Carbon or low alloy steel tubes in steam generators (or boilers) are potentially vulnerable to under deposit corrosion (UDC), arising from the formation of porous magnetite deposits on the waterside heat-transfer surfaces. Beneath these deposits, “wick-boiling” causes a concentration of contaminants (such as chlorides), which eventually leads to rapid corrosion. In this work, the corrosion of carbon steel has been investigated in hot acid chloride solutions that simulate the concentrated local environments formed during UDC. Trisodium phosphate (TSP) is sometimes dosed into boilers for pH control. This work has shown that TSP addition such that the phosphate concentration equals the chloride concentration dramatically reduces the corrosion rate in these simulated environments from >20 mm/y to <0.1 mm/y. Additionally, a model of wick-boiling beneath deposits has been used to analyze the concentration of chlorides and phosphates during the initiation stages of UDC, suggesting that dosing of only 100 ppb of TSP into bulk boiler water should be sufficient to increase the critical deposit thickness (required for UDC) by >100 µm across a wide range of operational scenarios.