Self-Healing Conversion Coating Of Microcapsules Containing Inhibitor

Magnesium alloy is the lightest structural metallic material with many excellent properties,but they are liable to suffer severe corrosion in the field of aerospace and transportation during service,which severely limits their potential application in industrial practice.Chemical conversion coating is one of the easiest ways to enhance the corrosion resistance of magnesium alloys.Chromate conversion coatings for magnesium alloys are applied to provide corrosion protection.Due to the toxicity of Cr⁶⁺,numerous measures have been made to develop environmental alternative corrosion inhibitors.Thereinto,the most promising alternatives is the rare earth conversion coating.But localized corrosion always occurs on the RE-based conversion coating in a corrosive environment.With additional 0.2-0.6wt.%of rare earth,the anti-corrosion effect of RE-based conversion coatings improves,however,excess rare earth ions lead to a porous structure.Under the condition of keeping the integrity of coating,adding as many film-forming substances as possible is the key to enhance corrosion resistance of rare earth conversion coating.The aim of this paper is to present a new carrier to rare earth conversion coatings,that is,microcapsules,which can effectively avoid this porous state by releasing active and repairing material immediately after changes in coating integrity.The microcapsules also release inhibitors to the target area at a certain rate under ideal conditions.The main function of the conversion coating with microcapsules is one of a self-healing nature.In present work,gelatin-chitosan microcapsules containing inhibitor?La?NO₃?₃ or Ce?NO₃?₃?were prepared by complex coacervation,the cerium conversion coatings with microcapsules were deposited on magnesium alloy by electro-brush plating.The preparation process of self-healing conversion coating was optimized through orthogonal experiments.The coating formation mechanism was put forward by means of X-ray diffraction?XRD?,fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?,demonstrating coating rate and bonding position of microcapsules.The corrosion resistance,microcapsules distribution and breakage of conversion coating were carried out by means of electrochemical workstation,scan electron microscopy?SEM?,auger electron spectroscopy?AES?,electro-probe micro analyzer?EPMA?and weight increase method of scratches.The failure or protection mechanisms and self-healing mechanism of coatings were further clarified.The process and optimize parameters to prepare self-healing conversion coating are determined by the orthogonal experiments.The results are:electro-brush plating?10V,10min?,corrosion inhibitor of La?NO₃?₃,microcapsules amount of 20wt.%,microcapsules coating quantity of 4g.The self-healing conversion coating formation mechanism was put forward:coating is composed of amorphous phase by XRD analysis;the bonding position of cerium ions and gelatin-chitosan microcapsules may be on oxygen of the carboxyl and nitrogen of acylation groups by FTIR analysis.The XPS results demonstrated that coating is mainly composed of CeO_x and La³⁺ from microcapsules.In the formation process of conversion coating,the valence of cerium ions transforms from Ce³⁺ into Ce⁴⁺?CeO₂?.The self-healing mechanism of conversion coating is as follows:in corrosion environment,the coating is damaged and microcapsules migrate into the damaged area,triggering release of inhibitors,supplementary to the conversion coating.The cerium conversion coating with microcapsules shows a much higher corrosive resistance than the coating without microcapsules,and demonstrates the achievement of a self-healing performance.