In Situ High-Temperature TEM Observation of Inconel Corrosion by Molten Chloride Salts with N₂, O₂, or H₂O

Abstract

In situ TEM is used to understand corrosion of Inconel-625 by chloride salts (MgCl2 − NaCl − KCl) at 500-800°C in 1.0 atm inert N2 or pure O2, or hydrated salts. The isothermal corrosion rate R in inert N2 increases from 203 ± 30 μm year-1 at 700°C to 463 ± 30 μm year-1 at 800°C. An oxygen ambient causes a six-fold increase to R = 1261 ± 170 μm year-1. Salt hydration dramatically accelerates corrosion to R > 3×105 μm year-. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600°C. To reduce the rate of corrosion it is important to both avoid incorporation of H2O into the system and ensure proper flushing before increasing the temperature beyond 600°C. In addition, steps should be taken to tailor chloride melt compositions that has low solubility for HCl and O2. All of our corroded samples exhibit passive-protective oxide layers of Cr, Mg, and Ni. In addition, distinct volatile compounds are detected. To minimize corrosion formation of volatile by-products should be avoided.

Intergranular Attack of Low Carbon Steel in Molten Aluminum Chloride

“…[63][64][65][66][67][68] Further research has identified through microstructure observations the role of oxygen and moisture in drastically increasing corrosion. 69,70 Additionally, the presence of secondary phases and carbides at the grain boundaries is able to induce fast propagation of localized corrosion by forming microgalvanopores that exhibit a difference in electronegativity. 54,55,[63][64][65][66] These mechanistic explanations can find potential application in the present corrosion phenomenon.…”