Influence of halide ions and alloy microstructure on the passive and localized corrosion behavior of alloy 22

Abstract

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...

The crevice corrosion of Alloy 22 in the Yucca Mountain nuclear waste repository

“…30 Another type of specimen that contains an unintentional artifi cial crevice formed by a PTFE compression gasket is a variation of the ASTM G 5 specimen usually referred to as prismatic. 19,[53][54][55] This type of PTFE compression gasket is not considered demanding enough to study the crevice corrosion of Alloy 22. 55 The crevice depth (x) and the crevice gap (g) defi ne a crevice geometry.…”

“…64,65 This LRO is thought to have little or no effect on corrosion. 53 At temperatures higher than 593°C, molybdenum-and/or chromium-enriched tetrahedral close packed phases (TCP, , , and P phases) precipitate at grain boundaries. These phases could have a detrimental effect upon corrosion resistance.…”

Modified Point Defect Model for the Electrochemical Behavior of the Passive Films Formed on Alloy C (UNS N10002) in Borax Solutions

“…The composition of the passive films formed on the Ni-Cr-Mo alloys has been characterized by using different methods such as atomic force microscopy (AFM), time of flight secondary ion mass spectrometry (ToF-SIMS), and X-ray photoemission spectroscopy (XPS). These studies indicated that the composition of the passive films depends on many variables such as anodic potential, presence of aggressive anions, pH and aerating conditions [5,[11][12][13].…”

“…Therefore, there is a permanent interest in A. Fattah-alhosseini (B) Faculty of Engineering, Bu-Ali Sina University, Hamedan 65178-38695, Iran e-mail: arash.fattah@gmail.com; a.fattah@basu.ac.ir studying the passivation of these alloys in various conditions [4][5][6][7][8][9][10].…”

Electrochemical Behavior of the Passive Films Formed on Alloy 22 (UNS N06022) in Acidic Solutions

“…Moreover, the excellent passive behavior of these alloys is B Arash Fattah-alhosseini a.fattah@basu.ac.ir 1 Department of Materials Engineering, Bu-Ali Sina University, 65178-38695 Hamedan, Iran well known in numerous solutions with pH values ranging from 1 to 13 [1][2][3]. For that reason, it is important and interesting to study the electrochemical behavior of the passive films formed on these alloys in diverse environments [3][4][5][6][7][8][9][10].…”