Chapter 1: Corrosion Applications of Nanomaterials (NMs)

Nanomaterials (NMs) are defined as materials which are designed and produced with at least one dimension measuring smaller than 100 nm particle size.^1–7  In addition to their very small size, NMs exhibit superior qualities that are entirely different from their bulk counterparts, making them powerful candidates for a wide range of applications.^8–17  These qualities are high mechanical and thermal stability, large surface area and chemical reactivity, etc.

Coatings are extensively used for the effective protection of metals against corrosion.^18–20  However, because of the weak resistance of traditional coating materials against the penetration of solutions with corrosive features to the interface of the metal and coating, the long-term corrosion resistance effect of the coating material is gradually decreased. To overcome this drawback, recently, coatings were integrated with NMs to significantly improve their mechanical, optical and chemical features.^21–26  NM-based coatings either have constituents in the nano-scale or are made out of thin layers which have a size lower than 100 nm. NM-based coatings can be successfully employed to decrease the undesired effects of a corrosive medium because of various preferences including the hardness of the surface, adhesive quality, long haul and high-temperature corrosion opposition, etc. In addition, NM-based coatings can be effectively used with smoother thickness, which enables adaptability in the design and development of the metal equipment as well as a reduction of process expenses.

In this chapter, an overview of the recent progress in the corrosion applications of NMs is provided. It starts with the features of NMs, then the types of NMs are briefly presented. Finally, corrosion applications of NMs are demonstrated.

Among the number of great features of NMs,^27–34  being very hard is one of the most intriguing. Very hard nanocomposite materials can be successfully prepared by using borides, nitrides and carbides through plasma-induced chemical and physical vapor deposition techniques. In the suitably prepared binary systems, the hardness feature of the prepared nanocomposite material significantly exceeds that given by the rule of mixtures in bulk. The discovery of carbon nanotubes (CNTs) stimulated extensive studies due to their great mechanical features.^35–42  The strength of the carbon-based nanofibers enhances with the graphitization along the fiber axis. CNTs that are formed of seamless cylindrical graphene nanolayers represent the ideal carbon fiber material and have excellent mechanical properties among the carbon fiber species.

Rapid progress in the development of the mechanical properties of NMs is very important when we incorporate these NMs (i.e. CNTs, nanocrystals, nanoparticles, nanowires, etc.). When taking into account the detailed analysis of the mechanical properties of NMs, there are a number of factors that should be carefully considered including structure of the NMs surface, functionalization, porosity, synthesis techniques, chemical modifications, etc. These factors have crucial effects on the mechanical properties of the NMs.

In the literature, the unique superiorities of NMs including electrical,^43,44  optical^45–52  and magnetic features⁵³  have been researched in detail. On the other hand, the thermal features of NMs⁵⁴  were only recently given serious consideration. This is especially because of the drawbacks of experimentally measuring and screening the thermal transport in nano dimensions. For this purpose, an atomic force microscope (AFM) was proposed for the efficient measurement of the thermal transport of NMs with high resolution that enables a promising route to probe the thermal features of NMs.

Investigation of the electronic features of NMs is a crucial issue in the design and development of electronic tools and systems.⁵⁵  However, it is not very easy to understand the electrical conductivity of NMs because of their smaller dimension and based on their distinct mechanisms. There are different types of mechanisms such as Coulomb charging and tunnelling, surface scattering, widening and discrete band gap. In addition to these, the enhanced perfection (i.e. decreased impurity, structural defects and dislocations) may impact the electrical conductivity of NMs. NMs can also hold significantly higher energy than traditional materials due to their large grain boundary area.

NMs have also received increasing interest because of their unique optical features that significantly differ compared to bulk forms.^56,57  Various contributory parameters include quantum confinement of electrical carriers within NMs, effective charge and energy transfer throughout the distances in nano-dimensions. The non-linear and linear optical features of NMs can be successfully tailored by careful control of the dimensions of NMs as well as their surface chemistry.

Another interesting superior feature of NMs is their great adsorption, which has gained significant interest from researchers in recent years because of their potential environmental applications.^58–61  NMs enable chemically inert surfaces for physical adsorption processes. In addition, their large surface areas stand in comparison with those of traditional adsorbents such as activated carbon. Various NMs such as CNTs are essentially different from activated carbon. For example, their structure at the nanodimension is more uniform and well-defined. Various factors including pore diameter and adsorption energy are required to investigate the adsorption on activated carbon while one can deal directly with different well-defined adsorption sites available to the adsorbed compound/s on CNTs.

