Wear-resistant ceramic coatings
Reactive maintenance procedures force the plant to make quick fixes where parts are reinforced, patched, rebuilt, or completely replaced to get back up and running as soon as possible. If repaired or replaced parts are put back into service without somehow protecting the parts against recurrent damage, the service life of that equipment will be less predictable.
Reliability and predictability are pivotal to the operational success of industrial equipment. In aggressive industrial applications, preventive and scheduled maintenance maximize the life and structural integrity of large and small metal components such as mixing vessels, pumps, hoppers, housings, tanks, chutes, pipes, and centrifuges. These heavy-duty parts are often exposed to aggressive slurries, sand aggregates, particulates, and large stones that can damage and abrade even the hardest reinforced steel substrates.
Damage to components can be caused by mechanical attack, chemical attack, corrosion, or by a combination of these three modes. Mechanical attack involves dry particulates or slurries dropping onto and traveling through chutes, pipes, pumps, and other components. Over time, even soft flowing materials can abrade and wear away at hard alloys such as AR400 steel. As the surface metal weakens, thin layers are gradually stripped away from the parent substrate, reducing its thickness and structural integrity.
Chemical attack and corrosion physically change substrates through chemical reactions. For example, corrosion occurs when iron oxidizes, leaving a very weak and loose layer of oxide on metal surfaces. Corrosion can be concentrated locally, forming a pit or crack, or can extend across a wide area, uniformly corroding an entire surface. If left untreated, active corrosion below the surface of a high build coating can cause the substrate beneath to weaken, ultimately causing a critical failure in the structure.
In chemical attacks, damaging chemicals passing through metal equipment can react with and eat away at the surface of the metal, causing layer-by-layer damage. As damaged layers are continuously stripped from the parent substrate, the thickness and structural integrity of the metal part gradually decrease.
Examples of wear are easy to find in any industrial facility. Pump casings and impellers wear as a result of abrasive slurries and solids, cavitation, and chemical attack, wearing down internal sections of the equipment. Some of the common wear areas include the cutwater, wear ring seats, impeller vane tips, and inside the volute. In the case of pipes and ducts, most wear occurs at elbow bends where fluid flow changes direction. Some plants are forced to repair or replace duct elbows every three months, adding significantly to labor and material costs. In quarries and mines, iron ore can severely abrade reclaimer buckets due to the continuous sliding caused by the digging and reclaiming action.
Many facilities only conduct maintenance on large components once damage becomes extremely apparent or a catastrophic failure occurs, such as a hole in a sidewall or a part dropping off of equipment. When critical failure occurs, equipment must be shut down for an unscheduled, costly, and time-consuming repair. Reactive maintenance procedures force the plant to make quick fixes where parts are reinforced, patched, rebuilt, or completely replaced to get back up and running as soon as possible. If repaired or replaced parts are put back into service without somehow protecting the parts against recurrent damage, the service life of that equipment will be less predictable and the chances of another unscheduled critical failure increase.
To deliver long-term protection from wear, abrasion, chemical attack, and corrosion, epoxy-based wear-resistant coatings containing ceramic fillers can be applied to vulnerable metal surfaces to protect them and minimize planned and unplanned downtime. These coatings act as a sacrificial and renewable working surface preserving the structural integrity of the base substrate and preventing mechanical attack, chemical degradation, and corrosion caused by abrasive particles, slurries, and chemical exposure. These engineered coatings can be applied to worn equipment for restoration or to new equipment before it is placed into service.
How does a wear-resistant coating work?
Ultra-smooth and high gloss, epoxy-based wear-resistant ceramic coatings increase equipment efficiency and life expectancy. These coatings can be a quick and cost-effective remedy to damage because parts are returned to service with little interruption or process downtime.
Wearing compounds are two-part epoxy systems containing ceramic beads or ceramic powder, and silicon carbide. The epoxy-base polymer that adheres the ceramic material to the substrate is formulated to be extremely robust, offering excellent performance under high-compression and high-impact loads. These coatings cure in temperatures between 55 F and 90 F, with cure time dependent on mass and temperature—the larger the mass, the faster the cure. This is a result of the exothermic heat generated by the epoxy reaction during the cure process. Higher substrate and air temperatures will also result in accelerated cure.
