Corrosion is expensive. Whether you're talking failure costs, inspection and repair costs, or replacement expenses, the costs add up to billions of dollars. At last estimate, corrosion was predicted to cost the U.S. economy over $1 trillion in 2015. That's why coatings are often applied to metallic and non-metallic substrates to prevent material deterioration due to corrosive electrochemical and chemical reactions. Due to changes in regulatory requirements as well as the severity of working environments, there is a constant need to develop new coatings that are durable, cost-effective, safe and eco-friendly.
Coating producers have been constantly endeavoring to develop new products to meet these challenges. Present trends are related to advanced phosphorus-free surface pre-treatment, the development of silanes, and nanoparticles in coating systems. (Silanes are discussed in The Science of Anti-Corrosion Thin Films.) Cold spray techniques are also being developed as a replacement for thermal sprays for improved surface strength and corrosion protection. Smart coatings, which can sense the pH of corrosive species and release anti-corrosive chemicals just in time to counter the attack, are also being developed.
The Parameters of Corrosion Protection
The corrosion protection provided by a coating is generally evaluated based on the following parameters:
- Coating adhesion to the substrate surface
- Cohesion of the layers
- Film thickness and uniformity
- Thermal stability
- Chemical stability
- Mechanical strength of the coating
- Wear resistance
- Free from holidays and disbonding
- Other specific requirements
Good preparatory processes of surface cleaning and conditioning are essential for ensuring strong adhesion of the different layers of coating with the substrate surface.
Water-based coatings are increasingly replacing some of the solvent-based coatings, as they are safer and more eco-friendly. They are generally known as conversion coating products. Recent environmental regulations also mandate the application of low-VOC coatings. Powder coatings and radiation-cured coating would also meet the regulator's requirements. Other offerings include thin ceramic paint products for protection against corrosive chemicals and abrasion.
Apart from developing new coatings for minimizing surface deterioration, specialists are also focusing on improving the substrate surface cleaning and pre-treatment processes. The use of titanium, vanadium, silicon and zirconium-based chemical compounds with polymers has led to improvement in pre-treatment effectiveness. Many automakers have successfully replaced some of their pre-treatment processes that use zinc-phosphating with a pre-treatment process using ceramic products with nano-sized particles of zirconium compounds as well as many other organic materials, which are more eco-friendly.
Developments in Sacrificial Protection
In some cases, a coating may be continuously consumed through a reaction with a corrosive species. When the coating acts as a sacrificial anode, as in the case of the layer of zinc on steel, improvements can be effected by trying out zinc-aluminum as well as zinc-magnesium alloys as coatings. Similarly, inorganic and organic resin-based, zinc-magnesium-rich paints and coatings have been developed and tested for the protection of steel and other metal substrates.
Developments in Barrier Coatings
Many organic coatings provide a strong barrier to a corrosive species and prevent the transfer of ions from the corrosive species to the substrate. In these barrier-type polymeric coatings, which are applied on metallic substrates, permeability becomes the key issue, as holidays (a type of defect), inclusions and damage to the coating layers generate pathways through which the corrosive matter can reach the substrate and local corrosion can begin.
In order to lengthen the diffusion path of corrosive matter, pigments with shapes of plate-like flakes are added to organic coatings. Mica as well as glass flakes are added for this purpose. The orientation of the pigment flakes ought to be parallel to the substrate surface. These must be fully compatible with the resin used in the coating as well. Montmorillonite (bentonite clay primarily consists of montmorillonite) and other layered clay platelets are added into the polymer resins in order to create a strong protective barrier against common corrosion species such as corrosive gases, oxygen and water molecules.
Developments in Inhibitor Coatings
For protecting metals from corrosion, mostly lead and chromate-based corrosion, inhibitors have been extensively used as primers in the past. However, owing to regulatory restrictions, new corrosion inhibitors are being developed and tested for various applications. Corrosion inhibitors containing silicates, phosphates and molybdates have been used in significant quantities to replace chromates and lead.
Thermal Nano-Spray Coatings
Tungsten carbide/cobalt (WC/Co) thermal spray coatings have been adopted for the corrosion protection of equipment and component surfaces for many decades. In recent years, researchers are focusing their efforts on the development of nanomaterials and spray processes to deposit nano-structured composites of ceramics with metals on substrates. The advantages of nanoparticle coatings include high corrosion protection as well as abrasion protection, while retaining the advantage of good fracture toughness. Nano-structured WC/Co particles are processed in the lab with grain growth inhibitors as well as alloying additions and used as thermal spray feedstocks.
