Nanocoatings have been discussed for long in the paint and coatings industry. Many from the industry have speculated its significance. Nanocoatings show superior enhancements in the anti-corrosive performance of surfaces compared to micromaterial coatings like silver, copper and aluminium which are generally recognised. In this article, Abhidnya Kumra discusses future of nanocoatings in the coatings industry.

 

Anti-corrosive coatings have become significant since last four decades due to heavy wear and tear, rust and financial losses experienced in the past, across the globe. It is estimated that corrosion accounts to around $2.5 trillion losses per year globally in terms of worth. Nanocoating modifies the properties of a surface or substrate. Nanocoatings are in liquid or in solid form. They are used to protect, seal, or colour the surface or substrate. Nanoparticles used in nanocoatings have metabolic properties. Nanocoatings are penetrative in nature and they have characteristics such as scratch resistance, high hardness, and resistance to molds and bacteria. These characteristics make them durable and high-performance coatings. Nanopaints and coatings are largely applied in industries such as marine, automobile, warships, hospitals, oil & gas, electronics & optics, biomedical, packaging, and aerospace.

 

Nanocrystalline structures are superior over microstructures for corrosion enhancement due to the fine grain sizes, which provide better space filling and a higher integrity of the coated surface. Applying nanocoating onto the surface of the substrate makes it harder, tougher, and improves its adhesive properties. However, the coating thickness and composition should be designed so as not to decrease its protective characteristics towards corrosive and eroding influences.

 

Nanocoatings act through different mechanisms to provide enhanced corrosion resistance, and in some cases, they might bring up adverse effects. The fine sizes of nanocoatings, form a uniform physical barrier on the surface of the material. These nanoparticles possess improved adhesion properties due to the high density of their grain boundaries, thus increasing the corrosion resistance of the substrate. On the other hand, the higher grain boundary fraction and uneven surface generated from the agglomeration of the fine particles can foster the chance of forming anodic sites, which will make the surface more susceptible for corrosion attack. Hence, it is important to consider all of the surrounding factors related to nanocoating and substrates,in order to achieve the expected corrosion protection.

 

The corrosion of metallic nanocoating has been studied by many companies across the globe. It should be noted that there is no factor that affects the corrosion resistance alone in one dimension, neither one can alone contribute to the corrosion behaviour of the nanocoating. Nevertheless, they all play a role in determining the corrosion performance of the nanocoating. Nanocrystalline Ni and its alloy have great potential as a promising metallic nanocoating, especially in the form of the nanocrystalline Ni–P alloy. Range of 14-17 wt. per cent of nickel in Zn–Ni alloys shows the best corrosion resistance. 

 

The addition, phosphorus improves the corrosion behaviour in neutral and acidic media, with a composition of around 9-11 wt. per cent in the alloy. In regard to the grain size, nanocoating composition and the acidity of the media can dominate the effect of the grain size. Pulse electrodeposition provides better corrosion properties than direct current, as the former technique produces a finer surface. Moreover, the concentration of additives encountered in the nanocoating should be optimised for the best corrosion properties.

 

The corrosion behaviour of ceramic nanocoating on different kinds of oxides was unique with specific corrosion characteristics depending on the substrate, surroundings, and nanomaterial type and characteristics. Alumina nanocoating shows enhanced corrosion characteristics when it is deposited with a plasma-enhanced ALD technique compared to thermal ALD deposition. Pre-treatment processes improve the surface properties and corrosion characteristics of the coated surface. Pre-annealing the copper substrate before coating it with Al2O3, doping TiO2 with nitrogen anions, or pre-etching the surface before coating it with Ta2O5 reported better corrosion resistance. Both TiO2 and Ta2O5 show high resistance towards corrosion in NaCl solutions. For nanocomposite coatings, a filler of ceramic or metallic nanoparticles enables it to be used as an effective nanocoating. 

 

Corrosion protection with nanocomposite coating is achieved by building a compact barrier and preventing charge transfer such as oxygen permeability and ion transportation. In addition, nanocomposite coating improves some of the other properties that help in enhancing the corrosion behaviour of the nanocomposite, such as: cohesive and adhesive properties, hydrophobicity, agglomeration, and dispersion and distribution properties. The corrosion of nanocomposite coating is affected by the same factors as those mentioned above. In addition to those, the corrosion behaviour of nanocomposite coatings is influenced by the nanocomposite synthesis method, type, and concentration of the filler, as well as whether the coated substrate is heat-treated or not. TiO2, SiC, and SiO2 have positive corrosion behaviour when added to the Ni–P matrix in a NaCl solution for electroless Ni coating, while for electrodeposition coating, Al2O3 shows good corrosion resistance when blended in Ni matrix. For polymer nanocomposite coating, conductivity has become a point of interest.

 

Challenges: New development of the nanocoatings

The corrosion resistance of a material defines its stability and durability. Nanocoating contains ultrafine constituents that might influence the resulting surface regarding aspects of lattice structure, grain size, porosity, intermetallic particles’ distribution, surface state, etc. 

The ultrafine constituents in the nanocoating constituents have very small and dense grain boundaries that make it challenging to develop new corrosion theories for their interaction with the surface. For example, smoothening the surface increases the integrity, uniformity, and fatigue performance of the ultrathin coatings, which would decrease the possibility of pit initiation at such surfaces. At the same time, having nanoparticles covering the surface provides excessive smoothness that might weaken the adherence of the coating and cause detachments of parts of the coating. In addition, lowering the surface roughness might increase the possibility for preferential intergranular corrosion that allows the growth of a more defective and permeable coating at the triple-junction grain boundaries. These two mechanisms allow the nanomaterials to interact with the surface in two opposite directions, and it is difficult to identify which theory is applicable. In addition, due to presence of such nanomaterials on the surface of the substrate, oxide formation is affected; hence, the transition mechanism of the surface state differs, and will be difficult to detect. Surface state transition from passivation to pit initiation and then to the breakdown of the film is influenced. Depending on the initial conditions of the uncoated surface and the cleanness of the final coated surface, the new state is defined, which will affect the overall surface corrosion. Due to the non-uniform distribution of the nanoparticles on the surface of the coated substrate, ions might accumulate and create weak points of higher potential that cause pit initiation. On the other hand, an accumulated coating might physically isolate the substrate surface from electrolyte ions.

--

About the author:

Abhidnya Kumra is a senior research associate at Delhi University and is works jointly to develop nanocoatings under industry-university partnership programme. She has worked with Advance Paints as part of R&D team and with IIT Kharagpur as a research associate.

--

--------------------------

The ultrafine constituents in the nanocoating constituents have very small and dense grain boundaries that make it challenging to develop new corrosion theories for their interaction with the surface.

---------------------------