CSL Silicones - A step change in anti-corrosion technology

CSL Silicones' Si-COAT elastomeric silicone anti-corrosion coatings demonstrate that the limits of silicone chemistry are bound only by our imagination.

Have we truly exhausted our collective base of knowledge in chemistry? Why is the issue of corrosion growing when it should be shrinking? Where is the step change in anti-corrosion technology?

We have seen the advance of silicone (polysiloxane) co-polymers for incremental improvements in coating performance. An understanding of the inherent properties of silicone leaves one asking why the material has not found wider application as a coating for protecting valued capital investments from corrosion.

The limits of silicone chemistry are bound only by our imagination in respect of its application. Such imagination, creativity and ingenuity were employed by the chemists and engineers of CSL Silicones to develop the patented Si-COAT elastomeric silicone anti-corrosion coatings. Targeted primarily for the maintenance of corroding assets, Si-COAT anti-corrosion coatings are a primerless one-coat system of unmatched longevity that may be applied directly to rust, without the need for abrasive blasting. The benefit to the end-user is lower installed and lifecycle costs. In addition, the highly elastic nature of Si-COAT, combined with its superior bonding chemistry, eliminates rust creep.

Adhesion mechanisms

Traditionally, it has been rather fashionable to draw two broad categories of adhesion mechanism: chemical adhesion or mechanical adhesion. Progressive thinking, however, discounts the theory of mechanical adhesion, except in cases of extremely porous surfaces. Instead, adhesion of a coating to its host surface is explained via various forces that exist at the molecular level. With respect to the adhesion of coatings to their host surfaces, the forces taken into consideration are covalent bonds, van der Waals forces and hydrogen bonding. We can further classify these bonding forces as follows:

  • primary valence attractions (covalent bonds)
  • secondary valence attractions (van der Waals forces and hydrogen bonding).

In terms of the strength of the bond, primary valence attractions are far greater in strength than secondary valence attractions.

The adhesion of conventional coatings is highly reliant upon weaker secondary valence attractions. Thus, conventional chemistry has required the preparation of the metal substrate to a level that is free of dust, oils and all traces of rust, and which has been profiled to increase its actual (versus apparent) surface area.

"Si-COAT has a very low surface free energy, which yields excellent wetting properties comparied with coatings of alternative chemistries."

In comparison, Si-COAT prefers the natural - in other words, oxidised - characteristics of exposed metal surfaces. The coating scavenges for metal oxides on the substrate, and, via a chemical reaction during the curing (vulcanising) process, converts these sites to primary valence bonds between itself and the substrate.

Certainly, the fundamental requirement of any protective coating is to adhere to the surface it is protecting. Before adhesion can take place, however, the coating must approach the substrate closely enough so that the bonding forces that hold the two together may take effect. Irrespective of the nature of the bond, the distance between the coating and the substrate that must be achieved is very small (less than five Ångstrom units or no more than three times the diameter of an oxygen atom). Furthermore, as the distance between the coating and the substrate increases, the attractive forces decrease exponentially.

A much stronger bond

This is where the low-surface free energy of Si-COAT factors in. Without doubt, the first requirement for good adhesion is to eliminate surface contaminants that could otherwise bond with the coating. All the surface preparation in the world, however, is useless if the second requirement for good adhesion isn't met: the need to get the coating as close to the substrate as possible so that it may bond. Si-COAT, being engineered from a pure silicone backbone, has a very low surface free energy, which yields excellent wetting properties compared with coatings of alternative chemistries. Quite simply, the lower the surface free energy of a coating, the better its wetting characteristic and its ability to get very close to the substrate.

As might be expected, the true chemical bond formed by Si-COAT via primary valence bonding with the substrate provides a much stronger bond than the physiochemical bonds of secondary valence bonding. In the case of primary valence bonding, the bond is so strong that the interface between coating and substrate may be regarded as disappearing entirely.

Of course, no coating system is immune to damage from external forces such as accidental impact. The underlying steel that is exposed in such an event is known to expand in volume by as much as ten times as it corrodes. This force from underneath the coating is akin to a cancer that can quickly metastasize. When compared with Si-COAT, conventional coating systems with their relatively weaker adhesion mechanisms and inelasticity are unable to cope with this force and 'pop' off the surface of the steel to expose more virgin steel for further corrosion. This cycle continues and expresses itself as blisters of corrosion that determinedly creep and grow under the surface of the coating.

When Si-COAT is used, however, corrosion creep is arrested. The elasticity of Si-COAT allows it to conform to the corroding steel, while its strong adhesion keeps it firmly secured to the substrate. Effectively, corrosion of the exposed steel is firmly held in place by Si-COAT until repair of the damaged coating is convenient.

The time has arrived to question how the industry will avoid the wasteful and repetitive expense of combating corrosion. The answer lies in Si-COAT elastomeric silicone anti-corrosion coatings.

"Si-COAT" and "CSL Silicones" are registered and/or unregistered trademarks of CSL Silicones Inc.

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Email: [email protected]
URL: www.cslsilicones.com

Si-COAT coatings may be applied directly onto rust, without the need for abrasive blasting.
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