R. S. Lillard

Influence of dichromate ions on corrosion processes on pure magnesium

The corrosion behavior of Mg is of interest because of its growing use as an alloy in the transportation industry and also because it is a major component of some intermetallic phases in Al alloys, such as the deleterious S (Al 2 CoMg)-phase found in AA2024-T3. Pure Mg corrodes rapidly in a chloride-containing solution and even dissolves in water if the surface hydroxide is damaged by scratching the surface, for example. Uniform dissolution is drastically reduced in NaCl solutions (from 0.01 to 0.5 M) with the addition of very dilute concentrations of dichromate (10 -4 M). However, it is replaced by a strong localized attack in the form of fast filiform-like attack. On a large-grained sample with a defined defect structure, the attack can be seen to propagate at twin boundaries. Orientation imaging microscopy analysis found that corrosion was limited to planes near {0001} orientations with propagation being in prismatic directions. Auger electron spectroscopy analysis shows that interaction of chromate with the Mg hydroxide results in incorporation of reduced chromium ions in the hydroxide surface layer. Formation of a more resistant surface film could explain the very local nature of the corrosion in this case. The interaction between dichromate ions and Mg hydroxide can also explain the higher corrosion resistance of S-phase particles in chloride solutions containing dilute dichromate, although differences in the surface film formed compared to pure Mg are observed. Sputter-etching of the surface in order to assess the depth of the attack revealed that very hard or isolating corrosion products difficult to sputter are produced along the filiform path and that chromium compounds are not integrated in the corrosion products. Focused ion beam sectioning followed by scanning electron microscopy investigation of the sectioned area, demonstrates the presence of a continuous protective surface film. Adhesion between the Mg hydroxide and the metal is lost at the location of the corrosion filament, suggesting that the mechanism of propagation is similar to filiform corrosion under a coating. The depth of attack is a couple of micrometers with large cracks present within the corroded area that could induce severe surface damage.

Factors Influencing the Transition from Metastable to Stable Pitting in Single-crystal Beryllium

The pitting potential (E pit ) of single-crystal beryllium in 0.01 M sodium chloride decreased with crystallographic orientation in the order (0001) > (1010) > (1120). Orientations which were associated with the lowest E pit, (1010) and (1120), were characterized by crystallographically oriented parallel plates of unattacked Be on the pit interior and square pit walls. While pit geometries in the (0001) surface appeared to be random, corrosion propagation was often in the (1010) and (1210) families of directions. Potentiostatic current/time data revealed that metastable pitting was characterized by two types of transients: (i) transients with a lifetime <1 s, similar in shape to the current/time data observed in the repassivation process, and (ii) transients whose lifetimes lasted 5-140 s and were associated with physical damage on the sample surface. The nucleation frequency of repassivation events was directly proportional to passive current density and inversely proportional to applied anodic potential. Although the frequency and magnitude of damage events increased with increasing applied anodic potential for any given orientation the total accumulated damage was greatest for orientations with the highest E pit . Prepassivation resulted in a decrease in the frequency of repassivation events but had only minimal effect on the frequency and magnitude of the damage events. The results are discussed in terms of physical bulk metal properties as well as critical pitting environments (i,e., ohmic and mass transport models).

The Role of Metallic Bonding in the Crystallographic Pitting of Magnesium

The role of metallic bonding in the crystallographic pitting of magnesium (Mg) has been investigated using atomistic simulations. To assess the degree of metallic bonding in Mg (0001) and (1010) surfaces the modified embedded atom method (MEAM) has been used. The interatomic potentials developed with MEAM were then used by a Monte Carlo code, standard Metropolis algorithm, to calculate atom removal probabilities. Simulations for the Mg (1010) surface found that atom removal formed geometric surface structures, the orientation of which were always at a 58° or 116° angle with respect to the [1210] direction or 90° with respect to [0001]. These results are identical to published experimental results from Mg (1010) single crystals. Simulations of Mg (0001) surfaces found no preferred orientations similar to experimental results. Finally, the threshold potential for atom removal in our simulations follows the same trend as the critical electrochemical potential for pitting in single-crystal Mg surfaces. This result indicates that experimental differences in the pitting potential for Mg as a function of crystallographic orientation are related to the metallic bond strength (chemical potential) associated with each surface orientation.

The Kinetics of Anodic Dissolution and Repassivation on Stainless Steel 304L in Solutions Containing Nitrate

In this paper we investigate the influence of nitrate (NO ― 3 ) on the dissolution and repassivation kinetics of freshly bared stainless steel 304L surfaces generated during scratch tests. To differentiate anodic dissolution from film formation during early decay times, t < 20 ms, we have developed a mathematical expression for fitting this portion of the transient. The expression assumes the total current owes to dissolution plus film formation where the oxide coverage is derived from Avrami growth kinetics. In this manner the approach allows the investigator to separate the film formation and dissolution currents from the decay transient and analyze them individually. It is shown that film formation in 0.9 M NO ― 3 occurs more rapidly as compared to 0.1 M chloride (Cl ― ). In addition, NO ― 3 was characterized by thinner films (< 0.6 nm) and higher electric field strengths (1.1 x 10 7 V/cm) at early times. Correspondingly, increasing Cl ― concentration from 0.1 to 0.9 M resulted in thicker passive films (0.6―2.2 nm) and lower electric field strengths (3.7 x 10 6 V/cm).

