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

Abstract

In situ transmission electron microscopy (TEM) diffraction and imaging techniques are used to monitor and quantify corrosion of Inconel-625 by pure molten chloride salts (MgCl2 − NaCl − KCl) at 500 °C–800 °C in 1.0 atm inert N2 or pure O2, or by salts which are controllably hydrated in a high vacuum chamber. 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 at 700 °C. Salt hydration dramatically accelerates corrosion to R> 3 × 105 μm year−1 at 700 °C while it leads to a more moderate R = 95 ± 20 and 486 ± 30 μm year−1 at 500 °C and 600 °C, respectively. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600 °C, where corrosion is aggravated by increased generation and solubility of corrosive HCl gases. Hence, to reduce rate of corrosion it is important to both avoid incorporation of H2O into the system at each stage and ensure proper flushing of the system before increasing the temperature beyond 600 °C. Compositional analysis of the corroded cells indicate that corrosion in O2 ambient is dominated by oxidation of metals by O2 gas dissolved in the chloride melt, but corrosion in H2O ambients is caused by chlorination of metals by dissolved HCl gas and MgOH+ ions. So, to reduce rate of corrosion, 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 of Ni, Mo and Cr involving NiCl2, (Na,K)2MoO4 and CrO2(OH)2 are detected in N2, H2O, and O2 ambients, respectively. We believe that corrosion acceleration can be minimized by minimizing formation of volatile by-products or promoting reactions that could convert these volatile compounds to solid phases, as these volatile compounds led to destruction of protective oxide layers.