E-Coating (Electrocoating)
Also: e-coat, electrocoating, electrocoat, electrophoretic deposition, EPD, electropainting, cathodic electrocoat, CED coating, KTL coating, ED coating, Powercron, CathoGuard
E-coating deposits paint from a water bath onto a charged metal part: an even 15-30 µm epoxy or acrylic film that reaches inside tubes and recesses.
24 shops tagged E-coating among 3,387 finishing and heat treating companies in the Noramark directory
E-coating is painting by the rules of electroplating. The part is cleaned, usually zinc phosphated, and lowered into a tank of waterborne paint, an emulsion of resin and pigment at roughly 10-20% solids. In the cathodic process the part is the cathode; a DC voltage of roughly 200-400 V drives the charged paint particles to it, where they coagulate into a film. The film insulates as it grows, so the current shifts to the bare areas and deposition stops by itself at a set thickness. The part is rinsed with ultrafiltrate, which returns the undeposited paint to the tank, and baked at about 160-200 °C (320-390 °F) to cure.
That self-limiting behaviour is the point of the process. The film comes out 15-30 µm (0.6-1.2 mils) and nearly the same on a flat face, inside a box section, under a hem flange and down a tube, which spray paint and powder cannot match. The ability to coat recesses is called throwpower, and it is why car bodies and most truck frames, chassis parts, agricultural stampings and appliance brackets are e-coated. Transfer efficiency is above 95%, and one line runs thousands of parts a shift.
Cathodic epoxy is the standard chemistry: excellent corrosion resistance and adhesion over zinc phosphate, but it chalks and fades in sunlight within months, so it is a primer under a topcoat or a finish for parts kept out of the sun. Cathodic acrylic is UV-stable and serves as a one-coat finish on appliance, lawn and garden and hardware parts. Anodic e-coat, with the part as the anode, is older, cheaper and now uncommon. A tank holds tens of thousands of litres of one colour, so contract lines run black, sometimes grey, and little else.
At a glance
| Typical tolerances | Film 15-30 µm (0.6-1.2 mils), commonly held to about ±5 µm (±0.2 mil) of a stated nominal, and nearly uniform across faces, recesses and the inside of tubes. The film adds its full thickness to every coated surface: a 25 µm coat closes a hole by about 50 µm (0.002") on diameter. Sharp edges come out thinner than flats, which is where e-coated parts rust first. Precision bores, ground points and fine threads are masked with caps and plugs; fastener and spring lines often coat threads that were sized to take the film. |
|---|---|
| Size limits | Contract lines are conveyorized; a typical hanger takes parts up to about 3-4 m (10-13 ft) long, 1.5-2 m (5-6.5 ft) tall and a few hundred kilograms. Automotive lines take whole car bodies. Small parts hang by the hundred on racks, and every part needs an electrical contact point. Parts must drain and must not trap air: an air pocket leaves a bare spot, and a trapped pool of paint drips and bakes into a run. |
| Surface finish | A smooth, thin, satin to semi-gloss film that follows the underlying surface: it does not hide machining marks, weld spatter or scratches. Black is standard; grey and a few colours exist on some lines, and custom colours generally do not. Epoxy e-coat over zinc phosphate on steel commonly gives 500-1,000 h of ASTM B117 salt spray; it chalks and dulls outdoors within months unless topcoated. Acrylic e-coat keeps its colour and gloss in sunlight. E-coat under a powder topcoat is the premium system for outdoor equipment. |
| Lead time | 3-7 business days at a contract line for black e-coat on standard hangers; 1-2 weeks for e-coat plus a powder or liquid topcoat. Lines run to a schedule, so small lots wait for a slot and carry a minimum charge. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Any conductive metal that survives the bake: cold-rolled and hot-rolled steel (pickled, or blasted to remove mill scale), galvanized and zinc-plated steel, aluminum with a zirconium or phosphate pre-treatment, zinc and aluminum die castings, and cast iron. Laser-cut edges carrying oxide scale from oxygen-assist cutting must be blasted, or cut with nitrogen, or the coating flakes off with the scale. Stainless takes it but rarely needs it. Plastics, rubber, and assemblies with seals, bearings or electronics cannot be e-coated: they do not conduct or will not survive 175 °C (350 °F).
