Paints and Coatings

Paints and coatings

Coatings for corrosion protection can be classed as either active, sacrificial or passive depending on their mode of action. Paints which prevent corrosion purely through barrier protection are known as passive corrosion protection, as they do not change the behaviour of the corrosive agent or change the tendency of the substrate to corrode. These paints can also utilise layers of metal or silicate flakes to provide a tortuous path – making it difficult for water which has breached the surface of the paint to reach the substrate. However, if the paint film is damaged, corrosion will occur very quickly due to lack of protection from the environment. Some of the commonly used passive corrosion paints include alkyds, acrylics, amine epoxies, polyamide epoxies, urethane and polyurethane.

These paints are also used in both sacrificial protection and active corrosion prevention. Sacrificial coatings utilise both barrier protection and cathodic protection to protect a substrate, with cathodic protection being used upon disruption of the paint film (e.g. scratch, chip). These coatings utilise the presence of metals which are more active, and will corrode preferentially to the substrate, helping to prevent corrosion of the underneath layer. Typical sacrificial corrosion pigments include zinc phosphate (ZnPO4), zinc dust or chromate 6+ (CrVI) compounds. There are a number of issues with the use of these compounds in anti-corrosion paints, with of serious health and environmental issues resulting from the use of both zinc and chromate compounds. This has led to the restriction in the use of hexavalent chromium in Europe and other countries, with European companies requiring specific and very limited authorisation from REACH to use these compounds. There is a strong worldwide encouragement to shift away from using these types of materials in anti-corrosion coatings.

Active corrosion protection paints contain an additive which can influence the corrosion reactions caused by exposure to corrosive elements, by disrupting the corrosion chemical reactions. These are frequently used in primers.

Inhibispheres® offer active corrosion protection when incorporated into a traditional barrier paint system and promote self-healing of the paint film upon damage, preventing corrosion from occurring. With their homogeneous dispersion throughout the coating, Inhibispheres® will provide protection regardless of where the coating is ruptured, diffusing to the corrosion site to quickly prevent further corrosion. Designed with specially selected corrosion inhibitors, Inhibispheres® can be used on either steel or aluminium substrates, in both water-based and solvent based paint systems, and the sustained release action of Inhibispheres® means that the coating will provide long lasting corrosion protection.

Coil coating

Coil coating is a continuous process where a coating is applied to a continuous metal sheet for the purpose of corrosion protection. The coil sheet moves at up to 200 metres/min through the process where it is cleaned, treated, dried, primer coated and top coated. The most common substrates that are coil coated are aluminium and steel with aluminium being the more popular. The continuous coil sheet with the applied coating is passed through an oven for the curing process and the coating which is a thermoset cures. These protective coatings are usually very thin. Anti-corrosion coil coating primers usually have a dry film thickness ~5 µm while the topcoats are ~15-20 µm. Coil coatings produce a high quality uniform coating to protect the substrate beneath. The coils produced in this process are often used in the construction and automotive industries to make cladding, roofing, guttering, etc. Polyester coil coatings are the most common kind of coil coating with 50-60% of the market with major demand and growth in the Asia-Pacific region.

The anti-corrosive properties of the coil coating pre-treatment are imparted mostly through the use of chromates. There has also been a number of non-chrome treatments introduced in the last two decades. This conversion coating helps to preserve the substrate surface and improve adhesion of the paint film. After the pre-treatment comes the primer coat. The primer coats are primarily anticorrosive coatings. They utilise a corrosion inhibitor in the coating to prevent oxidation of the substrate surface. The most common inhibitor used in coil coating in North America is chromate whether zinc or strontium. After primer coating comes the topcoat to create a smooth aesthetic on the surface and prevent penetration of moisture or corrosion initiating species though the coating to the substrate surface.

Inhibispheres® offer coil coaters the opportunity to formulate with a chromate free coating. The removal of chromates from coatings is a legal requirement in some regions like the European Union. Inhibispheres® offer an environmentally friendly alternative while maintaining the same level of corrosion protection with a unique organic or organo-metallic corrosion inhibitor. The use of organic corrosion inhibitors allows formulators to create coatings without the need for toxic chromates. Inhibispheres® M and Inhibispheres® B are recommended for aluminium substrates to help protect them against corrosion. Inhibispheres® M, Inhibispheres® A, Inhibispheres® ZB and Inhibispheres® ZS are all recommended for protection of steel substrates. Uniquely Inhibispheres® M is recommend for both and could be used in formulating a single solution for multiple metal substrates types. The different Inhibispheres® products from Ceramisphere vary in the type of corrosion inhibitor used as well and the release rates of the inhibitors. Inhibispheres® ZS is also recommended in applications where edge protection is especially required. These materials are easily incorporated into polyester and epoxy coatings and can be used in either a primer or topcoat to help prevent corrosion of substates. They can be used alone or in combination with one another to help formulate more versatile protective coatings. They can also be used in conjunction with traditional corrosion inhibitors to extend the corrosion protection performance of existing products. One critical requirement for the application of Inhibispheres in coil coatings and to ensure optimum results is to grind the Inhibispheres into the paint to an Hegman grind of 7-8. This ensures proper dispersion and good incorporation in the 5 microns coatings.

Inhibispheres® from Ceramisphere offer a step change in the corrosion protection solutions for coating formulators in the Coil coating industry.