Because ceramic is inert, it does not react with most materials that come in contact with metal components. Ceramic coatings resist harsh chemicals and withstand temperatures to 450 F. The size of the ceramic material—powder or bead—used in the wear-resistant coating directly corresponds to the size of the particulate that will ultimately damage metal components. For large particulates and stones, maintenance professionals should use coatings with the largest available ceramic beads. For fine slurries or wastewater, fine powder ceramics are sufficient to inhibit damage.
Designed for a wide variety of operating environments, wear-resistant coatings fall into seven distinct categories: putties with beads, high-temperature formulations, ultra-high-temperature formulations, fast-set materials, brush-on formulations, sprayable formulations, and impact-resistant materials.
Putties are thick pastes best applied using a trowel or a gloved hand on surfaces within arm’s reach. These wearing compounds withstand abrasion from fine to coarse particles in locations including elbows, cyclones, and other areas where fluid changes direction and causes turbulence. Various sized bead fillers incorporated into different putty formulations give them distinct strength and hardness characteristics to resist wear based on the fluids or solids running through the system.
High-temperature formulations that resist up to 350 F are standard performers. Ultra-high-temperature wearing compounds resist temperatures to 450 F, commonly found in ovens and hot water or hot fluid slurries. These wearing compounds are often used to protect the substrate from elevated temperature chemical attack.
Fast-set materials are used when equipment and parts need to return quickly to the production line. These compounds cure to functional strength in as little as three hours.
Brush-on coatings are self-leveling, low viscosity materials that can be applied with brushes or rollers, or simply poured onto a substrate and allowed to coat the desired area. At just 1/8- to 1/4-in. thick, brushable coatings work best on equipment carrying slurries with very fine particles such as sand, metal chips, shells, or seeds. They can easily be applied to areas that are out of arm’s reach, like the inside of pipes, and will reduce friction caused by moving fluids in applications like fluid pumps.
Sprayable coatings are low-viscosity materials similar to brush-on materials, but achieving just 0.020-in. thickness. When applied with a spray gun equipped with an atomizing mix tip, these ultra-thin coatings can be applied overhead, used on large application areas and on hard-to reach, intricate, and small-diameter parts like curved components and elbows. Because these coatings are so thin, they should not be exposed to high turbulence or impact.
Impact-resistant coatings are specially formulated to absorb shock and withstand damage from medium to large sized aggregate such as rocks, coal, and other substances falling onto or otherwise striking a substrate. These materials are often found protecting chutes at the end of conveyor belts in mining and construction.
Surface preparation and application
Metal surfaces must be dry prior to rework and must be thoroughly cleaned before coatings are applied. Maintenance professionals should remove all corrosion, chemicals, contaminants, rust, paints, and residues from the surface of the metal using abrasive blasting, grinding, grit blasting, or other mechanical cleaning techniques. Alcohol will degrease the surface in the final cleaning step before the new sacrificial coating is applied.
To bond the wear-resistant coating to a badly degraded surface or to fill large voids, weld wire mesh can be tacked over the damaged area to act as a backbone, rebuilding structural integrity (see Figure 2). The coating is then applied over the mesh. To prevent adhesion when coating a large area, undesired surfaces can be masked off using lubricants or mold release agents. In these treated areas, any unwanted wear-resistant coating can be easily removed post-cure as it will not permanently adhere to the surface.
After cleaning, the metal surface is roughened to encourage adhesion of the coating. Abrasive blast using an angular grit such as aluminum oxide or silicon carbide will provide sufficient roughness. High-velocity water blasting with an abrasive medium will achieve a similar result. The surface can also be roughened using a needle gun or a coarse grinding wheel with 60 grit or coarser. Using coarse sandpaper or a file is acceptable only if the first two methods cannot be employed.
After roughening, the surface is again thoroughly cleaned with a cleaner and degreaser, and repairs are made as soon as possible to avoid rusting.
