Top Nano Coatings Developments to Know for Corrosion Protection
While regulatory restrictions on the use of VOCs is driving the development of water-borne coatings, nanotechnology-enabled hybrid and composite ceramic thermoplastic coatings are gaining acceptance for many applications.
The new technology of nanomaterials has changed the way we look at pipe corrosion. It is now feasible to greatly extend the service life of oil field pipelines and drill pipes. Nanotechnology coatings are also making their way into other industry sectors including automotive, aviation, maritime and construction. In this article we'll look at these promising new coatings and the applications for which they are suitable.
Nanotechnology Composite Hybrid Coatings
Advanced hybrid nanotechnology composite coatings contain molecules that establish a unique durable bonding with the molecules of the steel substrate. These hybrid coatings are a combination of abrasion resistant glass-ceramics and corrosion proof thermoplastic material composites that are formulated with proprietary chemistry by the manufacturers.
Special features of these nanotechnology-enabled composite hybrid coatings include:
- The coating's molecules form a durable covalent bond with metallic molecules on steel surfaces
- Long lasting ceramics and nanotechnology thermoplastics that are a synergistic combination of resilient and hard materials containing extremely abrasion resistant and corrosion resistant materials
- A unique combination of corrosion resistance and toughness due to a network of tough nano-enabled composite thermoplastic materials with ceramic-glass particles
- Stable high temperature service up to nearly 180°C (356°F) thanks to a hybrid technology that combines organic and inorganic high performance materials
- A smooth coating surface that provides low fluid friction to flowing water and oil, thus reducing oil pumping power losses
- A lower film thickness of around 10 to 20 microns ensures stable dimensions and increases the feasibility of coating threaded surfaces and complex surfaces
Marine coating manufacturers have developed nanotechnology-based high performance ceramic coatings that protect both the topside and areas below the waterline of seagoing vessels. (To learn more about ceramic coatings, read Top 5 Applications for Ceramic Coatings.) Nanotechnology ceramic coatings provide protection from salt water splashing as well as from fouling underneath the hull that normally results from algae and shells.
Procedures to clean a vessel's hull are expensive, unsafe and complicated due to the environmental concerns involved. Nanotechnology ceramic coatings can repel algae so that biofouling will not be an issue. This improves the vessel's operational efficiency along with providing corrosion protection. With a clear, bright, durable glow, these coatings eliminate the need for other glow maintenance products such as waxes.
The nanotech ceramic coating also reduces the flow resistance between the vessel’s hull and the sea water, resulting in an appreciable reduction in overall fuel consumption and CO2 (carbon dioxide) emissions.
Coatings to Prevent Cavitation Corrosion
Cavitation corrosion is a destructive process that affects fluid system components such as pump impellers, casings and valves. Huge water pumps are used extensively for offshore oil extraction to pump large quantities of sea water into crude oil wells. Cavitation is a major degenerative process in the pump, resulting in lower water output, vibrations and unexpected breakdowns. (Cavitation is discussed extensively in The Science of Cavitation: Diagnosis & Resistance Methods.)
Cavitation corrosion is a specific type of erosion caused by gas bubbles imploding on metallic surfaces. It is usually connected with abrupt pressure variations associated with a fluid's hydrodynamic parameters around propellers, stirrer blades and turbine blades. Low and high pressure areas get induced due to fluid velocity changes in the flow path, and the abrupt low pressures cause the sudden release of vapor bubbles. When the bubbles are carried to high pressure zones in the flow path, they bombard the impeller and casing surfaces, causing cavitation corrosion on these surfaces. Bubbles collapse after reaching a high pressure zone, with tremendous pressure, sound and vibration. A well-designed pumping system can reduce cavitation corrosion.
Cavitation corrosion occurs in a pump when the static local pressure is temporarily below the vapor pressure of the fluid being pumped, at the given temperature. Cavitation corrosion is a function of fluid temperature, velocity and local pressures. Using a larger diameter pump inlet pipe, controlling water contaminants and eliminating air ingress into the pump can reduce cavitation corrosion damage.
Glass Flake Coatings
Glass flake coatings have been used for many decades and have fair chemical resistance and corrosion protection properties. However, the volatile organic compound (VOC) base of glass flake coatings makes them a health hazard and its cavitation resistance is inadequate. Generally a coating thickness of 2 mm is required, which may interfere with the pump's flow rate.
