(Invited) Advanced Coating with Self-Healing Property By Healing Agent in Pore of Porous Film Formed on Al Alloy
Authors: Chiba Makoto , Tomioka Yumino , Hirasawa Kota , Tsuji Yuki , Yanagimoto Haruno , Hyono Atsushi , Takahashi Hideaki 
Published: December 2020

Al and Al alloys are used for many purposes, e.g. automobiles, heat exchanger, electronics, because of their good properties such as lightness, high heat conductivity, good processability, hardness, and so on. However, corrosion protection of these materials are not so high, corrosion of Al, such as pitting corrosion and atmospheric corrosion, will occur during operation. From this, some kinds of surface treatments are needed for improvement of corrosion protection of Al used for long-term. A polymeric coating is one of popular technique to improve the corrosion protection of substrate metal. However, corrosion protection of substrate covered by polymeric coating will easily lose by mechanical damage of the coating, leading to exposure of substrate metal to surroundings and to the local corrosion of substrate under damaged area of coating.
From above, new surface treatment techniques for Al substrate to keep high corrosion protection, even the surface was damaged, are developed in our research group. This is a coating, with self-healing property by dispersion of micro-capsules containing the healing agent of coating. When the coating is damaged, the healing agent flow out to the damaged area of coating and react with water vapor to form the self-healing structure and this can be covered to the damaged area of coating. However, the procedure of synthesis of micro-capsule containing healing agent is too complex and yield of this is too low to synthesize the capsules with low cost.
From above, we start to develop the new type of self-healing coating. This coating can be formed on Al alloy anodized. As you know by anodic oxidation of Al alloy, electrochemically in some kind of acidic solution, porous type of anodic oxide film can be formed on Al alloy, which have many small pores arranged regularity. Pores of porous film are used as a container of healing agent of coating. Firstly, the pores in porous film formed on Al alloy were filled by healing agent. And then, this sample was covered by polyurethane coating. If this coating was damaged, mechanically, healing agent contained in pores of porous film, flow out to the damaged area and reaction with water to form polyurea as a self-healing structure and cover to the exposed Al substrate. In this study, healing property of this type of self-healing coating is investigated by observation of the surface of specimen with scratchin.
#1050 Al alloy (size : 20 mm × 20 mm × 1.5 mmt) were electro-polished in CH3COOH / HClO4solution. And then anodic oxide films were formed on pretreated specimens by anodizing in (COOH)2 solution with a constant current density of 200 Am-2 for 1 hr. The anodized specimens immersed in IPDI for 100 min with ultrasonic bath, in order to promote that IPDI soln. to penetrate to pores of the porous film. After wiping the surface of the specimen by soft tissue, then a mixed solution of ethylene glycol and prepolymer of polyurethane coating was purged to specimen surface and leave for 24 h to form the polyurethane coating on anodized Al alloy surface. The prepolymer was synthesized by using the mixture of TDI, glycerol and cyclohexanone. Mixture was agitated under 600 rpm of stirring rate for 24 h. In order to remove the water dissolved in this mixture, the first 1 h of agitation, the mixture was bubbled by passing pure N2 gas. During the agitation, TDI reacts with glycerol to form the precursor of polyurethane.
After specimen damaged by scratching with 7 N of load and ageing, corrosion protection of damaged Al alloy specimens with polyurethane coating were evaluated by immersion tests. The coated specimens were immersed to CuSO4 / KCl solution kept at room temperature for 1 day. After the immersion tests, to remove the coating, the specimens were immersed to the coating remover solution. And then, specimens were immersed to H3PO4 / K2CrO4 solution. Surfaces after immersion tests and removing of surface layer were observed by scanning electron microscopy (SEM).
From the SEM observation for damaged specimen covered with porous film and normal coating, as shown in Fig. 1-a, sharp and deep scar can be seen at the canter of image of specimen surface. The scar can be seen on the image of damaged specimen with self-healing coating on anodized specimen, however, unique structure also can be observed at the bottom of scar, shown in Fig .1-b. This structure may be formed by the reaction between water in air and IPDI contained in the pore of porous film on Al alloy, thus, this coating has a self-healing property.

Figure 1