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The corrosion mechanism of type 316 SS wire was investigated with and without sol−gel-applied TiO2 coatings on exposure to HCl under SCWO conditions. The SCWO experiments were carried out in a semi-batch, continuous-flow tube reactor with 5 vol % air and feed streams containing 0.1, 0.2, or 0.5 N HCl. The corrosion was characterized using mass loss, scanning electron microscopy, and energy-dispersive X-ray analysis. Inductively coupled plasma atomic emission spectroscopy was used to analyze the reactor effluent. Sols were prepared from titanium isopropoxide and applied by a dip coating process. The thermal and phase transition behavior of the powders obtained from the sols was characterized using simultaneous TGA/DTA, wide-angle XRD, and micro-Raman spectroscopy. A pitting corrosion mechanism in which the HCl acts as both a catalyst and a reactant was proposed. The results obtained with the TiO2-coated wire suggest that the applied coatings were permeable to super- and subcritical water.