Epoxy Coating (Solvented)
Depending upon the application, epoxies are designed with a variety of properties by changing the binders. They are modified by using hydrocarbon resins to improve mechanical properties and chemical resistance. Solvented-epoxies, however, contain undesirable solvent VOCs that are associated with health and safety issues. The solvent also means some shrinkage-related problems in the long run.
Polyurethane coatings can be designed for flexibility to provide cavitation corrosion and impact and erosion protection. However, over time they may be moisture sensitive.
Solvent-free Epoxy Coatings
The solvent-free coatings of epoxy provide all the advantages of solvented epoxy coatings, such as cavitation corrosion protection and erosion resistance. In addition they enable the designer to modify the coating's properties based on the application; for example, the mechanical strength, chemical resistance, flexibility, toughness, adhesive strength, erosion resistance and cavitation resistance can all be modified. (See Review of Solvent-Free Epoxy Protection for Secondary Containment Concrete for more information about these coatings.)
Proprietary products are available that can be applied with lower thicknesses. A thin film coating will not interfere with the designed flow rate of the pump, but can still effectively protect the surfaces against cavitation corrosion and erosion corrosion. Solvent-free coatings also eliminate the risk to health and safety. A smoothly coated surface, free from surface irregularities, enhances pump efficiency by minimizing hydraulic losses and cavitation within the pump. To obtain the best results, the rotating part (the impeller) and the static part (the inner surface of the casing) should receive a layer of coating. Nano coatings used for turbines may further improve the productive life of water pumps.
The aviation sector is continuously trying to minimize its environmental footprint by using lighter, more efficient materials and eco-friendly processes. Despite being a trendsetter in advanced materials, it continues to account for the highest carbon footprint among all large-scale industries. Therefore, aviation is developing more efficient engines and lighter coating materials that have effective corrosion resistance and high mechanical strength.
Some jet engine producers are using proprietary elevated- temperature ceramic matrix composite coated components for the next generation of jet engines. The ceramic matrix composites are UV-resistant and high-temperature corrosion resistant. They are built from silicon carbide ceramic fibers placed in a silicon carbide matrix and finally coated with a proprietary ceramic material.
Advanced coatings developed for the aviation sector are meant to protect the component assemblies and top surfaces of the aircraft from extremely corrosive environments. There is a rising demand for advanced coatings with properties such as extreme temperature corrosion resistance, erosion corrosion resistance and abrasion resistance for engine components and wearing parts. (For more about aviation coatings, read Aviation Coatings for Corrosion Prevention.)
Nano coatings offer numerous advantages including improved long-term corrosion protection, surface cleanliness, surface hardness, low flammability and better fuel economy due to drag reduction. This results in a lower carbon footprint, improved cleanliness, and lower maintenance and operation costs.
Halogenated flame retardants used with earlier coatings were not environmentally friendly, so nanotechnology additive fillers in the coatings effectively replaced halogenated fire retardants. Because the nano coatings are applied directly on the aircraft's surface, in the event of a fire the coating's film immediately creates a ceramic layer on the substrate, thus eliminating the risk of generating smoke and heat.
The aircraft manufacturers also find that nano coatings add to the fire safety and aesthetic quality of the aircraft's interiors, frames and engine component surfaces.
Polyester Urethane (Low VOC)
Aviation coating manufacturers are offering hazard-free low VOC polyester urethane coatings with faster drying and curing capabilities combined with chemical resistance against the hydraulic fluids used in aircraft. With the additional advantages of UV resistance, greater durability, abrasion resistance and gloss retention, these can also provide the superior gloss needed for aircraft interiors.
New Nano Coatings in Tribology: Friction, Wear and Corrosion Prevention
Another development in aviation materials is related to the use of nano coatings to improve the durability of metals by reducing friction and wear. Nanotechnology has enabled the use of lighter magnesium alloys in place of aluminum and steel by providing effective corrosion resistance to magnesium alloy surfaces. Nanomaterials used in new coatings include silicon oxides, boron oxide and nano crystals of cobalt phosphorus.
