1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP)-Zn complex thin films for the corrosion protection of cold-rolled steel (CRS)

Self-strengthening HEDP-Zn films were constructed on the CRS surface and the corrosion protection was afforded by electrochemical impedance spectroscopy (EIS) tests. After immersion in NaCl solution, HEDP-Zn-coated CRS underwent three stages: strengthening, balance and weakening. During immersion, a portion of unstable HEDP-Zn complexes were destroyed, accompanied by the intercalation and departure of Na+, thereby resulting in the changes in microstructure, chemical compositions and thickness of the films. Thus, anticorrosion performance increased due to the increased surface coverage by the complexes and additional binding sites during the first stage but decreased because of the reduced film thickness in the third stage.