Corrosion Protection of Substrate Metal by Formation of Self-Healing Coating
Authors: Chiba Makoto , Takada Rin 
Published: December 2024

Organic coating is one of the most popular and powerful technique for the corrosion protection of substrate metal. However, the coated layer is damaged, the corrosion protection by the formation of coating will lose soon. Therefore, self-healing coating are strongly demanded from many industrial fields, such as automobile industry. Some kinds of self-healing coatings healed by some mechanisms, developed in our group. In this study, we focused on a following coating. This is electrodeposition coating dispersed with many micro-capsules containing in healing agent of coating. When this coating was damaged, capsules dispersed in this coating also break, simultaneously. And healing agent in the capsules is released to the damaged area of coating. Moreover, the healing agent reacts with moisture in the air to form the self-healing structure and the damaged area of coating is covered by this self-healing structure, without any treatment. In this study, corrosion protection of damaged Al materials with self-healing coating by electrochemical impedance spectroscopy. And self-healing property of this coating was also evaluated by the three-dimensional optical microscope observation, of the shape of defects formed by using the micro-Vickers indenter.
AA1050 plate after electropolishing is employed as a specimen. Electrodeposited coating was formed on pretreated specimen surface in the commercialized electrodeposited coating solution with a constant potential of 30 V for 30 min. Prepolymer solution as a precursor of shell of micro-capsule, was synthesized under these conditions. Isophorone-Diisocyanate and Xylene were added to this solution and dripped to sodium dodecyl sulfate and glycerol solution under these conditions to form the capsules containing healing agent. Then these capsules dispersed in acetone was dripped to the coated surface and specimen was aged for 24 h. After aging, electrodeposited coating was formed for 30 min again, to form the self-healing electrodeposited coating. After formation of shape defined indent formed in the specimen surface by using equipment for micro-Vickers indentation test, and aging for 24 h, the surface of the specimen was then observed by three-dimensional optical microscope.
Corrosion protection of damaged specimens with self-healing coating was also evaluated by EIS measurement in boric/borate buffer solution (pH 8.4) after bubbling N2 gas for 20 min. Pt-mesh and Ag/sat-AgCl. electrodes were used as counter and reference electrodes, respectively. Potentials of 50 mV to a rest potential of -550 mV (vs. R.E.) were applied in the range of frequency between 10 mHz and 100 kHz.
From the optical images of defect formed by micro-Vickers indenter with 20 N of load, on the surface of specimen covered with electrodeposition coating without and with micro-capsules. The defect formed on normal coating is sharp diamond shape with 270 µm of diagonal length. The volume of this defect evaluated by 3D-optical microscope, is about 900,000 µm3. In contrast to this, the defect formed on self-healing coating is dull diamond shape with rounded edge. And the length of diagonal of this defect is about 220 µm and the volume of this defect is about 450,000 µm3. From these results, the volume of defect on self-healing coating is almost half of that on normal coating. Thus, the self-healing property of this coating can be estimated to approximately 50% under these conditions. And corrosion protection of the corrosion protection of damaged specimens with self-healing coating evaluated by EIS is much higher than that with normal coating.