To provide a visual indicator of the amount of wear a coating has endured and when it may require reapplication, epoxy coatings are available in different colors. When the first coat begins to wear, the second color will show through, indicating that the maintenance team will need to reapply the coating at some future point.
Cylindrical bowls in decanter centrifuges
Decanter centrifuges have a cylindrical bowl with discharge hubs flange-mounted to either end. The bowl outside is often exposed to aggressive chemical environments. The application of wear-resistant coatings saves the large, expensive bowl from regular replacement.
To apply the coatings, the bowl must first be blasted with abrasives to remove visible surface rust and contaminates and create a rugged, miniature mountain-and-valley finish. This surface roughness is known as surface profile, a condition that improves adhesion by increasing surface area and providing a keyed anchor pattern.
Surface profile will vary depending on the type of abrasive particles, equipment, and technique employed. It is critical to achieve the correct profile depth specified for a particular coating. Inadequate quality control and lack of restriction of large abrasive particle sizes for thin coats can lead to peaks of the surface not being adequately covered.
After the bowl’s surface is abraded and cleaned with a solvent such as industrial strength isopropyl alcohol, the exterior of the bowl is coated with a chemically resistant brushable ceramic coating. In areas where the cylindrical bowl is subject to both chemical attack and wear from abrasive particulate, the epoxy-based ceramic coating will increase the life and efficiency of the bowl. For large bowls or intricate areas where trowel-applied epoxy-based putties are not appropriate, a sprayable coating protects hard-to-reach regions.
Sprayable epoxy-based ceramic coatings are solvent-free, have low VOC content, and achieve a smooth, consistent finish. On a centrifuge, the sprayable ceramic coating is applied over another epoxy-based wearing compound to provide a low-friction wear surface.
Pump casings and impellers
Industrial pumps are used at virtually every manufacturing site to remove water, slurries, and other fluids from area to area. For example, in alloy and steel mills, fluids that collect in holding tanks contain some amount of sludge residue. When fluids are pumped out of the holding tanks, some of the sludge passing through the pump sticks to interior components including the casing, cutwater, wear ring seats, impeller vane tips, and volute.
As sludge builds up over time, the gap between the pump’s outer wall and the impeller becomes smaller and smaller. This gap is designed to preserve the pump’s efficiency, and efficiency decreases as the gap gets smaller. Also, the friction from the sludge-filled slurry wears down the pump’s interior over time, causing degradation of the casing and impeller. Without the use of maintenance techniques to clean away the sludge and control abrasion, the pump will eventually fail.
Applying brushable ceramic coatings to the pump’s wetted interior components will protect parts from damage by
- Reducing friction and allowing slurries and sludge to slide through the casing and impeller without sticking;
- Adding a layer of armor that rejects abrasion and wear related to friction and flow; and
- Protecting cast iron parts from aggressive chemicals that can eat away at and react with the metal substrate. The coating’s epoxy base and ceramic beads are both inert and highly resistant to chemical attack.
By making wear predictable, ceramic coatings vastly improve the lifespan of pumps and reduce incidents of unplanned maintenance. These coatings allow large pumps to run at their expected wear-rate and ensure that repairs can be made during scheduled, routine downtime. The catastrophic and unplanned failure of uncoated, unprotected equipment often requires the maintenance team to obtain replacement parts that can be expensive and hard to procure, making downtime longer and much more costly.
Rachel Nashett and J. Adam Lyman are application engineers for Henkel.
The Bottom Line:
- Epoxy-based wear-resistant coatings containing ceramic fillers can be applied to vulnerable metal surfaces to protect them and minimize planned and unplanned downtime.
- By making wear predictable, ceramic coatings vastly improve the lifespan of pumps and reduce incidents of unplanned maintenance.
- Ceramic coatings increase equipment efficiency and life expectancy. These coatings can be a quick and cost-effective remedy to damage because parts are returned to service with little interruption or process downtime.
Here are some of the articles at plantengineering.com, KEYWORD: CERAMIC COATINGS that further discuss this topic:
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Temperature sensors: Make the right choice, RTD vs. TC
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