No abstract

No abstract

The study of Alloy 22 was undertaken in several selected nitrate/chloride (NO 3-/Cl-) electrolytes with chloride concentrations [Cl-] of 1.0, 3.5 and 6.0 molal with [NO 3-]/[Cl-] ratios of 0.05, 0.15 and 0.5 at temperatures up to 100 o C. Results showed that the repassivation potentials increased with increase in [NO 3-]/[Cl-] ratio and decreased with increase in temperature. The absolute [Cl-] was found to have less of an effect on the repassivation potential compared with temperature and the NO 3-/Cl-. Regression analyses were carried out and expressions were derived to describe the relationship between the repassivation potential, temperature, [Cl-] and [NO 3-] for the conditions tested.

We introduce an acid addition technique for the rapid assessment of the influence of solution pH, anion ͑such as chloride͒ concentration, and temperature on the dissolution of metals. We demonstrate the technique with the characterization of the dissolution of alloy 22 ͑Ni-22Cr-13Mo-3W-3Fe͒ exposed to chloride-containing hydrochloric, sulfuric, and nitric acid environments as a function of pH ͑from pH 5 to pH −1͒ and temperature ͑25-90°C͒. A combination of electrochemical techniques ͑electrochemical impedance spectroscopy and linear polarization resistance͒ and atomic force microscopy are used to characterize the influence of the various solutions on the dissolution of alloy 22. Below 50°C, corrosion rates are less than 1 m/yr independent of acid type, pH, and temperature. In contrast, between 50°C and the upper explored limit of 90°C, dissolution rates in sulfuric and hydrochloric acid scale approximately linearly with temperature at rates that depend on the solution pH. In nitric acid, corrosion rates are lower at comparable pH values due to the oxidizing effects of nitrates. An increase in the open-circuit potential in conjunction with electrochemical impedance spectroscopy data suggests that nitrates promote a stable passive oxide film that inhibits alloy dissolution in all environments tested.Nickel-based alloys are employed in a wide range of applications where long lifetimes are vital such as in implant materials and nuclear waste storage. The robustness of alloys in these types of environments is due to the formation of a protective "passive" oxide film at the alloy interface. The nature and composition of these oxide films depends on the alloy composition and the pH, temperature, and electrolyte composition of the formation environment. Alloys can be tailored to fit a specific set of environmental conditions by selection of constituent elements that have strengths that compensate in the areas where other elements fail. For example, for Ni-Cr-Mo alloys, it is generally well established that the Mo provides resistance against reducing acidic environments, Cr against oxidizing conditions, Cr and Mo against localized attack, and Ni, Cr, and Mo against stress corrosion cracking due to chloride ions. [1][2][3][4] This combination of corrosion resistance in both reducing and oxidizing environments makes these alloys particularly well suited as long-term storage materials. As a result, the Ni-Cr-Mo alloy, alloy 22 ͑Ni-Cr22-Mo13-W3-Fe3͒, is the candidate material for the outer wall of nuclear waste packages at the proposed Yucca Mountain nuclear waste repository ͑Nevada, USA͒. Over the past decade, a number of studies have been undertaken to characterize its general and localized corrosion properties. 5-8 These studies demonstrate that alloy 22 is exceptionally corrosion resistant in a broad range of concentrated brines including chloride, fluoride, carbonate, sodium, and calcium with pH values ranging from ϳ2.7 to 13 and temperatures from 25 to 120°C. 9 To date, little work has been done to characterize the allo...

The influence of nitrates and chlorides on the passive film of alloy 22 ͑Ni-Cr22-Mo13-W3-Fe3͒ in chloride-containing solutions ͑1 and 4 m NaCl͒ from pH 5 to −1 at 60 and 90°C is characterized. The combination of nitrates and chlorides in low pH solutions leads to a variety of different corrosion states, including: active, passive, active/passive, and transpassive. For each corrosion state, electrochemical impedance spectroscopy ͑EIS͒ is utilized to characterize the passive film properties. Active corrosion is a sensitive function of the nitrate-to-chloride ratio, rather than the total amount of anions in solution. At pH Ͻ 0, transpassive dissolution is initialized in all nitrate solutions independent of the tested temperatures and nitrate-to-chloride ratios. Raman spectroscopy analysis shows that Mo is a significant dissolution product during the initial stages of transpassive dissolution. The dissolution is attributed to a transformation from an insoluble Mo oxide to soluble Mo complexes at high open-circuit potentials and low pH. Finally, the results are correlated with critical nitrate/chloride ratios that lead to crevice corrosion in occluded volumes.The interaction of anions with metal surfaces plays an essential role in the application lifetime of industrial alloys. Aggressive halide ions such as chlorides promote general and local breakdown of metals leading to increased dissolution rates and often result in pitting corrosion. 1-3 Conversely, anions such as chromates, phosphates, nitrates, and sulfates have been shown to inhibit corrosion. 4-6 Industrial application environments typically consist of a combination of aggressive and inhibitive anions. Consequently, the ability to characterize the complex interactions of both aggressive and inhibitive ions in contact simultaneously with passivating surfaces is an important part of assessing materials' lifetimes. In this paper, we characterize the corrosive effects of both nitrates and chlorides simultaneously in contact with the nickel-based alloy 22 ͑Ni-Cr22-Mo13-W3-Fe3͒, the candidate material for the outer wall of nuclear waste packages at the proposed Yucca Mountain, Nevada nuclear waste repository. Several recent studies have characterized alloy 22 corrosion in a variety of concentrated brine environments including chloride, fluoride, carbonate, sodium, and calcium over a large pH and temperature range. 7 However, no work has been done toward understanding the combined effect of chlorides and nitrates on the corrosion properties of alloy 22. Moreover, to date, very little work has been done in low pH environments ͑pH Ͻ 3͒ that may exist in occluded environments, such as crevices. 8-10 It has been shown that the solutions inside creviced stainless steels in chloride environments undergo hydrolysis reactions that lead to an increased acidity and chloride concentration in occluded volumes, which can eventually lead to rapid passive film breakdown. [8][9][10] In the presence of nitrates, however, creviced alloys have been shown to have diminished or in some ...