The Inhibition of Pitting Corrosion in Stainless Steel 304 L During Proton Irradiation

In 0.1 M sodium chloride, proton irradiation resulted in a 220 mV increase in the pitting potential of stainless steel 304 L (0.425 vs 0.644 V saturated calomel electrode). In addition, the passive region of the polarization curve during irradiation was associated with a drop in metastable pitting activity by a factor of 100. Mott-Schottky experiments in pH 1.6 H 2 SO 4 found that irradiation was associated with an increase in oxygen vacancy concentration (Vo = 2.94 X 10 21 vs 3.41 X 10 21 cm -3 ). However, electrochemical impedance spectroscopy experiments found that the Warburg coefficient (σ) increased during irradiation (47 kΩ cm 2 /s 0.5 vs 118 kΩ cm 2 /s 0.5 ). An increase in film impedance was also observed. Given that σ is inversely proportional to V o , one would expect that an increase in V 0 would result in a decrease in a. This apparent dichotomy, an increase in oxygen vacancies in the space charge region at the film/solution interface and a corresponding increase in σ, can be explained if the film is composed of inner Cr-rich p-type and outer Fe-rich n-type semiconducting layers. It is proposed that changes in the inner Cr-rich layer of the oxide are responsible for the observed increase in pitting potential during irradiation.

Influence of water radiolysis products on passive film formation and reduction in a mixed radiation environment

Abstract Spallation neutron sources generate a mixed radiation environment when a beam of high energy particles (e.g. protons or deuterons) hits a heavy metal target. Radiolysis results when these primary and secondary (spallation) particles lose energy by Coulomb interaction with the electrons in the hydrogen and oxygen atoms of water. The results of cyclic voltammetry experiments on gold in sulphuric acid (pH 1.6) and exposed to a spallation environment were consistent with the presence of water radiolysis products. However, no changes in the Au OA1 - OA4 oxidation peaks or the OCIII reduction peak were observed in irradiated specimens. These findings indicate that proton irradiation and the resulting water radiolysis products do not influence passive film formation and reduction. Cyclic voltammetry experiments have also shown that a modest quantity of hydrogen bubbled into the solution had little influence on the concentration of radiolysis products that are produced during spallation near the electrochemical double layer.

Advances in PSII deposited DLC coatings for use as barriers to corrosion

Abstract Plasma source ion implantation (PSII) is a non-line of sight process for implanting complex shaped targets without the need for complex fixturing. The breakdown initiation of materials coated with diamondlike carbon (DLC) produced by PSII occurs at defects in the DLC, which expose the underlying material, thus establishing a galvanic couple between the coating and exposed material at the base of the defect. Pitting and oxidation of the base metal leads to the development of mechanical stress in the coating and eventually spallation of the coating. This paper presents current progress in attempting to mitigate the breakdown of these coatings by implanting the parent material before coating with DLC. Ideally the parent material would be implanted with chromium or molybdenum, which are known to improve corrosion resistance, however the necessary organometallics needed to implant these materials with PSII are not yet available. Here the effects of carbon, nitrogen, and boron implantation on the susce...

A review of corrosion issues related to uranyl nitrate base aqueous homogeneous reactors

Abstract Aqueous homogeneous reactors (AHRs) are liquid fuel nuclear reactors, typically consisting of an acidified nitrate or sulphate base solution of enriched uranium-235 (235U). The goal of this paper is to review both the report and research literature on the topic of materials corrosion issues related to uranyl nitrate base AHRs. The materials of interest are stainless steel (SS)347, SS304L, SS304L-NAG, SS316L and zirconium (Zr) base alloys. In nitric acid solutions, SS and Zr alloys are passive, although there is some increased susceptibility of SS347 to intergranular corrosion (IGC) and knife-line attack as compared to other alloys. This is likely a result of the higher carbon (C) content in SS347. In uranyl nitrate fuel simulants corrosion rates increased as compared with nitric acid alone with the corrosion rate of SS347 being the highest while SS316 was the lowest of the austenitic stainless steels tested. In comparison, corrosion rates of Zr based alloys remained relatively unchanged in uranyl nitrate base solutions. The most prominent issue for these materials appears to be iodide (I−) induced pitting corrosion on the walls of the off-gas extraction system with the potential for stress corrosion cracking. For Zr alloys, vapour phase I− pitting and SCC are well documented failure mechanisms with the threshold I− concentration being on the order of 1–2 ppm. In the off-gas extraction system of a 200 kW AHR calculations indicate that the I− concentration may be as high as 0·4–0·6 ppm warranting further study in this area. The influence of a prototypic AHR neutron flux on corrosion rates and susceptibility to underfilm corrosion, due to precipitation of UO2(OH)2, was not available from the literature.

Modeling of water radiolysis at spallation neutron sources

In spallation neutron sources neutrons are produced when a beam of high-energy particles (e.g., 1 GeV protons) collides with a (water-cooled) heavy metal target such as tungsten. The resulting spallation reactions produce a complex radiation environment (which differs from typical conditions at fission and fusion reactors) leading to the radiolysis of water molecules. Most water radiolysis products are short-lived but extremely reactive. When formed in the vicinity of the target surface they can react with metal atoms, thereby contributing to target corrosion. The authors will describe the results of calculations and experiments performed at Los Alamos to determine the impact on target corrosion of water radiolysis in the spallation radiation environment. The computational methodology relies on the use of the Los Alamos radiation transport code, LAHET, to determine the radiation environment, and the AEA code, FACSIMILE, to model reaction-diffusion processes.