What is and how works e-coating?
What Is E-coating?
E-coating (electrocoating, electrophoretic deposition) is an immersion wet paint finishing process using electrical current to attract the paint particles suspended in a bath to the metal surface, in the context of microchannel heat exchangers – to aluminum.
Prior to paint application, the pretreatment process includes cleaning heat exchangers from oils and contaminants. The pretreatment process is necessary to improve electrocoat performance, ensure a high-quality finish, and keep the paint bath clean of unwanted contaminants. Zinc phosphate is the preferred pretreatment choice as it provides the best corrosion resistance and paint adhesion.
The paint adheres electrostatically to the aluminum surface through electrophoresis. It results in a uniform film of paint, as the process is self-limiting and slows down as the build-up of applied paint electrically insulates the part from the bath.
The system is cathodic: heat exchangers are negatively charged to attract the positively charged paint particles. Paint solids deposit initially in the surfaces closest to the counter electrode, and as these surfaces become insulated to current, solids are deposited in further bare aluminum surfaces providing complete coverage (throwing power phenomenon). When it comes to microchannel heat exchangers, especially ones with louvered fins, enough current should be applied to drive paint particles to omit voids within the interior of the fins. At the same time, overcurrent may lead to fin bridging with excessive paint. Thus, DC-rectifier voltage should be precisely controlled to ensure the exact amount of paint will be deposited onto the heat exchanger surface.
In anodic electrocoating, where the part to be coated is the anode with a positive electrical charge, a small number of metal ions can migrate away from the part and into the paint film, contaminating it and limiting the anti-corrosive properties of the coating. Anodic coatings are economical and allow for a lower cure temperature.
Post rinses, usually two or three, are necessary to remove any residual paint build-up and other loose materials. The final post-rinse stage, made of deionized water, helps to improve the appearance of an electrocoat finish.
Following post rinses, the electrocoated heat exchangers enter the bake oven to crosslink and cure the paint film, making it hard and durable to ensure maximum performance properties. End-product quality is largely dependent on the time spent in the oven and applied temperature (selected based on paint technology and varies from 80°C to 240°C).
E-coating can be done with different types of coating material. One of these is epoxy which provides superior protection against corrosion, including exceptional resistance to salts and humidity. However, epoxy electrocoatings display low resistance to UV light, thus requiring a UV-resistant topcoat to protect epoxy primer.
Compared to other types of coatings, electrocoating is not too expensive; however, the construction of e-coating lines is costly, and its maintenance is also expensive, which adds to the cost of end products. E-coating lines are set up for one paint product to run constantly. Replacement of the paint material and clean up of the line is again expensive and measured in days, so color changes of electrocoated product are next to impossible.
Summing up, e-coating offers crucial advantages for all-aluminum microchannel heat exchangers, including:
- High corrosion protection. Cathodic epoxy electrocoating with a film thickness of 20 microns withstands more than 6000 hours of salt spray test performed in accordance with ASTM B117 standard, more than 4000 hours SWAAT performed in accordance with ASTM G85 Annex A3.
- Environmental friendliness: electrocoat process produces little to no hazardous air pollutants, volatile organic compounds and is RoHS, OSHA and EPA compliant.
- Uniform, even coat and perfect aesthetic quality.
Where Is E-coating Mandatory?
Kaltra strongly recommends e-coated microchannel heat exchangers for areas with corrosivity categories C5-I (industrial areas with high humidity and aggressive environment, mass loss of aluminum between 5 and 10 g/m² after one year of exposure) and C5-M (coastal environments, off-shore installations with high salt content, aluminum mass loss more than 10 g/m²) as per ISO 12944-5:2019, as well as for C4 zones (e.g., industrial areas and coastal areas with moderate salinity, aluminum mass loss 2 to 5 g/m²).
For further reading, details of chemical resistance of e-coating, and performance test results, refer to our Protective Coatings brochure.