A Pre-Treatment Coating with Redox-Responsive Self-Healing Function for AZ31B Magnesium Alloy
Authors: Ding Chendi , Fu Jiajun 
Published: February 2020

Benefiting the major advantages such as high strength to weight ratio and good formability, magnesium alloy, as a significant light alloy, has shown great potential in modern machine manufacturing field. Unfortunately, limited by their poor corrosion resistance and fast degradation rate, magnesium alloy materials face predicament in stability and durability. Considering that the degradation rate of magnesium alloy is very quick, the repairing speed should also be fast enough to overtake the corrosion process. Hence, sensitive, rapid responsiveness and fast repair process is necessary for magnesium alloy protection.
Electrochemical potential was chosen as indicator to monitor the corrosion process: It decreases as soon as the corrosion took place, and it only declined with corrosion reaction. This character guarantees the sensitivity of self-healing coating based on electrochemical potential mechanism to sense the very beginning of corrosion reaction.
Disulfide bond unit was frequently introduced into controlled release systems for redox-responsiveness due to that they could easily be reduced and cleaved into thiol groups. Inside human body, there exists abundant glutathione (GSH) in the cytosol. This reducing substance could realize rapid cleavage of disulfide bonds. According to previous literatures, the half-cell potential of GSSG/GSH was -0.26 V, while for magnesium alloy, their corrosion potential is much lower. Therefore, it is expected that the redox reactions during corrosion of magnesium alloy would have the same effect in cleaving disulfide bond as GSH. Following this biomimetic idea, corrosion environment combined with disulfide could realize redox-responsive self-healing agent release, composing the most dependable self-healing mechanism.
Herein, we report a sensitive, intelligent anticorrosion sol-gel pre-treatment coating based on redox-responsive motif. As key component, self-healing agent, the inhibitor molecules benzotriazole (BTA) was located inside mesoporous silica materials, sealed by supramolecular complex gate. Composed by novel macrocyclic molecules, water soluble pillar[5]arenes (WP5) and pyridine cation derivative molecules (G), the formation of supramolecular complex is driven by electrostatic force and hydrophobic function. Disulfide bonds were used as linker connecting surface of mesoporous silica nanospheres and the “gate” molecules. To shorten the travel path of inhibitor molecules, we added Fe3O4 nanoparticles inside mesoporous silica layer to fabricate magnetic intelligent nanocontainers. To our knowledge, the magnetic properties have been widely used for controlling the location of materials. Therefore, under the effect of external magnetic field, the magnetic nanocontainer with BTA wrapped inside could be enriched to coating-metal interface. Through shortening the travel path, the inhibitor molecules could reach the corroded areas in minimal time, further accelerating repairing speed. Through evaluations by in-situ scanning kelvin probe technique (SKP), the reported coating exhibit the rapid self-healing functionality, give the timely feedback and maintain the reliable long-term protection of magnesium alloy. 15 μL of 0.05 M NaCl electrolyte was covered on artificial defect (deep into the alloy layer, simulating the coating deterioration at the beginning of corrosion) and NiCr SKP probe was positioned 100μm above the sample surface. Through non-destructive, in-situ measurement of electrode potential at metal surface, a function of the potential over time was recorded (Figure 1B). At the beginning of immersion, the potential at defect site remains level, at the corrosion potential of AZ31B (-1.5 V), indicating that the contact between metal and electrolyte leads to active corrosion in all the samples. After four hours of immersion, the corrosion potential of doped sample rockets to -1.05 V, entering into the passivation range, while for undoped sample, it did not display self-healing function.
This is the first time that precise control over location of repair factor inside coatings is achieved.

Figure 1