This article discusses the use of thin films for anti-corrosion applications and the most effective anti-corrosion thin films available.
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The Importance of Corrosion Resistance in Metals
Corrosion refers to the degradation of a metal due to electrochemical/chemical reactions with the environment, which adversely affects the durability and strength of the metal and shortens its lifespan.
Improving the corrosion resistance using anti-corrosion coatings can increase the lifespan and better the performance of metals. Coatings act as a sacrificial material to the metal surface, preventing corrosion of the metal.
Anti-corrosion coatings can also improve the efficiency of metals, enable the creation of new metal surfaces with enhanced functional properties and features, and reduce replacement and maintenance costs.
Anti-Corrosion Thin Films for Corrosion Resistance
In the last several decades, thin films have been investigated extensively for corrosion protection in metals. Chromate-free anti-corrosion thin films were introduced in the late 1990s to improve the metallic component corrosion resistance.
These coatings were formulated by focusing on the statistical design and functionality of polymers to augment the film performance and ensure better uniformity of crosslinking during curing.
The sol-gel technique for thin film production received more attention than others due to the economically viable production of sol-gel thin film coatings for corrosion resistance. Additionally, the structure, thickness, and texture of thin film coatings can be tailored using the sol-gel method.
The addition of nanoparticles in hybrid sol-gel coatings can improve the corrosion protection properties of these coatings by reducing the cracking potential, increasing thickness, and lowering porosity.
In a study published in the journal Ceramics International, researchers used the sol-gel method to prepare silica-alumina, silica-titania, and zirconia anti-corrosion thin film coatings for corrosion protection of 316L stainless steels.
The coatings were deposited on the 316L stainless steel substrates using dip-coating. The coated anti-corrosion thin films were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction.
Corrosion tests were performed to evaluate the performance of the synthesized anti-corrosion thin films in 15% deaerated sulfuric acid and 3% sodium chloride solutions using a potentiodynamic polarization technique.
The corrosion potential, corrosion rate, and polarization resistance were determined by extrapolating the Tafel lines. The results demonstrated that coated samples more effectively prevented corrosion compared to uncoated samples in both aqueous and basic media at room temperature.
Additionally, the corrosion rate of the anti-corrosion thin film coatings was ten times smaller compared to the corrosion rate of the 316L substrate. The films reduced the critical current density for passivation, which increased the ease of passivation and decreased the corrosion current density.
Silica-titania and silica-alumina thin film coatings were more effective for corrosion resistance compared to zirconia when the coated 316L steel substrate was exposed to a 15% sulfuric acid solution.
The polarization resistance of the coated samples was nine times higher compared to the uncoated samples. Moreover, the overall lifespan of the substrates was increased by ten times in the 3% sodium chloride solution and by five times in the 15% sulfuric acid solution due to the application of anti-corrosion thin film coatings.
Thin film application techniques can also influence the effectiveness of anti-corrosion thin film coatings. In another study published in the Journal de Physique IV, researchers investigated the feasibility of using the atomic layer deposition method to synthesize anti-corrosion thin film.
Thin films of alumina, titania, tantalum oxide, and alumina-titania multilayers were deposited in stainless steel substrates at 150-400 oC. Electrochemical impedance spectroscopy was employed to investigate the corrosion behavior of the coated samples in one and 0.1 mol/l hydrochloric acid and 3.5wt% sodium chloride solutions.
The findings demonstrated that alumina-coated samples protected the substrate for a limited duration in sodium chloride solution. Titania provided better corrosion protection when combined with alumina in a multilayer structure. In the hydrochloride solution, the tantalum oxide films prevented the corrosion in the substrate to a limited extent.
Recent Studies on Anti-Corrosion Thin Films
In a more recent study published in the journal Surface and Coatings Technology, researchers applied copper/diamond-like carbon (Cu/DLC) composite thin films on mild steel using magnetron sputtering in a mixed atmosphere of argon and methane and investigated their anti-corrosion and mechanical performance in terms of argon/methane and Cu/C ratios.
The Cu/DLC thin films formed through magnetron sputtering demonstrated excellent anti-corrosion performance. Raman spectra of the films displayed typical D and G band features, which indicated the DLC phase formation. The Cu/DLC anti-corrosion thin films with higher Cu content showed a higher degree of sp2 carbon clustering and lower diamond-like sp3 bonding.
Additionally, the Cu/DLC thin film internal stress values were reduced with the increasing Cu/C ratio. Although the addition of CU to DLC increased the plastic hardness and the H3/E2 ratio of Cu/DLC composite anti-corrosion thin films, the optimum value was observed for films with an intermediate Cu concentration.
Effective anti-corrosion thin films have also been developed for metallic implants. In a study published in the journal Surface and Coatings Technology, researchers demonstrated that plasma polymerized hexamethyldisilazane (ppHMDSZ) thin films with specific density and thickness can promote corrosion resistance in metallic implants.
The plasma polymerization method provided an effective, simpler, and dry process, which allowed the synthesis of dense thin films with a few hundred-nanometer thickness. In the study, researchers showed that the anti-corrosion behavior was closely correlated to the density of thin films coated on the stainless steel.
They controlled the plasma deposition time to prepare dense thin films. The corrosion resistance of the coated stainless steel implant was improved significantly against Hank’s solution by depositing 140 nm thick ppHMDSZ thin films with 1.15 g/cm3 in density. The film effectively blocked the charge transfer between the electrolyte and stainless steel surface, leading to better anti-corrosion performance.
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References and Further Reading
Khamseh, S., Alibakhshi, E., Mahdavian, M., Saeb, M. R., Vahabi, H., Kokanyan, N., Laheurte, P. (2018). Magnetron-sputtered copper/diamond-like carbon composite thin films with super anti-corrosion properties. Surface and Coatings Technology, 333, 148-157. https://doi.org/10.1016/j.surfcoat.2017.11.012
Ting, W., Chen, K., Wang, M. (2021). Dense and anti-corrosion thin films prepared by plasma polymerization of hexamethyldisilazane for applications in metallic implants. Surface and Coatings Technology, 410, 126932. https://doi.org/10.1016/j.surfcoat.2021.126932
Matero, R., Ritala, M., Leskelä, M., Salo, T., Aromaa, J., Forsén, O. (1999). Atomic layer deposited thin films for corrosion protection. Journal de Physique IV. https://www.researchgate.net/profile/Markku-Leskelae-2/publication/42364774_Atomic_layer_deposited_thin_films_for_corrosion_protection/links/54e1b11d0cf24d184b112459/Atomic-layer-deposited-thin-films-for-corrosion-protection.pdf
Atik, M., de Lima Neto, P., Avaca, L. A., Aegerter, M. A. (1995). Sol-gel thin films for corrosion protection. Ceramics International, 21(6), 403-406. https://doi.org/10.1016/0272-8842(95)94466-N
Zaferani, S. H. (2016). The Science of Anti-Corrosion Thin Films. Corrosionpedia. https://www.corrosionpedia.com/2/5354/coatings-and-lining/the-science-of-anti-corrosion-thin-films#:~:text=Since%20the%20early%201900s%2C%20thin,many%20types%20of%20industry%20applications.&text=Nanocoatings%20are%20defined%20as%20the%20nanostructures%20with%20thickness%20in%20the%20nanometer%20scale.
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