Nanoparticles typically have a size between 1 to 100 nm. They have found applications in the fields of biomedical, environmental, agriculture, coatings and food research. Primarily, the properties like high surface-to-volume ratio, mechanical strength, optically active nature and chemical reactivity make nanoparticles attractive for various commercial applications.
Steel is an important and well-known structural material. Due to their low cost, carbon steels are preferred construction materials in oil, gas, chemicals and marine industries. The problem with carbon steels is that they are susceptible to corrosion under industrial operating conditions. As a result, they need to be protected from corrosion and one way to do that is the use of anti-corrosive protective organic coatings.
In the field of anti-corrosive organic coatings, nanoparticles of various types have attracted the attention of researchers because they can act as a good water barrier. In combination with the binder and additives, they change the surface energy of the anti-corrosive coating and effectively block water absorption. Consequently, protective anti-corrosive coatings formulated with specific nanoparticles can enhance the service life of metals like steel.
Nano-sized titania, graphene, graphene oxide, silica, iron oxide, zinc oxide, alumina, carbon (nanotubes) etc., have been investigated by various researchers to enhance the anti-corrosive properties of organic coatings with binders like epoxy, polyurethane (PU), polysiloxane, polystyrene etc.
Despite the fact that nanoparticles have shown promising results on a lab scale, they are not employed significantly in commercial anti-corrosive coatings. According to a recent study, only 2% of nanoparticles used in the coating industry serve as corrosion inhibitors. Different reasons for the hindered approach of nanoparticles to the anti-corrosive organic coatings market will be discussed in this blog post. But first lets have a look on how steel corrodes and what are the financial costs related to corrosion.
How Steel Corrodes?
Generally speaking, when two metals with different electrode potentials are contained in an electrolyte, electrons start to transfer from the metal with higher negative potential and corrosion occurs at the place from where the electrons have left.
In the case of steel corrosion, it is not necessary to have a separate metal in contact with steel through an electrolyte to start the corrosion process. Steel is microscopically non-uniform. It contains numerous small areas with particles like graphite (or others) which can become small cathodes whereas the bulk of steel can act as anode. Such areas, thus, form small corroding batteries in the presence of electrolyte (i.e., water containing dissolved salts, oxygen etc.).
We can say that corrosion of steel occurs in the presence of water, oxygen and ions like chloride (Cl-1) and fluoride (F-1). In addition, gases like carbon monoxide (CO), carbon dioxide (CO2), Sulphur dioxide (SO2) and Nitrous oxide (NO2) influence the corrosion process by speeding it up. Corrosion and its rate on other known metals is also dependent on similar factors.
What is the Cost of Corrosion?
- Recent (2016) estimates from the National Association of Corrosion Engineers (NACE) indicate the global annual cost of corrosion to be $2.5 trillion.
- NACE estimates also show that globally the total cost of marine corrosion amounts between $50-80 billion each year.
- For pharmaceuticals, chemicals, and petrochemicals industries the estimated direct cost of corrosion (excluding maintenance and operation) is $1.7 billion.
- NACE estimates also indicate that by implementing the available corrosion controls, 35% savings can be made on actual costs of maintenance related to corrosion over the life of an asset. Revenue lost due to downtime is not included in this estimation.
These numbers clearly show the importance of corrosion control.
Challenges Associated with Using Nanoparticles in Anti-corrosive Coatings
Uniform Dispersion of Nanoparticles
Poor or non-uniform dispersion of nanoparticles in organic coatings is a major problem. Agglomeration of nanoparticles happens due to their high surface energy. Changes in surrounding temperature can also trigger the agglomeration of nanoparticles. Desired anti-corrosive performance of an organic coating containing nanoparticles (as the corrosion inhibitor) can only be obtained if the nanoparticles are uniformly dispersed throughout the polymer matrix. It is easy to understand that non-uniform dispersion of nanoparticles means that there will be some regions in the coating from where the corrosion causing ions can diffuse and reach the substrate, thus, initiating corrosion.
To solve the problem of agglomeration, nanomaterials are usually functionalized with chemicals e.g., silanes. This functionalization changes the surface properties of the nanoparticles, consequently providing better dispersion in the coating and improved anti-corrosive properties of the coating. However, functionalized nanomaterials are expensive than their non-functionalized analogs (see below an example of price difference). Therefore, the price of coatings loaded with nicely dispersed (functionalized) nanoparticles is high. This means that the coating manufacturer has to find a customer willing to pay the price of such coatings. Relatively low usage of nanoparticles in commercial anti-corrosive coatings indirectly indicates that the customers are not willing to pay for such coatings.
Development of Novel Coating Material
Protective coatings with good anti-corrosive properties can have other properties which do not meet the desired criterion. For examples, epoxy coatings are well-known for their good anti-corrosive properties, but they show aging issues. Same is true for nanoparticles. As mentioned above, nanoparticles can enhance the anti-corrosive properties of the coatings only when dispersed uniformly. Therefore, development of novel binders and nanoparticles is necessary. With respect to nanoparticles, nanohybrids can provide optimized properties of interest.
Cost of Nanomaterials
In general, the price of nanomaterials is higher than their micron-sized analogs. A good idea about price difference can be obtained from the website of Sigma-Aldrich. Micron sized amorphous silica costs around € 80 per 500 g, whereas, the price of amorphous nanosilica is around € 550 per 500 g.
Functionalized nanomaterials are more expensive than their non-functionalized analogs. To illustrate the difference between the prices of non-functionalized and functionalized nanosilica, we take the example of US Research Nanomaterials Inc., which supplies different types of nanomaterials. For a non-functionalized nanosilica (20-30 nm particle size), price per kilogram is $185. The same material when functionalized with a silane coupling agent costs $358 per kilogram. We understand that prices (strongly) depend upon volumes required by a customer but still, in most cases, the price of a functionalized nanomaterial will be higher than the non-functionalized one. Therefore, the high prices of nanomaterials is a hurdle in their usage in commercial anti-corrosive coatings.
Health & Safety Issues
There are safety and health concerns regarding the use of nanomaterials. It is believed that they can enter the body of workers at the coating production site, coating application site or into the body of consumers once released from the coating. It is still unknown how the nanoparticles can influence human body and the environment. Studies conducted so far to evaluate the health effects of nanoparticles in coatings industry have shown mixed results. Ultrafine particles can catalyze reactions generating products which might be dangerous for the human body.
There is also a lack of standardized test methods for characterizing nanomaterials released from surfaces dependent upon specific treatment processes. This leads to significant variance among the obtained results.
In the dry powder form, nanoparticles need special attention and arrangement for safe handling. Production workers need to be trained for handling such materials. Due to this reason nanomaterials are mostly sold as dispersions. But dispersions are sometimes not compatible with the preferred solvent used to produce the coatings. Therefore, the coating producer may have to consider changes in the formulation. Consequently, the price of such an anti-corrosive coating will be definitely higher than its traditional counterpart.
In conclusion, it will take some time before the nanomaterials can actually replace the existing corrosion inhibitors used in commercial anti-corrosive coatings. The factors (inhibiting the use of nanomaterials in anti-corrosive coatings) discussed here require significant research and development work to provide alternatives which will facilitate the financially viable incorporation of nanomaterials in commercial anti-corrosive coatings.