Magnesium and magnesium alloys are susceptible to stress corrosion cracking in various environments, including distilled water. There is compelling evidence to conclude that SCC is assisted, at least in part, by hydrogen embrittlement. This paper reviews the thermodynamics of the Mg-H system and the kinetics of hydrogen transport. Aspects of magnesium corrosion relevant to hydrogen absorption are also discussed. Crack growth mechanisms based on delayed hydride cracking, hydrogen adsorption dislocation emission, hydrogen enhanced decohesion, and hydrogen enhanced localized plasticity have been proposed and evidence for each of them is reviewed herein.

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Impedance measurements were performed for alloy 22 in the passive and transpassive range, in 1 M NaCl at 90°C. A RΩ-(R//CPE) circuit model was applied in the full passive range, where R was the film resistance. This model also applied for a wide range of chloride concentrations and pH values, at the open circuit potential. Two time constants were observed at the beginning and at the end of the passive range. In these cases, the resistances for the ion transfer might be located at the film interfaces, and not in the film itself. The protective properties of the film improved with polarization time due to the thickening and ageing of the film. The film resistance and the space charge layer thickness increased with the potential. The oxidation of Cr3+ to Cr6+ occurred in the film at high potentials, followed by the transpassive dissolution. In the pre-transpassive range of potentials, the film showed a p-type electronic character, while the ionic properties were that of a passive film. The passive film of alloy 22 was an n-type semiconductor, which changed to a p-type for the high passive potentials. ND = 2.7 × 1020 cm−3 and EFB = −0.551 VSCE were determined.

Corrosion tests with gaseous H 2 S require special facilities with safety features, because H 2 S is a toxic and flammable gas. The possibility of replacing H 2 S with thiosulfate (S 2 O 3 2− ), a non-toxic anion, for studying stress corrosion cracking of stainless and carbon steels in H 2 S solutions was first proposed by Tsujikawa in 1993. H 2 S production was detected in presence of carbon steel corroding in acidified thiosulfate-containing solutions. In this paper, the kinetics of H 2 S evolution are used to estimate the range of partial pressure of H 2 S that can be simulated with thiosulfate solutions. It was determined that acid brines containing 10 −4 M and 10 −3 M S 2 O 3 2− could be used for replacing continuous bubbling of dilute H 2 S/N 2 mixtures in tests of degradation of carbon steels, with H 2 S partial pressures ranging between 0.03 and 0.56 kPa. The kinetics of H 2 S production were compared with the amount of sulfur in side reactions, like formation of iron sulfide films and elemental sulfur.

Alloy 22 (UNS N06022) is a Ni-Cr-Mo-W alloy highly resistant to localized corrosion. Alloy 22 may be susceptible to crevice corrosion in pure chloride (Cl -) solutions under aggressive environmental conditions. The effect of the fl uoride (F -) over the crevice corrosion induced by chloride ions is still not well established. The objective of the present work was to explore the crevice corrosion resistance of this alloy to different mixtures of fl uorides and chlorides. Cyclic potentiodynamic polarization (CPP) tests were conducted in deaerated aqueous solutions of pure halide ions and in different mixtures of chloride and fl uoride at 90°C and pH 6. The range of chloride concentration [pH, microbial activity, volume of electrolyte, crevice former geometry, crevicing material, etc. Internal factors include 7 the metallurgical condition of the alloy (microstructure), presence of a weld seam, type of annealing, oxide fi lm formed, surface fi nishing, etc. A more detailed discussion regarding this topic can be found elsewhere. 7 Many of the factors listed above, such as chloride concentration, temperature, and inhibitors (namely nitrate and sulfate), have been studied in some detail. 6,[8][9][10][11] The infl uences of other factors still need to be investigated. 7 In particular, the role of halides other than chloride is still not 7,12-17 and will be discussed briefl y in this paper.Meck, et al., did not fi nd crevice corrosion in prismatic Alloy 22 specimens (a variation of the ASTM G5 specimen, 18 which contained an artifi cial crevice formed by a polytetrafl uoroethylene [PTFE] compression gasket) tested in 1 M sodium chloride (NaCl), pH 6 and in 1 M sodium fl uoride (NaF), pH 9 solutions at

Alloy 22 (N06022) is a Ni–Cr–Mo alloy that offers an outstanding corrosion resistance in a wide variety of highly corrosive environments. In the present work, the general corrosion of alloy 22 in the passive and active states and the transition from passive and active states were studied in acidic chloride solutions at 90°C . Electrochemical studies, including electrochemical impedance spectroscopy and polarization tests, were performed using mill-annealed and thermally aged (10 h at 760°C ) alloy 22 specimens. The depassivation pH (pHnormalD) was 1.7 in deaerated conditions and pHnormalD=0.3 in aerated conditions. The transition from passive to active states was characterized by a 3 orders of magnitude increase in the corrosion rate (CR) and a significant increase in the interfacial capacity. The CRs obtained via electrochemical tests for mill-annealed (MA) and thermally aged alloy 22 were comparable in all the tested conditions used in the present work. Intergranular attack was observed in thermally aged alloy 22 corroding in the active state due to the presence of precipitates and adjacent depleted zones of the protective alloying elements.