These nano coatings are applied onto the surfaces of turbine blades and many other rotating components that are subjected to high speeds, severe loads and very high temperatures, resulting in less frictional wear, energy losses and noise generation. Tribological nano coatings reduce the coefficient of friction and enhance resistance to abrasive wear as well as corrosive wear, thus enhancing the productive life and efficiency of jet engines.
Nano Technology for Dirt-free Scratch Resistant Coatings
Research in nanomaterials has enabled coating producers to develop scratch resistant coatings for the automotive industry and other applications. New coating products include polymer nano composites with water repellent properties and quartz nanotechnology particles that enable the coated surface to be self-cleaning, corrosion free and scratch resistant.
A water-resistant surface can be created on a vehicle's window glass by using a dirt resistive coating. As the rainwater automatically flows off from the glass, driver visibility is enhanced even without using the wipers because the air flow will dry or clear the beads of water droplets.
Applying nano coatings to the inside surfaces of the windows will discourage water vapor from collecting and condensing, thus ensuring full visibility in most cases.
Eco-friendly is the Latest Trend
The coating industry is adding new capabilities in response to customer demand and stricter regulatory requirements. The demand for modern functional eco-friendly coatings that protect metallic components such as threaded fasteners has been rising sharply across the manufacturing, automotive, utility and railroad sectors. Older corrosion protection processes, such as hot dip galvanizing (which requires the use of toxic materials and acids) are being replaced.
The patented eco-friendly coating processes include dry thermal diffusion processes based on zinc. Because these eco-friendly processes do not use hazardous materials and do not result in polluting emissions or byproducts, they do not require regulatory approval.
Waterborne Anti-corrosion Coatings
Waterborne anti-corrosion coatings contain a small amount of solvent and approximately 80% water. Solvents are applied to a substrate in order to effectively disperse the resin. Due to their high water content, these coatings are preferred as low VOC, nonflammable, odorless, eco-friendly alternatives; they are also convenient to apply. Because they are fast drying, more layers can be readily applied. These coatings are used in a variety of applications because of their unique properties (i.e., low toxicity, abrasion resistance, very good adhesion and nonflammability), which make them preferred primers. Application hazards are minimal.
Waterborne coatings are now widely used due to stringent VOC regulations prevalent in the U.S. and Europe. These coatings contain VOC content below 3.5 pounds/gallon of water, as required by regulations. A smaller quantity of this waterborne coating can be used to coat a substantially larger surface area. This comparison holds for equal quantities of waterborne coatings and for solvent-borne coatings. Generally, the waterborne coatings are less expensive because they don’t require additional chemicals such as hardeners.
Based on the resin used in the waterborne coatings, they are classified as polyurethane coatings, epoxy coatings, acrylic coatings, alkyd coatings, polytetrafluoroethylene (PTFE) coatings or polyvinylidene difluoride (PVDF) coatings, as well as some others.
Waterborne coatings are now extensively used in the automotive, architectural, marine, construction and packaging sectors. On the global level, demand for acrylic waterborne coatings is rising rapidly because this resin is popular in the automotive and architectural/construction sectors. Rapid urbanization in global regions such as the Middle East, Asia-Pacific and South America, with their booming real estate and automobile sectors, is driving additional demand for waterborne coatings.
The trend toward eco-friendly waterborne coatings along with the development of high performance nano composite coatings point to the future development of nanotechnology enabled smart coatings that are self-healing (self-repairing).
Written by Shivananda Prabhu
Shivananda Prabhu is a Graduate Engineer from the University of Mysore, Karnataka, India and PGDBM (Equivalent to MBA) from XLRI, a top-ten management institute. He previously worked for Tata Steel, Jamshedpur, in the area of maintenance as a Manager and Specialist in tribology, lubrication, wear prevention, corrosion prevention, maintenance management and condition monitoring. He has contributed to loss prevention and value engineering as well as knowledge management initiatives.
He later worked as a Technical Trainer, Safety Trainer, Lead Auditor of ISO 9001, ISO 14001, Management Trainer, and Training and HR specialist.
For about four years he worked in academics in PG institutions, as a Professor and later as Director of IPS (Management Institute) in Pune. He also worked for three years as an editor and writer for research papers, newspapers, trade journals and websites. Overall his experience spans more than 25 years.