Electrospun fiber as corrosion resistant coating

An interesting use of electrospun fibers is in anti-corrosion protective coating. Electrospun membrane from hydrophobic polymer is known to exhibit greater hydrophobicity than the same material in the form of film. Since the presence of water generally increases the rate of corrosion of metal, an electrospun hydrophobic layer on the metal surface will help to protect the underlying metal substrate from corrosion. Iribarren et al (2019) demonstrated the protective benefit of electrospun polyvinyl chloride (PVC) with nanoparticles of a corrosion inhibitor like ZnO on aluminum alloy 6061T6. Aluminum with electrospun PVC coating with and without ZnO nanoparticles showed reduction in corrosion current density by two orders of magnitude. Heat treatment was carried out to improve adhesion of electrospun coating to the metal substrate. Electrospun coatings with and without heat treatment showed a minimum water contact angle of 120° (at heat treatment temperature of 120°C) with higher heat treatment temperature lead to a corresponding reduction in its water contact angle. The reduction in water contact angle with increasing heat treatment temperature may be due to increased fusion between fibers and lowers the surface roughness. Interestingly, electrospun pure PVC fibers heated to 80°C (Tg) showed protection efficiency of 99.01%. However, the same temperature treatment on PVC/ZnO has a protection efficiency of 95.12%. While increasing the heat treatment temperature of electrospun pure PVC fibers to 100°C reduces the protection to 97.38%, the same heat treatment on PVC/ZnO increases the protection to 99.19%. The reduction of protection in the heat treated pure PVC fibers may be attributed to reduction in its water contact angle. In the presence of ZnO nanoparticles, the same molecular relaxation may improve the distribution of ZnO nanoparticles within the PVC matrix, hence increasing the level of protection.

SEM images of the electrospun PVC-ZnO nanocomposite fiber at different scale bar of 10 µm (a) and 20 µm (b), respectively [Iribarren et al 2019].

Vicente et al (2021) demonstrated that electrospun poly(vinylidene fluoride) (PVDF) coating across different average fiber diameters on bare aluminum substrate (AA6061T6) was able to reduce the corrosion current density and the corrosion rate of the aluminum substrate in three orders of magnitude. They also found that smaller fiber diameter instead of coating thickness was able to reduce corrosion rate. Smaller fiber diameter may increase packing density of the fibers and this may reduce possible migration of water vapor or contact with liquid water to the surface of the substrate hence reducing corrosion rate. Lesser number of beads was also found to improve corrosion resistance and this too may be attributed to greater packing density of fibers. As for coating thickness, greater fiber diameter will increase coating thickness at a faster rate but the packing density will be lower than coating of smaller fiber diameter.

Unlike titanium, magnesium alloys will degrade in physiological condition due to the presence of chloride ions which leads to the formation of magnesium chloride. Without any protection, this will lead to a rapid corrosion and degradation of the magnesium alloy. Electrospinning a layer of polycaprolactone fibers on the magnesium alloy has been shown to significantly reduce its degradation rate by up to five times lower than unprotected samples after 7 days immersion in simulated body fluid [Soujanya et al 2014]. Water contact angle test showed that magnesium alloy is hydrophilic while the alloy sheet with the polycaprolactone coating is hydrophobic. It is likely that the hydrophobicity of the fiber coating prevented or reduces the contact between the fluid and the alloy surface and therefore reduces the corrosion rate. Another consideration for increasing corrosion protection using electrospun fiber is the adhesion between the fibers and the base metal. Dabirian et al (2022) tested the corrosion control of magnesium alloy (AZ31 alloy using electrospun polycaprolactone(PCL)-curcumin(Cur) nanofiber coatings. Comparison was made with pure electrospun PCL nanofibers and sodium alginate (SA)-polyvinyl alcohol (PVA)/PCL nanofibers coating and bare metal as negative control. The SA-PVA/PCL nanofibers coating was formed by having SA-PVA nanofiber as an undercoat followed by an outer PCL nanofiber layer. Of the three electrospun fibers, pure PCL is the most hydrophobic. This is followed by the PCL-Cur layer and least hydrophobic is the PCL/Cur/PVA layer. While a more hydrophobic layer is better able to prevent water contact with the metal surface, the adhesion is also poorer. All three electrospun nanofibers coated metal showed less corrosion compared to bare metal. Of the three coatings, PCL-Cur nanofiber coating offered the best protection. The SA-PVA/PCL layer has the best adhesion with the metal but the hydrophilic SA-PVA undercoat layer degrades faster and results in a loss of protection. PCL-Cur provided the best balance between adhesion and hydrophobicity and hence showed the best corrosion protection for the Mg alloy. However, based on corrosion protection alone, a PCL dip coating on Mg which fully cover the surface of the metal offered better corrosion resistance than electrospun nanofibrous coating with corrosion rate at 0.3 ng per second for the former and 0.9 ng per second for the latter [Lee et al 2014]. Since electrospun nanofibrous coating is porous, water may still come into contact with the surface of Mg and this will certainly initiate corrosion while the same material completely covered by PCL will only start to corrode after PCL dissolved. For an implant, corrosion resistance is not the only consideration. More tests need to be carried out to determine the effect on cell biocompatibility on the fiber coated magnesium alloy and its impact on corrosion rate.