Inhibition ability of gluconates for fresh water corrosion of mild steel changed with metal cations were investigated by immersion tests and electrochemical impedance spectroscopy (EIS) tests. The results of the tests showed that the inhibition ability of gluconates was enhanced by metal cations that have large "corrosion inhibitory effect of cation, Y", in model fresh water. Surface analysis showed that gluconate ligands and metal cations that have large Y can bond on mild steel and that they form a protective film on the steel. EIS analysis with an equivalent circuit suggested that metal cations that have large Y would reduce the area of defects in the protective film of gluconates. (C) 2017 The Electrochemical Society. All rights reserved.