Powder Coating Systems for Improved Corrosion Protection

By Martin Neal, Kathryn Shaffer, Rebecca A.O. Short, John R. Schneider, and Paul Bradley, PPG

Introduction

Corrosion of parts due to environmental damage to exposed metal surfaces is a leading cause of structural failure in the field. Corrosion is defined as the slow oxidative deterioration of a metal by its reaction with the environment. Powder coating technologies can be used to mitigate and slow the onslaught of this deterioration. However, their effectiveness is dependent on their ability to fully cover and encapsulate the surface and maintain coating integrity over time, including at the sharper edges. Ensuring appropriate edge coverage of these coatings at their onset of application is critical for providing a successful protective layer. Multiple powder coating scenarios exist for providing corrosion protection, and the proper choice of layers and chemistry is dependent on the level of corrosion protection needed. This article provides a brief overview of some current powder multilayer protective approaches and takes a deeper dive into recent progress in the development of a one-layer powder coating solution.

A Brief Survey of Powder Primers for Corrosion Protection

Two-layer powder coating systems are highly recognized and accepted as state-of-the-art solutions for corrosion protection in the field. The choice of an appropriate protective system can be better assessed by considering specific customer requirements for performance. A tool for better understanding appropriate protection level is the ISO 12944 standard. This standard is a globally recognized system that provides a broad range of requirements for protection for steel structures in various environments. Although specific to structures, this standard can provide a directional path for considerations for other end uses as well, such as those in the construction equipment, industrial, and trailer industries. Figure 1 provides an overview of the ISO categories and typical environment associated with each category.

The ISO 12944 standard is separated into six distinct classes for corrosion level, with specific environments associated with each level.

By exploring these ISO 12944 standard levels from most challenging to least harsh, we can review the multiple powder primer offerings that exist today for use with powder topcoats to provide the necessary level of corrosion protection as a two-layer system:

  • Zinc-rich primers4, typically used for C5 corrosion level situations, are used when there is a need for exceptional corrosion protection through the incorporation of zinc into the formulation. The primer acts as a sacrificial layer as the zinc is oxidized during the corrosive mechanism, keeping the damage away from the metal part itself, and thus slowing the overall corrosion of the part over time.
  • High edge primers, which can either contain zinc or be zinc-free, provide better edge coverage through rheology control during cure, which provides an improved layer of protection. These primers are typically used for C3 corrosion level needs, unless they contain zinc, whereby they can be used in the harsher C5 situation.
  • When less stringent levels of corrosion performance are needed, there are many primers to consider that are free of zinc and provide adequate protection at the C2 to C3 levels.

These two-layer options provide good solutions over a diverse range of performance levels depending on the need, as shown in Figure 2. For certain end uses that include the C4 and C5 levels of corrosion protection, higher levels of corrosion protection continue to be a must, and two layers of protection for these powder systems remain the rule. However, as customers at the lower performance end (C1–C3) strive to save cost and reduce complexity, the capability to provide adequate corrosion protection in just one layer is becoming increasingly attractive within multiple industrial segments, including heavy duty equipment (HDE), trailer, and general industrial. But how can we ensure that proper corrosion protection can be met with just one layer of paint? To answer this question, we need to take a step back and consider the challenges in achieving proper edge coverage.

Examples of powder primers on CRS/HRS substrate and their placement across performance level and potential end use categories, as related to the ISO 12944 standard.

As mentioned previously, for a paint to perform effectively as a protective barrier, it needs to fully cover and encapsulate the part in its entirety, including the sharp edges. Full coverage is not a simple task for most paint systems. Why is protecting sharp edges on a metal part so difficult? The answer lies in the rheological changes that a paint (both liquid and powder) undergoes during the cure/bake process. Figure 3 demonstrates the effect of the “pull-back” that occurs during the curing process for a thermoset powder. The shrinkage of the paint during cure on an already-exposed edge causes it to flow even further away from the sharp edges, resulting in bare metal at the edge. This is the entry point for corrosion failure.

Maximizing powder coverage at the edges is a challenge for both liquid and powder thermoset systems. Pull-back of paint during the cure cycle further exposes these sharp edges.

To improve the coverage of the powder coating on these sharp edges, two strategies come into play. The first strategy involves careful control of the electrostatics of the application process to provide a more uniform application. Application control, i.e., ensuring appropriate gun settings, is essential to provide optimized edge coverage. The use of electrostatic modifying additives can also help ensure coating uniformity. The second strategy involves control of the rheology of the powder coating through the use of additives that increase the melt viscosity of the powder during cure to prevent the flow away from the edges.

 

Continue reading in the July-August 2026 Issue of CoatingsTech.