Industrial Nano-Structured Particle Coatings
Machine parts require effective surface treatment in order to enable them to work under various corrosive environments. Nanoparticles in some cases cannot be used in thermal spray on account of their very low mass. They cannot be directly carried in a high velocity gas stream to be uniformly deposited on a component surface.
Some processes have been patented for converting nanoparticles into spherical granules first and reproducing smaller nanoparticles in flight as they move closer to the surface. These nano-coatings display higher bond strength, corrosion resistance, toughness and abrasion resistance. Nanoparticle feed stocks used for corrosion and wear protection include tungsten carbide/cobalt (WC/Co and alumina/titanium oxide (Al2O3/TiO2). The latter is being used by the U.S. Navy as well as for civilian applications.
Replacing Hexavalent Chromium with Smart Coatings
In spite of the health and safety issues, hexavalent chromium has been a popular ingredient used by the metal industry for corrosion protection. But safety, health and environmental factors today prohibit the use of hexavalent chromium. Researchers are now replacing passive barrier coatings with multifunctional and smart nanoparticle coatings, which can sense the corrosion media and release anti-corrosive ingredients. Some of the nano structured particle coatings can heal the damaged coating and thus ensure that protection continues to be effective and durable. (Further reading: The Advantages of Smart Coatings and Nanotechnology for Corrosion Prevention.)
Smart nanoparticle coatings perform many essential functions that will ensure corrosion sensing and healing capabilities. The particles sense and react to external stimuli, such as a reduction in pH, changes in coating integrity or changes in corrosion potential. The nano-structured particles in multiple layers of the coating, reacting within their integrated structure, trigger the healing process.
Researchers have now devised advanced techniques of layer-by-layer deposition of coatings for corrosion protection and wear protection. In this method, corrosion inhibitors and polyelectrolytes are deposited on a pretreated substrate.
In Germany, scientists from the Max Planck Institute of Colloids and Interfaces claim that their advanced coatings are based on polyelectrolytes, which are sensitive to the pH of the corrosive media. These are sandwiched with inhibitors in a network of polymeric nano-structures. These advanced coatings show self-healing properties with long-lasting corrosion protection of metallic substrates, such as aluminum alloys used for aircraft.
The technique has potential applications in automotive, maritime and oil and gas pipeline sectors. This advanced system not only acts as a barrier to corrosive media, but also senses the changes in internal structure of the multiple layers of coatings on account of corrosive reactions, and responds to the changes to heal the damages and prevent further damages. Self-healing ability is provided by the mobility of the polyelectrolytes.
The process used is the ultrasonic pretreatment of the substrate (aluminum alloy) surface and precise deposition of layers of oppositely charged nanoparticles of inhibitors and polyelectrolytes of less than 5 to 10 nanometers thick. These smart multilayer coatings provide passivation of the substrate along with self-healing features and pH buffering.
Cold Spray Coatings
Cold spraying uses high-speed jets of inert gas to speed up the finely powdered anti-corrosive material particles that are directed toward the substrate. The particles plastically deform at a temperature that is lower than the melting temperature of the metal. This technique applies various metallic, composite and other powders, such as ceramic powders, to a metallic substrate by accelerating the powders to very high velocities (500 to 900 m/s). Upon impact onto the substrate surface, the powdered particles readily deform and adhere to the surface to create a strong bond with the substrate. Additional particles continue to impinge the solid surface, thus forming a thick coating.
Various combinations of corrosion resistant and wear resistant powders can be used for a given substrate in this technique. Composite cold coatings have been applied to the alloys of steel, copper and aluminum. Some ductile contents added to the powder enable the essential plastic deformation process.
Cold spray processes have been developed to further improve the corrosion resistance and other essential functional capabilities of metals and composites. It has been observed that the normal thermal spray techniques result in coatings with inferior functional properties compared to properties shown by parent materials. The thermal processes may suffer due to inclusions, in-flight oxidation, as well as re-melting at high temperatures, resulting in the onset of early corrosive deterioration due to interconnected porosity. Cold spraying creates effective coatings with capabilities closer to the properties of the parent metals. Studies indicate that corrosion and wear-resistant materials such as tungsten carbide can be cold sprayed on a variety of material surfaces.