Carbon-based NMs include single, double and multi-wall carbon nanotubes, graphene and C60 fullerene. They are extensively researched for their potential applications. These materials are often used in electronics, coatings, energy storage devices, industrial catalysts, and nanomedicine as a result of their unique shape and large surface-area-to-size ratio.⁶²  The most commonly studied and used carbon-based materials are single-walled CNTs (SWCNTs), which are a single layer of carbon in a fibrous form, multi-walled CNTs (MWCNTs), which are multiple layers of SWCNTs wrapped together and fullerene C60, which resembles a soccer ball and consists of 60 carbon atoms.^63,64  While their overall uses are plentiful, their use in consumer products may be limited due to previous studies indicating severe toxicity and asbestos-like pathogenicity observed in animal models induced following CNT exposure.^65–68 

Other nanomaterials that have uses in nanomedicine are polymer-based nanomaterials. These can consist of materials from plastics to proteins and DNA and show promise for quick drug delivery due to easy cell entry. Further, polymer-based nanomaterials are useful in food packaging as they can improve the barrier and antimicrobial properties of the packaging as well as be used as sensors to monitor the condition of the food.⁶⁹  Unlike many other nanomaterials available, polymer-based materials are relatively inert and often biodegradable.^70,71 

Ceramic-based nanomaterials are highly sought for use in engineering due to their improved physical and mechanical properties compared to their bulk-sized counterparts. For instance, magnesium aluminum can be used in harsh conditions due to an improved melting point and is ideal for lenses due to enhanced optical properties.⁷²  Further, they are useful as bioactive coatings on prosthetics and are important in tissue repair and implants.⁷³  Unfortunately, like many nanomaterials, their toxicity is not fully understood.⁷⁴ 

Metal-based nanomaterials have received a lot of attention over the years, mainly nanosilver, which is known to act as an antibacterial agent and is estimated to have the greatest number of commercial applications. Currently, these nanomaterials are used in a variety of medical applications, such as contraceptives and are used to treat burns and wounds. They are also being added to various household products such as paint, toothpaste, fabrics and toys for their antimicrobial properties.⁷⁵  Other metal-based nanomaterials commonly used are quantum dots and nanogold. Nanogold has found use in immunolabeling as well as tumor imaging due to increased penetration into tissues, greater sensitivity, and improved labeling.⁷⁶  The toxicity of metal-based nanomaterials has been heavily studied with many studies concluding that both silver and gold nanomaterials induce only mild toxicity.⁷⁷ 

Metal oxide-based nanomaterials, such as zinc oxide (ZnO) and titanium dioxide (TiO₂), are currently used in a variety of consumer products, such as sunscreen and cosmetics for coloring.⁷⁸  Unlike metal-based and ceramic-based nanomaterials, metal oxide nanomaterials are typically very toxic, induce high levels of reactive oxygen species (ROS) and inflammation, and pose significant health risks.^79–81  This is due to the effect of size on the electronic properties of the metal oxide. While the decrease in size is beneficial for stability, it can affect the electronic properties and reactivity of the nanomaterial, resulting in new, unstudied toxicological outcomes. The effect of size on metal oxide nanoparticles reactivity has been studied in depth and it was determined that a change in band gap, or the difference in charge between the tissue and the nanomaterials, had a profound effect on the toxicity of the materials.⁸² 

Corrosion is a natural phenomenon which leads to refined metal converting to a more chemically stable form (i.e. hydroxide, oxide, or sulfide).^83–85  The main crucial parameters which impact the occurrence of the corrosion process depend on the material and the environment. Various environmental conditions cause a material to be susceptible to corrosion including gases in dissolved form, especially O₂ and CO₂, medium pH, temperature and tensile stress, etc. There are various types of corrosion that depend on the chemical mechanism. These types are galvanic, uniform, pitting, crevice, environmentally-induced cracking, dealloying, intergranular and erosion-based corrosion. Among these, uniform corrosion is the form with the most incidences and the largest amount of metal-based waste. On the other hand, other types are localized corrosion, and may not cause so much loss of the material. It is very difficult to predict and control these types of corrosion.⁸⁶  Unless careful control and effective applications are performed in the field, corrosion in any types of forms can lead to undesired failures in the main stages of any processes including pipes, flanges and bolts (Figure 1.1).

