The erosion-corrosion environments found in fluid-conveying components such as pipelines, pumps, agitators, and chutes have important implications for both the service life of the components and the maintenance costs. The destructive synergy wrought when corrosion occurs in combination with erosion can be many times greater than the sum of each mechanism acting in isolation. To extend the service life and prevent catastrophic failure, the in situ application of cold-cure liquid epoxy coatings is recommended, but what type will be most suitable for the job?

Belzona 1331 and Belzona 1381 are new erosion-resistant coatings based on the latest filler-free polymer alloy technology. The polymer alloy contains an immiscible blend of a tough ductile phase within a hard epoxy matrix.

Traditionally, ceramic-filled epoxy coatings have been used for protection against erosive environments found in process industries. These coatings incorporate hard ceramic particles within an epoxy binder, creating a ceramic composite with excellent adhesion and chemical resistance. However, ceramic-filled coatings are recommended for brush application only because the hard angular nature of the ceramic filler can compromise performance and cause extensive damage to the piston ball valves, packing seals, and nozzle tips of spray equipment.

The new coatings, however, are able to resist erosion and cause considerably less wear on the spray equipment than conventional ceramic-filled epoxy coatings. Belzona 1331 is specified for use at ambient service temperature, while Belzona 1381 is the elevated temperature equivalent.

A single solution for two types of erosion

Erosion due to the scraping along of solids in the fluid’s current is categorized into two types of erosive mechanisms: ductile and brittle. Ductile erosion involves material removal due to cutting or plowing via sliding abrasion (e.g. along pipeline straights and risers). The angle of impingement is small in ductile erosion, typically less than 30›. Brittle erosion involves material removal due to radial crack formation via impact abrasion (e.g. pipeline elbows, bends, and protrusions). This type of erosion occurs at angles of impingement nearer to the surface.

Hard ceramic-filled coatings resist ductile erosion mechanisms but suffer from brittle erosion at impingement angles above 60› due to their low fracture toughness. However, the tough ductile phase of the new polymer alloys is capable of elastic deformation and

is therefore resilient to brittle erosion mechanisms. They also can resist ductile erosion mechanisms because of a low coefficient of friction.

In addition to their erosion resistance, the coatings can be applied at wet film thickness up to 2,000 µ without sagging because of their low densities and pseudo-plastic rheology. They are substantially thicker than conventional epoxy coatings that can only achieve a maximum of about 700 µ before sagging occurs.

The more ‘sprayable,’ the more economical

The polymers can be applied in one or more coats using off-the-shelf heated airless spray equipment. The volume mixing ratio is compatible with single and plural component spray units.

As “spray-friendly” coatings, the polymer alloys can be used on large areas or spin-sprayed inside tubing where application by brush or roller is impractical. Consumables such as nozzle tips, piston ball valves, and packing seals also will last longer and require less downtime for maintenance when used with these coatings.

Wear to the nozzle tip of spray equipment will cause the orifice and internal flow passages to enlarge. The flow of product will then increase as the fan angle decreases, resulting in a narrower applied band width and reduced coverage per pass that will not only require more time to complete the job but more product as well.

Testing the theory

Application trials were carried out to demonstrate the spray-friendly characteristics of the filler-free polymer alloy technology. A Graco King 65:1 heated airless spray unit was used to apply both a “sprayable” ceramic-filled epoxy coating and Belzona 1331. A Graco XHD523 nozzle tip was used with a .58 mm orifice.

Upon application of the ceramic epoxy coating, the fan angle halved within 10 minutes of continuous spray. When Belzona 1331 was applied under identical conditions, the fan angle remained consistent. After 45 minutes continuous spray, without observable deterioration to the fan angle, the test was stopped with the conclusion that erosion on the spray nozzle tip is negligible.

Damage to ball valves and packing seals in the positive displacement pump also was assessed after six hours of continuous spray application. Belzona 1331 and a ceramic-filled epoxy coating claiming to be sprayable were tested. The volume of product displaced by the piston after six hours was approximately 1,000 l (264 gal), equivalent to a coverage area of more than 2,000 sq m (21,528 sq ft).

Three hours into the test, the ceramic-filled epoxy coating leaked from the piston, resulting in catastrophic failure. No problems were observed with the new product during the six-hour procedure. On completion of the test, the piston was stripped down and the packing seals and ball valves were recovered, cleaned, weighed, and examined for visual signs of wear.

Proof positive

The packing seals did not erode in either test. Visual inspection of the ball valves, however, exhibited marked differences between the new polymer and the ceramic-filled epoxy coating that was labeled “sprayable.”

The ball valves exposed to the Belzona coating were still glossy and as good as new except for a few isolated marks. The ball valves exposed to the ceramic-filled epoxy coating were matted in appearance and evidently well worn. When weighed, the ceramic-filled epoxy coating was found to cause 100 times greater erosion to the ball valves than the Belzona product.

The test demonstrated that large volumes of the new polymer can be applied without damaging spray equipment.

Unlike some ceramic epoxy coatings that claim to be sprayable, these new coatings represent a class of coatings that, because of their chemical makeup, are truly “sprayable.” Using these epoxy coatings will likely translate into more cost-effective and low-maintenance erosion-corrosion protection.