Effect of Heat Treatment on Corrosion Behavior of Alloy 690 and Alloy 693 in Simulated Nuclear High-Level Waste Medium

Abstract

Nickel-based alloys are being considered as candidate materials for the storage of high-level waste generated from the reprocessing of spent nuclear fuel. In the present investigation, Alloy 690 (UNS N06690) and Alloy 693 (UNS N06693) were assessed using the potentiodynamic anodic polarization technique for the corrosion behavior in the solution-annealed and sensitized conditions, in 3 M nitric acid (HNO3) and simulated high-level waste (HLW) at 25°C and 50°C. From the results of the investigation, it was found that the polarization curves for the solution-annealed specimens were characterized by a long passive range and low passivation current density, at 25°C. Increasing solution temperature to 50°C led to a corresponding increase in corrosion potential as well as passivation current density. The solution-annealed specimen showed improved corrosion resistance compared to the sensitized specimen in both media, for both alloys. Double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was carried out for both solution-annealed and sensitized Alloy 690 and Alloy 693 in 0.5 M sulfuric acid (H2SO4) containing 0.0001 M potassium thiocyanate (KSCN), to measure the degree of sensitization (DOS). Microstructural examination was carried out by optical microscopy and scanning electron microscopy (SEM) after electrolytic etching. The results of the present investigation are discussed in the paper.

Enhancement of corrosion resistance of nickel based superalloys by laser surface melting

“…The DOS of the LSM specimens was found to be comparable to the solution annealed specimens, reported by us elsewhere. 25,27 Although the DOS of LSM specimens was found to be in the same range of the solution annealed specimens, I a was found to be very low for the LSM specimens as compared to the solution annealed specimens. The lower I a value for the LSM specimen was associated with a homogenised microstructure.…”

“…The ratio of I a and I f was found to be less for the LSM specimen (13?96) compared to the heat treated specimens, reported by us elsewhere. 27 The I a and I f values were also found to be low in the LSM specimen when compared to the solution annealed specimen. These results indicate that by laser surface melting treatment, all the pre-existing Cr carbide precipitates were dissolved, and the Cr depleted zones disappeared, making a homogenous solid solution.…”

“…27 The optical and SEM images of the as received specimens showed Ti and Cr rich precipitates, 25,26 while a homogenised microstructure with some traces of precipitates was noticed for the solution annealed specimens. 25,27 For the LSM specimens, the micrographs showed homogeneous cellular microstructures with all precipitates dissolved in the solid solution matrix. The improvement in corrosion behaviour of LSM specimens as compared to that of the as received specimens could be attributed to the elimination of microstructural inhomogenities, such as precipitates, inclusions, impurity segregation and the chromium depleted zone along the grain boundaries.…”

Thermal Barrier Coatings Overview: Design, Manufacturing, and Applications in High-Temperature Industries

“…The melter pots used in the waste vitrification process experience high-temperature corrosive environments of approximately 1273 K and must be protected from potential material failure during this process. These melter pots often use nickel-based alloys that show naturally good mechanical properties and resistance to corrosion and stress corrosion cracking . However, the operation lifetimes of these alloys could be increased via diffusion barrier coatings.…”

Magnetic and Electrochemical Techniques as Tools to Detect Deleterious Phases in Duplex Stainless Steels

“…[1][2][3][4][5][6][7][8][9][10][11][12] Later on, several groups adapted this technique to other materials and extended its scope of identification. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] The use of the DL-EPR technique has been proposed for duplex stainless steels (DSS) to detect several phases, such as sigma, chi, and others. However, the complexity of this class of stainless steel can be easily seen in the complete lack of consensus on the proper parameters for the operation of the DL-EPR tests, such as electrolyte composition and scan rate.…”