Abstract
During the process of supercritical water oxidation (SCWO), organic chemical streams are oxidized at temperatures and pressures in excess of 647 K and 221 bar. Due to high operating temperatures and pressures, severely corrosive environments often ensue and eventually lead to SCWO reactor tube failures. This case study looked at one such failure of Alloy 625 (61Ni-21.5Cr-9Mo [UNS N06625]) tubing, which occurred at the UBC SCWO pilot plant while treating a feed of waste-water containing ∼0.1 m 2,4 dinitrophenolate (C6H4N2O5), ∼0.25 m ammonium sulfate ([NH4]2 SO4), and ∼1.3 m free ammonia (NH3). The feed pH was approximately 10 and therefore not expected to be corrosive; in fact, the tube failed when exposed to this feed (with oxygen) for a period of about 1 h at two sections subjected to temperatures in the 650 K to 655 K range. Through a detailed thermodynamic examination of the Ni-H2O-NH3 system at 373, 473, 573, and 653 K, as well as scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis of the ruptured portions of tubing, it was found that the addition of ammonia and sulfate lead to solution acidification and NiO instability, which resulted in little protection for the alloy when subjected to the oxidative high-temperature medium. The alloy ultimately failed by transpassive dissolution.
Original language | British English |
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Pages (from-to) | 301-314 |
Number of pages | 14 |
Journal | Corrosion |
Volume | 64 |
Issue number | 4 |
DOIs | |
State | Published - Apr 2008 |
Keywords
- Ammonium
- High temperature
- Nickel
- Potential-pH diagram
- Pressure
- Sulfuric acid
- Thermodynamics