How would you……describe the overall signifi cance of this paper?This paper demonstrates that much research and understanding has been done in the last fi ve years regarding the localized corrosion resistance of Alloy 22. This information is crucial for predicting the lifetime performance of the high-level nuclear waste containers of Yucca Mountain.…describe this work to a materials science and engineering professional with no experience in your technical specialty?This is a review of results obtained using electrochemical and corrosion tests to assess the environmental and metallurgical variables that affect the localized corrosion resistance of the nickel-based Alloy 22 (N06022). The main experimental result discussed here is the repassivation potential value, a parameter used to predict if Alloy 22 may be prone or not to suffer localized corrosion. …describe this work to a layperson?This work deals with the degradation of an alloy (metal) due to its interaction with the environment. This process is called corrosion (rusting) in which a metal reverts back to oxides of minerals by a mechanism of oxidation. This metal is planned to be used as a barrier between nuclear waste and the environment in a mountain in Nevada. The susceptibility of Alloy 22 (N06022) to crevice corrosion may depend on environmental and metallurgical variables. This paper summarizes the current fi ndings regarding the effect of many of these variables, such as pH, other inhibitive species and types of crevicing material, geometry, and applied torque. There are still contradictory results regarding the effect of metallurgical factors such as the presence of weld seams. Crevice corrosion stifl ing and arrest must be considered in evaluating the life expectancy of components made of Alloy 22.

Alloy 22 (N06022) is the current candidate alloy used to fabricate the external wall of the high-level nuclear waste containers for the Yucca Mountain repository. It was of interest to study and compare the general and localized corrosion susceptibility of Alloy 22 in fluoride and chloride solutions at 90°C. Standard electrochemical tests such as cyclic potentiodynamic polarization, amperometry, and electrochemical impedance spectroscopy were used. Studied variables included the solution pH and the alloy microstructure (thermal aging). Results show that Alloy 22 is highly resistant to general corrosion in all the solutions tested. Thermal aging is not detrimental and even seems to be slightly beneficial for general corrosion at the higher solution pHs. Pitting corrosion was never observed. Crevice corrosion was found only for high chloride-containing solutions after anodic polarization. The presence of fluoride ions together with chloride ions seems to increase the susceptibility of Alloy 22 to crevice corrosion compared to pure chloride solutions.The maximum allowed temperature by design specifications is 350 °C. [1] Previous studies have shown that the mechanical and corrosion properties of this alloy did not change when it was aged for up to 40,000 hours at 427 °C. [5,6,7] Microstructural changes that occur in the base material have been evaluated at temperatures from 427 °C to 760 °C. Tetrahedral close-packed (TCP) phases precipitate in the Alloy 22 at temperatures of 593 °C and higher. [8,9,10] These phases could have a detrimental effect upon corrosion resistance and cause loss of mechanical ductility. A long-range ordering (LRO) reaction can occur at lower temperatures and produce an ordered Ni 2 (Cr,Mo) phase. [7,8] This ordering reaction is thought to cause little or no effect on corrosion and causes only a slight loss in ductility.Alloys that rely on passive oxide films for protection against corrosion are susceptible to localized corrosion, especially in the presence of halide ions. [11,12] Different concentrations of fluorides and chlorides can be naturally found in ground waters. While the effects of chlorides on the passive state and localized corrosion have been extensively studied for austenitic alloys that form chromium oxide films, the effects of fluorides have not been fully characterized. [13][14][15][16] The aim of this study was to investigate the effects of pH and thermal aging on the susceptibility of Alloy 22 to general and localized corrosion in chloride, fluoride, and mixtures of chloride-fluoride solutions. The results presented in this study correspond to solutions containing a much larger amount of chloride and fluoride than in the ground water at Yucca Mountain, which are approximately 7 and 2 mg/L, respectively. One M fluoride represents 19,000 mg/L and one M chloride represents 35,000 mg/L; these values are 5000 to 10,000 times the concentration of halide in the ground water. [12] II. EXPERIMENTAL PROCEDURE Specimens of Alloy 22 were prepared from wrought mill annealed plate stoc...