Corrosion of Paint 235
where R symbolizes any of the widely used polymers (such as acrylics, epox-
ies, alkyds, etc.). The electrodeposition process is dened as the utilization of
synthetic, water-dispersed, electrodepositable macroions.
Metal ions, typically 0.5Ni
2+
, show an electrical equivalent weight equal to
approximately 29.5 g while the polymeric ions typically used for electrode-
position exhibit a gram equivalent weight (GEW) of approximately 1600.
Therefore, IF plates out 30 g nickel and deposits 1600 g macroions.
The formation of uniformly thick coats on all surfaces of a formed work
piece, including the extreme recesses, such as the inside of car doors, is prob-
ably the reason for the rapid industrial growth of this process. The ability to
extend coats into recesses is known as “throwing power.”
Another advantage is the very small emission of volatile organic com-
pounds (VOCs), making electrodeposition with powder coating and radia-
tion cure the least polluting coating process.
The anodic deposition process for paint coating systems was introduced in
the early 1960s, and the cathodic deposition process in 1972. Electrodeposition
processes are widely used because they possess the advantages of unmanned
coating, automation, energy savings, and lower environmental pollution.
This process is used to apply coatings to automotive bodies and parts, domes-
tic electrical components, machine parts, and architecturals such as window
frames. Schematic illustrations of anodic and cathodic electrodeposition of
paints are shown in Figure 7.4.
The primary paints used in the electrodeposition process are anionic-type
resins with a carboxyl group (RCOOH polybutadiene resin) and cationic-type
resins (R-NH
2
epoxy resin). Hydrophilic groups and neutralizing agents are
added to the water-insoluble or undispersed prepolymers to convert them to
soluble or dispersed materials.
The dissolution of metal substrate in the cathodic process is much less than
that in the anodic process. The primary resins used in the cathodic process are
epoxy; and because epoxy resins provide good water and alkali resistance as
well as adhesion, cationic paint coatings are superior to anodic paint coatings.
7.7.2 Multilayer Coatings
The thicker a coating, the better the protective ability. However, the thickness
of a single coat is restricted because thick paint lms tend to crack as a result
of the internal stress. When a product is to be used for an extended period
of time in a severe environment, multilayer paint coating systems are usu-
ally employed. Automotive bodies and steel structures are typical products
receiving multilayer coatings. A two- or three-coat system is employed for
automotive bodies, and a general or heavy-duty coating system is adopted
for steel structures. A typical paint coating system for an automotive body
is as follows: