A theoretical study of factors affecting corrosion in supercritical water reaction vessels

This paper reports a molecular dynamics study of the pKW of water, pKa of HCl and pKb of NaOH in supercritical water at a pressure of 1000 atm. The extent of acid/base dissociation is known to be an important factor in the corrosion of supercritical water reaction vessels. This work describes classical molecular dynamics simulations using the SPC/e water model to gain insight into the controlling factors. The results indicate that the dissociation of water has a maximum at 448 K, whereas both the dissociation of HCl and NaOH become less favourable with increasing temperature due to a decrease in entropy. The simulation results are compared to values calculated from the generalised Born theory, and there is a qualitative agreement in the trend.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► At 1 katm, pKW shows an ususual trend, first falling, then increasing above 573 K. ► pKa of HCl continually increases with temperature at 1 katm. ► pKb of NaOH continually increases with temperature at 1 katm. ► Increasing trends are due to unfavourable entropy terms due to clustering. ► Generalised Born theory give similar qualitative trends.