What Is The Importance of Film Thickness?

Protecting metal from the environment (preventing corrosion etc) with a paint film normally involves three different types of protection – inhibition, cathodic and barrier. (Normally a paint system will use at least two of these, if not all three to some degree). 

Film thickness does affect the performance of all 3 types though to varying degrees

• Inhibition - probably where film thickness is least critical. Inhibitors form a passive layer that stops or greatly reduces corrosion. It’s very important that the film is complete and fully adhered (no voids or bare spots) but once this is achieved the passive layer should form and increasing the film thickness will not greatly increase the inhibition effect.

• Cathodic - uses zinc metal content to act like galvanising. The proportion of zinc content is the most important factor for this type, but it is also affected by film thickness – a film that is too thin will not contain sufficient zinc to fully produce the sacrificial anode effect that it relies on.

• Barrier - film thickness is most critical here – barrier protection relies on the physical strength and cohesion of the film to protect the substrate, both from impact and by sealing the substrate away from the elements.

Almost all coatings remain porous to moisture and chemicals, although only to a very minor degree. This means that a thicker film offers significantly more protection than a thinner one, simply because the moisture has to find its way through a much longer molecular route, greatly increasing the chances of it dispersing on the way. 

It also vastly improves the physical impact protection – paints rely on the bonding of the molecules within the film for their strength and a thicker film gives this the greatest chance, as well as providing a stronger shield to soak up damage to the surface while not affecting the coats underneath.

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What film thickness should be applied?

Any paint should come with a Technical Data Sheet (TDS), which will advise the recommended film thickness required, typically within a range. 

In addition, for most industrial coating systems there will be a paint specification (typically under the ISO12944 standard) which will state the protection level required. Paint manufacturers test their systems to this standard, and will advise which system will best suit your requirement – this will include stipulated film thicknesses. 

This is where it is really vital to achieve the advised thicknesses, as it has been proved via testing that those numbers are required in order to achieve the desired protection – if those thicknesses are not achieved, it is likely the system will fail sooner than the standard required.

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Minimum or Maximum?

A final note regarding the point mentioned above, that film thickness on a TDS is normally advised as a range – this is mainly because with different environments and equipment, a film may need to be built up differently. For example, if 300 microns is required for a particular coating, one user may be able to apply that in one coat, another may have to apply two coats of 150 microns each.

It is important not to simply aim for the minimum – this is because there will always be some low spots around corners, on high parts of a blast profile etc. If these points fall below the minimum, the whole film is regarded as being outside the standard, so will not achieve the stated protection and may fail quickly due to these weak areas.