In a work performed by Wang and colleagues,⁸⁷  inclusion of nanoparticle fullerene-C60 in nanocoatings for the corrosion control and mitigation of an oil-gas pipeline was investigated. For this purpose, nanocomposite coatings composed of fullerene-C60 and epoxy were prepared through a solvent-free dispersion approach using a high-speed disk and ultrasonication technique. The obtained results showed that the C60/epoxy composite coatings have great improvements in adhesion strength that enable excellent damage tolerance of coatings. Figure 1.2 shows the achieved data of the coating corrosion protection index.

As can be seen from Figure 1.2, the neat epoxy showed the lowest anti-corrosion efficiency. On the other hand, the materials which were reinforced with 0.5 and 1.0 wt% C60 nanoparticles exhibited maximum corrosion protection and high stability even after 200 h salt spray exposure.

Song et al. investigated the impact of MWCNTs on the adhesion strength and corrosion resistance of coatings based on acrylic nanocomposites.⁸⁸  In this study, incorporation of MWCNTs into the acrylic resin led to a significant rise in the adhesion strength (ca. 50%). The resistance values of nanocoatings based on an acrylic nanocomposite having 0.5 and 1 wt% MWCNTs were found to be 1.36 × 10⁵ and 9.27 × 10⁷ Ω cm², respectively, while the resistance value of the native acrylic-based coating was obtained as 235.8 Ω cm². These values were achieved by immersion in 3.5 wt% NaCl after 1.5 days.

Another crucial study on the corrosion application of NMs was performed by Arora and colleagues.⁸⁹  In their study, the researchers investigated the impact of the combination of MWCNTs with Zn-Co coatings that were electrodeposited on the surface of steel substrate. The results confirmed that the coating of ZnCo/MWCNTs with the concentration of 5 mg L⁻¹ MWCNTs exhibited the highest corrosion resistance, maximum intermetallic phase fraction and smoothest morphology. In addition, the optimum value of corrosion resistance was achieved when exposed to the electroactive environment for 2 days and 3 days.

Chen and co-workers reported the synthesis of polyaniline nanoparticles having excellent hydrophobic features and investigated their anti-corrosion behavior for epoxy coatings.⁹⁰  In this work, an ultrasonication-assisted oxidation polymerization technique was employed for the synthesis of polyaniline doped with perfluorooctanoic acid (PFOA/PANI). The synthesized PFOA/PANI nanoparticles showed excellent hydrophobicity and high dispersibility in EtOH that can uniformly distribute in epoxy (EP) resin coatings to significantly increase their anti-penetrant performance for a corrosive environment (Figure 1.3). The obtained corrosion resistance of the developed PFOA/PANI/EP coating was higher than 6 × 10⁸ Ω cm² after 9 days.

Wang et al. investigated the exceptional anti-corrosion features of the prepared graphene/epoxy coatings in harsh oxygen mediums.⁹¹  For this purpose, a spin-coating technique was designed and applied for the preparation of a graphene-based nanocoating that exhibits long term anti-corrosion efficiency. Figure 1.4 depicts the preparation process of the graphene-based nanocoating. The obtained data confirmed that the anti-corrosion efficiency of the prepared highly orientated graphene/epoxy coatings was maintained although they were immersed in 3.5 wt% NaCl for 60 days.

NMs enable high anti-corrosive efficiency because of a number of superior features including high surface area, extremely small size, high chemical and physical stability, etc. The incorporation of NMs into the coatings provides the formation of a denser covering and fewer and highly uniform pores compared to traditional coating materials. The absence of pores and flaws in the coating material as well as the formation of a barrier against electrolytes’ penetration with corrosive effect enable the corrosion resistance features of NMS-based coatings.

On the other hand, much effort is needed towards the investigation of the corrosion protection capability of NMs-based coating materials as well as the design and development of low-cost techniques for the utilization of the anti-corrosive features of NMs. In addition, it is also crucial to design and develop new environmentally-friendly approaches for the functionalization of NMs to obtain nanocomposite-based coatings having hydrophobic features as well as excellent corrosion resistance.