainless Steel Containers Used for the Long-Term Storage of Plutonium-Based Salts

Abstract This paper summarizes our efforts to assess corrosion-related failure in stainless steel long-term storage containers bearing plutonium oxides and electrorefining salts. Pitting corrosion of the internal can wall is believed to occur when these salt particles deliquesce forming the electrolyte necessary for corrosion-electrochemistry. Extrapolation of pit depths from coupon studies using generalized extreme value (GEV) statistics found that the probability of a through-wall corrosion pit is finite; the maximum pit depth after 50 years would be on the order of 1.7 mm where the container wall is only 1.6 mm thick. To assess susceptibility to environmental cracking fracture toughness (J1C), experiments were used in conjunction with a J-integral diagram constructed using the GE/EPRI method for linear elastic-plastic materials. As apart of this analysis, the residual stress associated with the weld was measured using the laser contour method. The hoop stress in the weld region was found to be on the o...

A review of the potential environmentally assisted failure mechanisms of austenitic stainless steel storage containers housing stabilized radioactive compounds

The degradation of stainless steel storage containers is a potential problem for 50-year storage of stabilized plutonium-bearing materials. Container materials and their welds will be exposed to ionizing radiation, elevated temperatures, embrittling and/or alloying agents (e.g., gallium and plutonium), chloride-containing compounds, and a limited quantity of moisture. All the above-listed environmental conditions have been shown to be deleterious to material integrity under certain conditions. Although the volume of the literature concerning failure of stainless steels is quite large, the interaction of the environment present within storage containers with stainless steels has not been comprehensively examined. This work attempts to detail selected relevant studies and assess the failure likelihood for storage of stabilized plutonium compounds.

Aqueous Corrosion Behavior of Plutonium Metal and Plutonium–Gallium Alloys Exposed to Aqueous Nitrate and Chloride Solutions

Previous work in this laboratory examined the polarization behavior of high-purity plutonium metal exposed to various solutions; however, the polarization behavior of plutonium-gallium alloys has not been studied. The objective of this work is to establish the effect of gallium on the electrochemical behavior of plutonium metal exposed to nitric acid, sodium nitrate, and sodium chloride solutions. Results indicated that plutonium-gallium alloys were not passive in HNO 3 solutions, unlike unalloyed plutonium. The dissolution of 0.5 and 1 wt % Ga alloys exposed to HNO 3 , and all materials exposed to NaCl, was found to be controlled by the presence of a corrosion product film. Relative oxide thickness measurements of unalloyed Pu exposed to HNO 3 were obtained. Chloride was found to aggressively attack Pu and its alloys with increasing chloride concentration resulting in increased corrosion. Nitrate ions were shown to have a weak effect on the corrosion behavior of Pu, as compared to protons and chloride ions.

Gallium-Suboxide Attack of Stainless Steel and Nickel Alloys at 800–1200°C

Failure of furnace parts composed of stainless steel or nickel-base alloys has been observed following treatment of gallium-containing compounds at 800 to 1200°C. This work examines the effect of gallium suboxide (Ga2O) and gallium oxide (Ga2O3) on the chemical and mechanical properties of 304 SS, 316 SS, and Hastelloy C-276 in an effort to elucidate a failure mechanism. Results indicate that all three materials are subject to attack by gallium compounds. Elemental segregation, oxidation, and Ga uptake all occur following exposure. Ga2O gas appears to play the dominant role in alloy attack under reducing conditions. Increasing temperature is shown to increase the magnitude of attack, as measured by oxide thickness and gallium-metal uptake. Calculations of the system thermodynamics suggest that Cr, Mn, Si, and V alloying components are responsible for metal oxidation and concurrent gallium absorption. A homogeneous, large (>30 wt.%) gallium uptake resulted in brittle failure of 304 SS. Therefore, exposure to gallium compounds can result in premature failure of iron- and nickel-base structural alloys.

Gallium Removal from Weapons-Grade Plutonium and Cerium Oxide Surrogate by a Thermal Technique

Currently, there is interest in fissioning weapons-grade plutonium in nuclear reactors, making use of its valuable energy while at the same time reducing certain dangers associated with its potential for nuclear weapons proliferation. In the process of dismantling nuclear weapons, the US intends to convert much of the Pu metal to oxide. This process yields a PuO{sub 2{minus}x} powder that potentially can be incorporated into mixed oxide nuclear fuel (MOX), a mixture of PuO{sub 2} and UO{sub 2}. This paper describes the process of gallium removal from Ga{sub 2}O{sub 3}-doped CeO{sub 2{minus}x}, a surrogate for weapons-grade PuO{sub 2{minus}x}. Gallium is removed from the surrogate feedstock material using thermal techniques. An Ar-6% H{sub 2} gas was used in order to reduce the oxide to gaseous Ga{sub 2}O. Experimental results were shown in the temperature range of 600 C to 1,200 C as a function of time and sample geometry. The results to date have shown that CeO{sub 2{minus}x} is a very good surrogate for PuO{sub 2{minus}x}.

