Electrocoating Process

ELECTROCOATING PROCESS
Introduction
Electrocoating, which is also referred to as electrodeposition, electrophoretic
deposition, or electropainting, is an organic finishing process that uniformly
applies
thin-film
primers
and
one-coat
finishes
to
metallic
substrates.
Electrocoating resembles electroplating in that it utilizes an electric current to

deposit a coating onto substrates. However, electrocoating deposits waterborne
paint onto substrates rather than metal ions. The overall process consists of four
main process steps which are pretreating, electrocoating, rinsing, and baking.
Substrate surfaces are commonly prepared for electrocoating by cleaning,
conversion coating, sealing, drying, and cooling. During electrocoating, an
electrical charge (direct current or DC) is applied to during immersion in a
waterborne paint bath. During the process, the part becomes the cathode (in
cathodic e-coat baths) and there are a number of anodes spaced evenly
throughout the tank. Paint molecules then travel to the substrate and adhere to
the surface. Adequate bath agitation is necessary during the process to prevent
bath separation and to ensure a uniform concentration of bath constituents.
Coating thickness can be precisely controlled, and is dependent on immersion
time, bath temperature, process voltage, and bath chemistry. Substrates are
typically immersed for 90 to 360 seconds at a bath temperature between 60 to
80F (15 to C). The DC power supply voltage typically ranges from 90 to 500V.
Processes
The electrocoat process can be divided into four distinct steps:
1. Pretreatment
2. Electrocoat Bath
3. Post Rinses
4. Baking
1. Pretreatment - cleaning and phosphating the metal
The pretreatment zone cleans and phosphates the metal to prepare the surface
for electrocoating. Cleaning and phosphating are essential to achieving the
performance requirements desired by today's end user of the product. A high
quality zinc phosphate system using the immersion method is primarily used
where steel and iron parts are to be coated.
2. Electrocoat Bath - applying coating in bath
The electrocoat bath and ancillary equipment zone is where the coating is applied
and the process control equipment operates. The electrocoat bath consists of 8090% deionized water and 10-20% paint solids. The deionized water acts as the
carrier for the paint solids which are under constant agitation. The solids consist
of resin and pigment. Resin is the backbone of the final paint film and provides
corrosion protection, durability and toughness. Pigments are used to provide color
and gloss.
3. Post Rinses - rinsing off excess paint solids
The post rinses provide both quality and conservation. During the electrocoat
process, paint is applied to a part at a certain film thickness, regulated by the
amount of voltage applied. Once the coating reaches the desired film thickness,
the part insulates and the coating process slows down. As the part exits the bath,
paint solids cling to the surface and have to be rinsed off to maintain efficiency
and aesthetics. The excess paint solids are called "drag out" or "cream coat."
These excess paint solids are returned to the tank to create a coating application
efficiency above 95%.
4. Bake Oven - thermally curing the paint film
The bake oven receives the parts after they exit the post rinses. The bake oven
cross links and cures the paint film to assure maximum performance properties.
The minimum bake schedule is 20 minutes with the part temperature at 375F for
most electrocoat technologies. However, there is also a "low temperature cure"
electrocoat material. This material has a minimum cure of 20 minutes at a part
temperature of 180F so that many assemblies containing seals, bushings,
bearings, or oil can use the electrocoat process.
Type of e-coating
The electrocoating process may be anodic or cathodic, depending on the charge
applied the substrate. Although the processes are virtually the same, properties
of the resultant coating are dissimilar. Anodic systems, which were the first to be
used for electrocoating, apply paint to positively charged substrates. The
negatively charged pigment and resin particles deposit onto the substrate
(anode). One disadvantage of this process is that substrate metals dissolve and
become incorporated into the coating, which affects surface properties. Cathodic
electrocoating deposits paint onto negatively charged substrates and offers
several advantages over anodic electrocoating. For example, metal dissolution of
the substrate does not occur, cathodic electrocoating has the ability to deposit
over contaminants, corrosion resistance is improved, and a better color
consistency occurs over welded areas.
1. Anodic electrocoating
In anodic electrocoating, the part to be coated is the anode with a positive
electrical charge which attracts negatively charged paint particles in the paint
bath. During the anodic process, a small amount of soluble iron can migrate away
from the part and into the paint film which results in a reduction of the
performance properties of these systems. Their main use is for products in
interior or moderate exterior environments. Anodic coatings are economical
systems and offer excellent color and gloss control.
2. Cathodic electrocoating
In cathodic electrocoating, the part to be coated is the cathode with a negative
electrical charge which attracts positively charged paint particles in the paint
bath. By reversing the polarities used in the anodic process, the amount of
soluble iron that enters the paint film during the cathodic process is greatly
reduced and the corrosion resistance properties of the paint film are improved.
Cathodic
coatings
are
high-performing
coatings
with
excellent
corrosion
resistance that can also be formulated to meet tough exterior durability

requirements.
References
1. Berger, D. M. and Wint, R. F. 1984.New Concepts for Coating Protection of Steel
Structures. ASTM Special Technical Publication 841, American Society for
Testing and
Materials.
2. Miles, M. & Peffer, R. 2007. Electrocoat Primers For Aerospace Industry. TriService
Corrosion Conference.

Electrocoating Process

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What are the main steps of the electrocoating process?

What is the difference between anodic and cathodic electrocoating?

ELECTROCOATING PROCESS

Electrocoating resembles electroplating in that it utilizes an electric current to

resistance that can also be formulated to meet tough exterior durability

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