Long-term corrosion testing of a mock-up dissolver vessel to be employed in the aqueous reprocessing of spent nuclear fuels of fast breeder reactors has been initiated. In this work, a Zircaloy-4 (Zr-4) mock-up dissolver vessel was used as the testing facility to evaluate the corrosion rate of several candidate materials based on zirconium and titanium in the boiling and vapor phases of simulated dissolver solution (SDS) comprising fission and corrosion product ions in 11.5 M nitric acid. Several campaigns of 100, 250, 500, 1000, and 2500 h of operation were completed. The corrosion rates of the candidate materials are expressed both in micrometers per year (μm/yr) and mils per year (mils/yr). Zirconium-702, Zr-4, autoclaved Zr-4, and commercial pure titanium (CP-Ti) exhibited low corrosion rates of 0.08 to 0.23 μm/yr (0.003 to 0.009 mils/yr) in the as-received and welded conditions exposed to the boiling liquid phase of the dissolver solution for 2500 h. Whereas the CP-Ti and CP-Ti weld exhibited marginally higher corrosion rates of 1.0 μm/yr (0.04 mils/yr) and 1.9 μm/yr (0.075 mils/yr), respectively, in the vapor phase of the dissolver solution, the lowest corrosion rate of 0.08 μm/yr (0.003 mils/yr) was obtained for the autoclaved Zr-4 sample exposed to boiling SDS. Scanning electron microscope investigations did not reveal any corrosion attack for the titanium and zirconium samples. Laser Raman spectroscopic analysis confirmed that the origins of passivity of zirconium and titanium samples were due to the formation of ZrO2 and TiO2, respectively. However, the CP-Ti/AISI Type 304L stainless steel (SS 304L) and Zr-4/SS 304L dissimilar welds had undergone severe corrosion. Visual inspection of the Zr-4 dissolver vessel revealed no corrosion attack after operation for 2500 h. The results of this 2500-h campaign would serve as the baseline data for the analysis of future long-term campaigns.