Gallium Suboxide Vapor Attack of Chromium, Cobalt, Molybdenum, Tungsten, and Their Alloys at 1200°C

Our prior work elucidated the failure mechanism of furnace materials (304 SS, 316 SS, and Hastelloy C-276) exposed to gallium suboxide (Ga2O) and/or gallium oxide (Ga2O3) during plutonium–gallium compound processing. Failure was hypothesized to result from concurrent alloy oxidation/Ga compound reduction followed by Ga uptake. The aim of the current work is to screen candidate replacement materials. Alloys Haynes 25 (49 Co–20 Cr–15 W–10 Ni–3 Fe–2 Mn–0.4 Si, wt.%), 52 Mo–48 Re (wt.%), 62 W–38 Cu (wt.%), and commercially pure Cr, Co, Mo, W, and alumina were examined. Preliminary assessments of commercially pure W and Mo–Re suggest that these materials may be suitable for furnace construction. Thermodynamic calculations indicating that materials containing Al, Cr, Mn, Si, and V would be susceptible to oxidation in the presence of Ga2O were validated by experimental results. The extent of attack (oxidation, Ga uptake, and elemental redistribution) cannot be predicted based on a simple rule of elemental mixtures—alloy composition plays a strong role. In contrast to that reported previously, an alternate reaction mechanism for Ga uptake, which does not require concurrent alloy oxidation, controls Ga uptake for certain materials. Due to the lack of thermodynamic data, calculations cannot quantitatively predict Ga uptake. However, a correlation between Ga solubility and uptake was noted.

On the Requirement for a Sharp Notch or Precrack to Cause Environmentally Assisted Crack Initiation in β-Titanium Alloys Exposed to Aqueous Chloride Environments

The requirement of a sharp notch or precrack to cause environmental crack initiation of metastable β-titanium alloys exposed to 0.6 M NaCI has been observed. The causal relationship has not been thoroughly examined, however. This paper seeks to explain the sharp notch requirement by examining notch acuity effects on a variety of parameters that affect HEAC susceptibility. These include the effects of a sharp notch on cation accumulation - hydrolysis - acidification, potential drop in solution and resulting hydrogen production, and localization of dynamic strain. It is shown that solution resistance down a sharp crack is two orders of magnitude larger for a fatigue precracked compact tension specimen than for a smooth bar. The potential drop down a sharp crack is severe enough to enable hydrogen production even when the applied potential is more positive than the reversible potential for hydrogen production. It also shown that a fatigue precrack results in an acidified crack tip chemistry (approximately pH 1) which is deleterious to HEAC resistance. The effects of a sharp notch on the interplay of mechanics, film rupture and hydrogen uptake are also examined. It is shown that the slip behavior at a sharp crack tip promotes localized film rupture and localized hydrogen uptake. Localization of hydrogen uptake may be critical for HEAC susceptibility in light of the large hydrogen concentration required to cause crack initiation (ca. 1000 wt. ppm) and the lack of significant hydrogen uptake on filmed surfaces.

An assessment of the validity of cerium oxide as a surrogate for plutonium oxide gallium removal studies

Methods for purifying plutonium metal have long been established. These methods use acid solutions to dissolve and concentrate the metal. However, these methods can produce significant mixed waste, that is, waste containing both radioactive and chemical hazards. The volume of waste produced from the aqueous purification of thousands of weapons would be expensive to treat and dispose. Therefore, a dry method of purification is highly desirable. Recently, a dry gallium removal research program commenced. Based on initial calculations, it appeared that a particular form of gallium (gallium suboxide, Ga{sub 2}O) could be evaporated from plutonium oxide in the presence of a reducing agent, such as small amounts of hydrogen dry gas within an inert environment. Initial tests using ceria-based material (as a surrogate for PuO{sub 2}) showed that thermally-induced gallium removal (TIGR) from small samples (on the order of one gram) was indeed viable. Because of the expense and difficulty of optimizing TIGR from plutonium dioxide, TIGR optimization tests using ceria have continued. This document details the relationship between the ceria surrogate tests and those conducted using plutonia.

Electrochemical Studies of Technetium–ruthenium Alloys in HNO3: Implications for the Behavior of Technetium Waste Forms

The electrochemical behavior of Tc–Ru alloys (Ru content, at. %: 3.2, 5.2, 20.1, 24.7) in 1 M HNO3 was studied. The transpassivation potentials (Etp) of Tc–Ru alloys were determined by linear voltammetry. The results show that the transpassivation potentials of the alloys increase with the Ru content. To understand the dissolution mechanism, electrolysis experiments at 1.2 V vs. Ag/AgCl were performed; the corrosion products of the alloys were characterized in solution by UV-visible spectroscopy and electrospray ionization mass spectrometry (ESI-MS). For Ru, a polymeric Ru(IV) species was detected, while for Tc the speciation was dominated by TcO4–.