The　under-deposit　corrosion　behavior　of　Ni-based　alloys　625,　825　and　Fe-based　alloy　800　in　supercritical　water　(SCW)　at　400　°C　and　500　°C　was　investigated.　Newly　designed　experimental　apparatus　was　used　to　ensure　that　the　surface　of　alloy　specimens　was　covered　by　NaCl　or　KCl　deposit　layers　during　the　corrosion　test　in　SCW　environment.　Based　on　the　cross-sectional　observation　and　surface　chemical　analyses,　the　oxide　film　on　the　samples　after　exposure　contains　an　outer　layer　containing　Fe-Ni-Cr　spinel　and　Fe3O4,　and　an　inner　layer　of　Cr2O3.　NiCl2　was　also　found　on　the　alloy　surface.　At　400　°C,　hydrolysis　of　the　metallic　ions　occurred,　forming　HCl　with　chloride　ions.　At　500　°C,　the　oxides　react　with　the　solid-phase　salt　deposit　via　the　formation　and　transformation　of　the　HCl　vapor.　The　salt　deposit　layer　acted　as　a　barrier　for　the　diffusion　of　SCW　and　HCl　vapor　at　500　°C.　©　2019　Elsevier　B.V.