In Situ High-Temperature TEM Observation of Inconel Corrosion by Molten Chloride Salts with N-2, O-2, or H2O

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 degrees C-800 degrees C in 1.0 atm inert N-2 or pure O-2, or by salts which are controllably hydrated in a high vacuum chamber. The isothermal corrosion rate R in inert N-2 increases from 203 +/- 30 mu m year(-1) at 700 degrees C to 463 +/- 30 mu m year(-1) at 800 degrees C. An oxygen ambient causes a six-fold increase to R = 1261 +/- 170 mu m year(-1) at 700 degrees C. Salt hydration dramatically accelerates corrosion to R> 3 x 10(5) mu m year(-1) at 700 degrees C while it leads to a more moderate R = 95 +/- 20 and 486 +/- 30 mu m year(-1) at 500 degrees C and 600 degrees C, respectively. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600 degrees 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 degrees C. Compositional analysis of the corroded cells indicate that corrosion in O-2 ambient is dominated by oxidation of metals by O-2 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 O-2. 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)(2)MoO4 and CrO2(OH)(2) are detected in N-2, H2O, and O-2 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. (C) 2022 The Electrochemical Society (ECS). Published on behalf of ECS by IOP Publishing Limited.

Automated summary

In situ transmission electron microscopy (TEM) techniques were used to study the corrosion of Inconel-625 alloy under different conditions. The results showed that the corrosion rate increased significantly at high temperatures and in the presence of oxygen. Salt hydration also accelerated the corrosion process. The researchers suggested that adjusting the composition of chloride melts could reduce the corrosion rate.