Cold sprays produce coatings with the advantage of very low porosity combined with a higher hardness compared to thermal sprays. They ensure a stronger substrate adhesion and cohesion of the particles, creating a harder surface. A comparatively thinner cold spray coating can be as effective as the thicker thermal spray coating for providing corrosion protection and wear resistance. Additionally, this new method is very convenient for in situ working and restorations as well as maintenance. This method has been tried for the corrosion protection and restoration of sensitive materials like magnesium and aluminum alloys. The hard chrome depositing process is now being replaced by a cold spray of WC-Co powder. Cold spray has a high potential for architectural applications due to the aesthetic advantages. It has medical applications as well.
Cold sprays of aluminum alloys have potential applications as anti-corrosive coatings in the petrochemical and oil and gas industries. Titanium, niobium and nickel alloys can also be used as anti-corrosion cold sprays. Anti-corrosive aluminum-tin alloy coatings can be coated on lead-free bearings and components in the automotive and aerospace sector. The U.S. military has tried cold sprays of:
- Nickel coating on steel and aluminum substrates
- Tantalum on aluminum
- Aluminum-zinc and copper-tin on door frames of military shelters
- Electromagnetic interference shielding at high altitude (not likely feasible by other processes)
- For shielding against the escape of electronic signals
- To seal joints with non-porous aluminum composite material coatings
- For wear resistant coats for ammunition boxes
- For restoration of mast supports
- For weapon systems
- For transmission housings
- For helicopter ring grove repair
The cold spray process results in higher Brinell hardness and lower oxygen content in the coating. It can be formed on complex geometric shapes with simple arrangement.
Nano Particles in Polymer Coatings
Nanotechnology is now helping the coating industry to produce more effective products that are also eco-friendly. The addition of nanoparticles to resins helps enhance corrosion resistance in a significant way. The greater surface activity of these additional nanoparticles on the coating surface helps absorb more resin as compared to conventional pigments. Clay as a nanoparticle strengthens corrosion protection of the substrate significantly.
Titanium dioxide (TiO2) nanoparticles dispersed in an epoxy resin system matrix reduce corrosive wear and tribological sliding and rolling wear. However, in many cases, stabilizers are needed to improve compatibility and reduce chemical reactions with other constituents of the formulation. (Learn more about using titanium dioxide in the article 3 Truths About Titanium Dioxide Corrosion Prevention.)
Nano fillers also improve:
- Scratch resistance
- Thermal stability
- Radiation resistance
- Protection from abrasive deterioration
They also reduce moisture ingress due to their low permeability.
Coatings Application with Nanoparticles
Polymer coatings with clay nanoparticles dispersed in them, when applied on the surface of cold-rolled steel, provide more effective corrosion resistance than any normal epoxy coating. Successful field trials of these nanoparticle dispersion coatings on bridges and other outdoor structures were generally exposed to harsh working environments.
Epoxy coatings with separate dispersions of nanoparticles of halloysite clay, zinc, SiO2 and Fe2O3 were successfully tried out on steel surfaces with room temperature curing. Salt solution immersion testing was carried on the coated test items. The beneficial effects of these nanoparticles on the corrosion protection of the coated steel surfaces were clearly established. Better microstructure of the coating improved the coating's mechanical properties as well as corrosion resistance.
Zinc oxide nanoparticles have been tried out on coatings for the automotive industry. This dispersion of zinc oxide nanoparticles can absorb ultraviolet light and stop it from reacting on the inner layer, thus protecting the coating and improving the longevity of protection. For producers of automotive coatings, this reduces servicing and repair costs.
A coating becomes oxidized when exposed to ultraviolet light, which finally results in flaking, weakening the protective barrier. During changes in climatic conditions and exposure to corrosive species, the coating is unable to provide stable protection. Zinc oxide nanoparticles quickly absorb UV beams and protect the coating from deterioration.
Coating producers have begun technology collaborations to develop and deploy nanotechnology for anti-corrosion coating applications. Zinc-free thin film coil coatings based on silica have been developed for steel as a result of such collaboration.
Coatings based on cerium oxide have been developed for use by aircraft producers. Cerium based coatings exhibit good adhesion strength on aluminum alloys used to build aircraft. Cerium hydroxide and cerium oxide phases are deposited after the usual surface preparation methods, and replace the traditional chromate coating, which are restricted by regulations in some localities.