What drives the cost
- Hanger or rack density: lines charge by the rack position or by surface area, so parts that hang close together cost less
- Pre-treatment: removing mill oil, laser scale, rust and weld spatter ahead of the zinc phosphate stage
- Masking of bores, threads, ground surfaces and electrical contact points
- Geometry: parts that trap air or paint need vent holes, special orientation or hand work
- Lot size: small orders pay a minimum charge; e-coat is cheapest at volume
- A topcoat (powder or liquid) as a second process
- Testing and certs: film thickness, cross-hatch adhesion (ASTM D3359), salt spray (ASTM B117), OEM specifications
When to use it
- Welded tube frames, box sections and assemblies whose insides need protection that spray paint and powder cannot reach
- High-volume stampings, brackets and chassis parts for automotive, truck and agricultural equipment
- A thin, uniform film on clips, springs, fasteners and parts with moderate fits, where powder is too thick
- A corrosion primer under powder coat or wet paint on outdoor equipment
- Parts with hem flanges, spot-welded seams and lap joints that trap moisture
When not to
- Custom colours or cosmetic finishes in small lots: most lines run black only
- Epoxy e-coat as the only finish on parts in direct sunlight; topcoat it or use acrylic e-coat
- Heavy abrasion or impact service, where a 20 µm film wears through; use powder, or e-coat plus powder
- Non-conductive parts, or assemblies with plastics, seals or electronics that will not survive the bake
- Parts that cannot be vented and drained, where trapped air leaves bare spots and pooled paint runs
Design tips
- Specify the chemistry, colour, film range, pre-treatment and performance test: "Cathodic epoxy e-coat, black, 18-30 µm, over zinc phosphate, 500 h ASTM B117".
- Add vent and drain holes at both ends of closed sections so the tank fills and empties completely and no air is trapped.
- Give each part a hang point: a hole or tab that can carry a hook and a small bare mark.
- Cut steel with nitrogen assist or blast the edges; oxide scale from oxygen-assist laser cutting flakes off and takes the coating with it.
- Break sharp edges to at least 0.5 mm (0.02"); the film pulls thin on a sharp corner and rust starts there.
- Mark every surface that must stay bare: ground points, bearing bores, and threads that cannot take 25 µm.
- Order e-coat before powder, not after: powder bonds well to cured e-coat, but a powder-coated part no longer conducts and cannot be e-coated.
Frequently asked questions
- What is e-coating?
- E-coating (electrocoating) is painting by electric current. A pre-treated metal part is dipped in a waterborne paint bath and charged; paint particles deposit on it until the film insulates the part, then stop. The result is a thin, uniform film, inside and out, cured in an oven.
- E-coating vs powder coating: which is better?
- E-coat is thinner (15-30 µm), more uniform and reaches inside tubes and recesses. Powder is thicker (50-100 µm), tougher, UV-stable in polyester, and available in any colour. For outdoor steel the best of both is e-coat as the primer with a powder topcoat.
- How thick is e-coating?
- Typically 15-30 µm (0.6-1.2 mils), with 20-25 µm the usual target. It adds that thickness to every coated surface, so holes close by about twice the film thickness on diameter.
- Is e-coat UV resistant?
- Epoxy e-coat is not: it chalks and fades in sunlight within months, which is why it is used as a primer or on hidden parts. Acrylic e-coat is UV-stable and can serve as a one-coat outdoor finish.
- Can you powder coat over e-coat?
- Yes, and it is a common premium system: the e-coat protects edges and recesses, and the powder adds thickness, colour and UV resistance. The reverse does not work, because a powder-coated part no longer conducts and cannot be e-coated.
