Electrocoating is a process that uses an electrical current to deposit an organic coating from a paint bath onto a part or assembled product. Due to its ability to coat complex parts and assembled products with specific performance requirements, electrocoat is used worldwide throughout various industries to coat a wide swath of products including those in automotive, appliance, marine and agriculture. Electrocoat, and specifically cathodic electrocoat, has enabled a dramatic improvement in corrosion resistance over that offered by anodic electrocoat or other more conventional methods of coating.
Positively charged cathodic paint is deposited from a bath containing electrocoat paint at the cathode (-) where reduction takes place. Negatively charged anodic ecoat takes place at the anode (+) where oxidation (corrosion) takes place. In a cathodic electrocoat tank, positively charged paint particles migrate to the oppositely charged cathode (substrate) and are neutralized by the electrolysis of water. The paint particles are deposited on the surface of the electrically conductive substrate to form an insulating film. The coated substrate is then baked to eliminate water and remaining volatile as well as to crosslink the polymer film.
Electrolysis of Water
The advantages and disadvantages of anodic electrocoat include:
- The ability to penetrate and coat interior portions of complex products and parts
- Poorer corrosion resistance than cathodic electrocoat as it traps acid at the metal/paint surface
- Oxidizes or corrodes the metal surface during the deposition process
The advantages and disadvantages of cathodic electrocoat include:
- Normally must be top-coated to provide a variety of colors and improved resistance to UV degradation
- Excellent corrosion resistance, even at lower film thicknesses
- Offers excellent resistance to bimetallic corrosion (when dissimilar metals are in direct contact)
Many cationic electrocoat resins are comprised of a bisphenol A (EU)-based epoxy resin (EU) comprised of amine groups that are neutralized with a low molecular weight acid such as formic, acetic or lactic acid. Since the coating bath has a pH of slightly below 7, bath components are comprised of stainless steel or other corrosion resistant materials. The most common crosslinker is a blocked isocyanate (EU) crosslinker, so once the coating is baked, the blocked isocyanate is activated and reacts with available hydroxyl (EU) and amine groups. Other components of a typical electrocoat bath include pigment, filler pigment, water, solvent and a low level of modifying resins such as plasticizers (EU) and flexibilizers (EU), flow modifiers (EU) and catalysts (EU).
Ecoat is used because it provides superior corrosion protection as it coats surfaces that are inaccessible by conventional means. Film thickness is uniform without any defects such as sags, runs or edge beads. Electrocoat is also very cost effective as it provides nearly 100% material utilization with good energy efficiency and a relatively low cost per square foot of applied coating.
Throwpower is the ability of an electrocoat to penetrate into “hard to reach” areas, such as the inside of a hollow metal object. Dependent on applied voltage, bath solids, conductivity, deposition time, bath temperature, solvent levels, and proper tank agitation, deposition time, throwpower and coating appearance can be optimized. A simple dip-applied coating cannot effectively coat the interior of complex shaped parts as during the bake process the water/solvent has a washing effect in the interior portions of the part that prevents adequate film build. At the time an electrocoated object is removed from the bath, most of the water and solvent is squeezed from the electrocoat so that during the bake the washing effect is minimal as compared to that of a simple dip-applied coating. The film build of electrocoat paint is self-limiting as the film becomes more insulative as the thickness of the film approaches its maximum. Higher voltage and longer immersion times will permit higher film builds until the maximum possible film build is reached which is normally about 1.0 mil and 1.2 mils. Voltage is normally between 225 and 400 volts. If the voltage is too high, there will be film rupture of the coating applied to the outer surfaces. This is called the rupture voltage. At a sufficiently high voltage, the current will break through the film, leading to gas generation under the film (Hydrogen for cathodic and oxygen for anodic). Other factors that affect film build include bath temperature and conductivity. Immersion times are normally on the order of 2-3 minutes.
Some considerations when choosing electrocoat include the fact that in most cases electrocoat needs to be top-coated to provide the best combination of corrosion resistance, light stability and color availability. In addition, capital cost, tank maintenance and tank turnover rates must be carefully considered.
In summary, a properly chosen and applied electrocoat can provide a corrosion resistant, near 100% application efficient, uniform defect-free film.
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An excellent article explains ecoat chemistry clearly and thoroughly