Gone in a flash – What is Flash-Rusting and how can it be prevented?

What is flash rusting?

In the field of coatings, flash rusting occurs when a water-based coating is applied to a metallic surface and manifests itself as rust spots. If these are left to their own devices they may ultimately lead to unsightly discolouration of the topcoat due to migration through the coating; gradual degradation of the substrate itself and delamination of the coating from the substrate.

Flash rust in coatings

In the field of industrial coatings, where this problem is the most relevant, there is a continuing transition from solvent-borne coatings to water-borne coatings and this poses extra challenges for the formulator to develop coatings with at least equal if not improved performance attributes. As you can imagine, using traditional solvent-borne coatings chemistry, the issue of flash rusting was not something which needed to be addressed but for the new generation of waterborne direct metal coatings, it is a significant consideration.

Is Flash Rusting just a Substrate Issue?

Well, no. Aside from when a water-borne coating is applied to a metal surface, flash rusting can also appear in metal packaging for water-borne coatings. Whilst most tins will have a protective internal lacquer, these can fail leading to rust contamination of the coating before use. In this scenario, it is not only industrial coatings which need to be protected but also decorative coatings which may not necessarily be intended for coating onto metal surfaces.

Figure 1: Flash rusting can occur when the water in water-borne coatings reacts with metal surfaces.

How to prevent Flash Rusting in a water-borne coating?

The good news is that there are additives which are readily available to the formulator to prevent flash rusting and are used at low dosage rates (approximately 0.2% for properly prepared ferrous substrates) to good effect. These additives can be a combination of amines, metal nitrites and organic acid salts.

Metal Nitrite and Nitrosamines Concentrations in Flash Rust Inhibitors 

Not all flash rust inhibitors are equal - whilst the presence of a metal nitrite is known to be effective in preventing flash rusting, too high a nitrite content can lead to increased water sensitivity which is undesirable and may also be an obstacle to eco-labelling requirements.  Also preventing the formation of carcinogenic nitrosamines (where amines are present) in production is also pivotal to the safe use of these additives.

Therefore, we wholly recommend the Ascotran range of flash rust inhibitors from our partners, Ascotec.  All the Ascotran range is developed not only with efficacy in mind but also with no VOC content and eco-label compliance as major considerations.

All Ascotran products are amine-free thus eliminating the potential for carcinogenic nitrosamine formation. As a consequence of their careful formulation, Ascotran inhibitors give enhanced hydrophobic properties not found in many inhibitors available on the market. Nitrite levels (where they are part of the formulation) are very low thus ensuring that performance is guaranteed without the disadvantage of water sensitivity.

What substrate can I use Ascotran inhibitors with?

The good news is that Ascotran inhibitors give good protection for coatings applied to ferrous, galvanized substrates and other non-ferrous substrates. In addition, Ascotran additives are compatible with a wide range of resin chemistries such as acrylic emulsion polymers, water-borne epoxy and water-borne alkyd emulsion polymers for example. Of course, we recommend carrying out the appropriate testing to confirm the suitability of your proposed coating formulation.

Figure 2: Applying paint to metal that is eco-compliant, VOC-free and won't flash rust.

Long-term Corrosion Prevention

It is important to bear in mind that when we talk about flash rust prevention, we are not talking about long-term corrosion prevention. If this is a part of your project brief, then we need to direct you to the Asconium range of high-performance corrosion inhibitors from Ascotec for which a separate article will follow shortly.

Summary and Contact Details

This article was designed to be an introduction to the idea of flash-rust prevention so if you have any more questions or want advice on your specific formulation, please get in touch with us or contact your account manager who will be able to help.