Powder coating

Powder coating is a thermoset coating process where a powder is sprayed on to a substrate surface and heated up in an oven which initiates the curing reaction and a polymer network forms between the crosslinker and the resin. Powder coatings have a number of chemistries that are used which include polyester, epoxy, fluoropolymer, etc. Each chemistry type has its advantages and disadvantages. In general powder coating offers excellent corrosion protection compared to liquid coatings. The impermeable nature of the powder coating makes moisture penetration through the coating very difficult (but not impossible). You still get corrosion initiating species that will penetrate even with powder coatings. To help mitigate this, corrosion inhibitors are employed. The most common corrosion inhibitor used in powder coatings is zinc dust. These zinc rich coatings are used on substrates that will be used in corrosive environments. However a zinc rich coating is a primer coating which is applied to a pre-treated metal. A topcoat is required to seal the primer coat and give the required aesthetic. These standard 2-coat system demands twice as much work from the applicator and use twice as much material.

Inhibispheres® can be used to create a one coat anti-corrosive powder coating for steel and aluminium substrates. Inhibispheres® M has been used to provide a one coat system for a super durable powder coating that can provide comparable corrosion performance to a 2-coat system. The 2-coat system being a zinc rich primer with a polyester topcoat. These systems are suitable for most corrosive environments. The benefits of a one coat system for the end user are significant. The cost of coating a substrate or asset with a single coat compared to a 2 coat gives greater savings in labour, materials and asset down time. With a single coat system there is no chance of interlayer incompatibility or interlayer delamination.

Inhibispheres® from Ceramisphere offer a step change in the corrosion protection solutions for coating formulators in the powder coating industry.

DTM coatings

Direct to Metal (DTM) coatings are applied directly to metal substrates without the use of a primer, forming a single coat in contrast to the two-layer primer and topcoat systems. DTM coatings can be applied with minimal surface preparation, making them useful where surface preparation may be difficult. These coatings are commonly used in the non-professional market, lending themselves to situations where only a basic level of corrosion protection and/or surface aesthetic is required. Most DTM systems can be classed as either solvent based, water based or high solids. Depending on the resin chemistry these are classed as acrylic, polyurethane or epoxy, and can be comprised of both one and two component systems.

In a conventional metal coating, the primer is used to provide primary corrosion prevention to the metal substrate, as well as improving adhesion for the topcoat layer. The topcoat is used to provide protection from weathering and UV exposure, as well as providing colour and gloss to the coating. DTM coatings provide the same adhesion, corrosion protection and UV and weathering resistance in a single coating, saving both time and money when compared to a two-coat system. The need to use only one coat to paint a surface also represents a smaller environmental impact when compared to a primer/ topcoat system, as a reduction in the number of coats put down translates to a reduction in the VOCs (Volatile organic compounds) being emitted.

The use of traditional corrosion inhibiting pigments in paint will usually give a matt, or reduced gloss finish to a coating. To overcome this, a two-coat system is required; the corrosion inhibitor is incorporated into the primer, and the topcoat provides a glossy finish to the coating. In DTM coatings, the optical properties of the paint have to be balanced with the corrosion protection in a single layer, usually leading to a compromise between the two features of the coating. As most corrosion inhibitors are typically pigmented, the use of these in a single coat system can lead to issues with the appearance of the coating, with obvious decreases in the gloss and smoothness. The effect of corrosion inhibitors on coating appearance means that a compromise between effectiveness in corrosion inhibition and coating quality is often made. This usually means that the corrosion protection ability of a DTM coating is lessened – leading to use in less exposed environments, or the gloss is reduced – leading to use in areas where coating appearance is less important.

To overcome these issues, a corrosion inhibitor which does not affect the optical properties of a coating is required to avoid compromising on corrosion ability through reduced dosage. Due to their size, shape and refractive index, Inhibispheres® are able to be used in single coat DTM without affecting gloss, while at the same time providing a long-lasting corrosion inhibiting effect due to a sustained release inhibitor formulation. Inhibispheres® utilise highly effective organic or organometallic corrosion inhibitors, which are released gradually through a tailored silica matrix, allowing for improved corrosion protection over time. Utilising specially selected inhibitors, Inhibispheres® are capable of working on aluminium and steel substrates, are able to be used in both solvent and water-based systems and can be use alongside traditional corrosion inhibiting pigments to provide long lasting corrosion protection.

Inhibispheres® from Ceramisphere offer a step change in the corrosion protection solutions for coating formulators in the direct to metal industry.

Extended protection

Inhibispheres can be used to extend the protection of current coatings. The addition of Inhibispheres to coatings already containing corrosion inhibitors such as zinc or zinc phosphate can help to increase their anticorrosion properties and extend the lifetime of the coating with little to no reformulation. The Inhibispheres mechanism of action, using organic and organometallic corrosion inhibitors, gives a complementary mechanism of action against corrosion when compared to traditional inhibitors. This helps to impede corrosion further while utilising the same paints system but adding additional functionality. Coatings normally rated for a mild corrosive environment can be upgraded to harsher environments or coatings that are designed to last any number of years can be made to last longer.



Inhibispheres® are submicron ceramic particles which can provide specific functionalities to classic coating formulations. Active materials, such as corrosion inhibitors, can be incorporated inside the ‘Smart Particles’, which can then simply be mixed into a paint or coating formulation. The particles are mechanically resistant, can survive paint formulation processes (e.g. mixing, grinding, extrusion) and will not adversely affect the mechanical properties of the coating.