Mercury corrosion of Si-deoxidized, low-carbon steel was studied in 5000-h tests using four natural circulation loops with once-through boilers operating at 593°C and with 111°C of superheat. The relative effects of very small quantities of Ti or Zr additions to the Hg and the effectiveness of steel pretreatment (for 500 h at 590–620°C with a liquid Hg-Zr solution), prior to contact with boiling Hg, were determined. A fourth loop, which had no additives nor loop pretreatment, was run simultaneously. With the exception of the Hg-Zr pretreated loop, maximum depth of corrosion did not exceed 50 µm and occurred near the superheater exit where the temperature was the highest. Boiler and condenser corrosion were less than 30 µm in these latter loops. Corrosion 300–1000 µm deep was found on the downstream side of the superheater of the Hg-Zr pretreated loop, and the severity of the attack was attributed to boiling instabilities, which caused liquid Hg to come into contact with the superheater walls. Adherent iron deposits were found in the boilers and cooler liquid regions of three of the loops, but no discernible iron deposits were found in the loop with Zr added. The total quantity of mass-transferred iron (deposits and particulate) was estimated to be approximately 0.2 gm in the Zr-added loop, 0.5 gm in the Ti-added loop, 1 gm in the loop with no additions and 2 gm in the loop pretreated with Hg-Zr solution.