Hexamethylene diisocyanate (HDI) biuret microcapsules were synthesized by modified in situ polymerization and used as self-healing additives in alkyd varnish coatings (AVCs). The water triggered self-healing ability of AVCs with artificial defects was evaluated through 3D laser microscopy, FT-IR spectroscopy, and electrochemical analyses. The FT-IR spectra confirmed that HDI biuret successfully sealed off the artificial defects. Self-healing was divided based on the electrochemical impedance spectroscopy into four stages: pre-self-healing, accelerated self-healing, self-healing-failure competition, and failure. This work investigated the self-healing activity of AVCs with synthetic HDI biuret microcapsules in protecting steel surfaces. (C) 2017 The Electrochemical Society. All rights reserved.