The primer is the most vital layer of a coating because it works as the base for the designated top coating that is needed for corrosion prevention. It also provides the subsequent layer a surface to grab and hold on to, thus enabling proper bonding of subsequent coating films. Hence, the primer contains binders that are needed to adhere to the substrate and to bind the next layer of the coating system.
Because the primer has to fill and hide the defects in the substrate's surface, it must be carefully matched to surface characteristics such as porosity. In the event that portions of the top coat becomes delaminated or damaged later on, the zinc content in the primer will continue to provide corrosion protection for a limited period of time.
Basics of Epoxy Zinc Rich Primers
Epoxy zinc rich primers belong to the family of zinc rich coatings that are applied to protect ferrous surfaces from corrosion. These primarily work as sacrificial cathodic protection products. Because zinc is anodic to steel, these particles corrode first, thus protecting the ferrous substrate beneath the primer from corrosion.
The corrosion product is a powdery zinc oxide that exposes the remaining zinc coating for further oxidation, thus providing continuity of protection. Therefore, zinc based primers provide sacrificial cathodic protection for the steel substrate.
Reduced zinc epoxy primers have around 55% zinc by weight in the cured primer film, whereas zinc rich epoxy primers have around 80 to 85% zinc by weight.
A zinc rich epoxy primer coating is an organic coating that necessarily requires a top coating for durable and reliable heavy-duty protection. It can be applied with airless spray, air spray or brush, and therefore the simplicity of application as well as the surface tolerance makes it suitable for onsite maintenance applications.
Inorganic zinc rich primers, on the other hand, have the advantage of not requiring a top coating. Normally silicate is used as the binder in this case. This primer can’t be applied on site, because perfect surface preparation by abrasive blasting of surface is a must. (For more on this topic, read Surface Preparation for Inorganic Zinc Silicate Coatings.)
The Protection Mechanism of Zinc Rich Primers
Even though the effective performance of zinc rich primers is attributed primarily to sacrificial corrosion prevention, the complex mechanism of protection must be understood in totality.
Initially, once the zinc rich primer and the top coating are applied on the substrate as a continuous layer, the coating system acts as a barrier to corrosive environments, and it can rightly be called a barrier coating. The coating fully isolates the substrate from the environment.
Only after a defect is formed in the coating, thus partially exposing the substrate to the corrosive environment, does the zinc rich primer layer play its part in providing sacrificial corrosion protection to delay the onset of the electrochemical reaction (corrosion) of the substrate. The amount of sacrificial protection provided depends upon variables such as the purity of the zinc dust used, the presence of moisture, and the electrical conductivity between the substrate and interconnected zinc particles. Thus, the zinc used in the primer is anodic to the ferrous substrate that acts as a cathode. Once the zinc particles are converted into zinc oxide, the sacrificial protection will end.
In his paper “Zinc rich primers for corrosion protection”, J Peter concludes that generally "inorganic zinc coatings can provide better corrosion protection than organic zinc coatings." The corrosion protection provided by coating systems with zinc rich primers were found to be superior to the protection provided by organic coatings without zinc rich primers.
Areas of Application
Zinc rich epoxy primers are extensively used to protect steel piping and structural members that are exposed to corrosive environments. These primers with advanced formulations are often designed for the severely corrosive environments faced by marine and offshore assets, such as bridges, oil refineries and mining machinery. These can also be used to repair damaged zinc silicate coatings and galvanized ferrous surfaces. (Galvanization is discussed in Galvanization and its Efficacy in Corrosion Prevention.)
Performance of Zinc Rich Epoxies
Compared to zinc rich silicates, the major advantages of zinc rich epoxies include:
- Quick curing ability, regardless of ambient humidity
- Ease of overcoating
- Capability for greater film thickness without the risk of mud cracking
- Suitability for using standard airless spray (without the need for the special spray equipment required for zinc rich silicates
- A greater tolerance to variable surface conditions and less stringent surface quality requirements for the substrate. For example, surfaces that are inaccessible for abrasive blast cleaning can be protected by a zinc rich epoxy primer.
Challenges Faced By Epoxy Zinc Rich Primers
These primers exhibit the following drawbacks:
- When exposed to a corrosive marine environment, zinc rich epoxy primers produce insoluble corrosion products that must be removed before overcoating the surface. (Related reading: Understanding the Special Precautions for Marine Coatings Applied Above the Waterline.)
- When steel is coated with a rich zinc primer, the weldability is negatively affected because the welding process produces zinc oxide fumes as well as other zinc salts. This is the reason why rich zinc primers can’t be used as a shop primer.
- Compared to zinc rich silicate primers, the zinc rich epoxies have inferior abrasion resistance, chemical resistance and heat resistance.
- Rust creep and blistering are the other major problems sometimes faced by zinc rich epoxies.
Causes of Coating Failures
Zinc rich epoxy primers may fail sometimes to protect the surface due to:
- Poor quality of the primer formulation due to the high filler content and consequently lower resin content.
- The use of lower film thicknesses less than 50 microns.
- The use of sea sand that contains chlorides for abrasive blasting, which adversely affects the substrate cleanliness. Often a chloride residue is left behind.
- Inadequate surface roughness, resulting in poor adhesion of the primer.
Advanced Coating Formulations
Coating manufacturers have developed innovative formulations for specific operational requirements, such as the addition of low temperature curing agents for temperatures below 5°C (41°F). However, low temperature curing agents added to the coating can increase the volatile organic compound (VOC) content.
One study that examined the performance of inorganic zinc rich versus organic zinc rich primers and activated zinc technology describes the performance of a proprietary activated zinc technology product that synergistically combines inhibitor, barrier and cathodic sacrificial protection mechanisms. It was found that in traditional zinc rich epoxies, only about a third of the zinc dust is actively utilized to provide sacrificial cathodic protection of the substrate. The new technology enhances the zinc dust utilization to protect the surface.
Furthermore, the study revealed that the new activated zinc epoxy technology product has superior rust creep resistance compared to conventional zinc rich epoxy primers and the performance is on par with that of zinc rich silicate primers. Additionally, the new activated zinc rich technology product was found to be superior to zinc rich silicate and conventional zinc epoxy primers with respect to impact resistance and flexibility.
Application Methods for Zinc Rich Epoxy Primers
The surfaces to be primed must be cleaned and abrasive blasted. Surface defects must be noted and rectified. The primer in two containers must be mixed according to the coating manufacturer's instructions, and a uniform content should be obtained.
The coating can be applied by brush, airless spray or air spray. For airless spray, the fluid pressure at the spray tip should be maintained according to the coating manufacturer’s specifications. In the case of brush application, the recommended film thickness should be maintained (typical figures per coat are 50 to 75 microns). The chosen film thickness must be matched to the surface profile. (Learn more about the importance of following the manufacturer's specifications in the article 5 Coating Defects That Can Be Avoided By Adhering To Coating Specs.)
If the film is to be cured at temperatures below 5°C (41°F), a product with low temperature curing agent should be chosen. The time between the primer application and the overcoating for the given ambient temperature must be kept in mind when planning the top coating application. (A typical interval for overcoating is 4 hours for 5°C.
While the use of zinc rich primers is an effective way of provide anti-corrosion protection, epoxy zinc rich primers have greater acceptance as maintenance primers.
The main advantage of these primers is their ability to produce higher film thicknesses without the risk of mud cracking.
Conventional airless type spray equipment can be used for the primer application. It is surface tolerant, but it can’t be used as a shop primer due to poor weldability.
Activated zinc rich epoxy products being developed through proprietary technologies have good creep rust resistance comparable to inorganic zinc rich silicate primers.