Page 301 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MOTOR VEHICLES 279

the treatment of vehicle parts with a mixture of zinc and phosphoric acid and some
proprietary additives to clean the surface for painting, which leave a thin layer of
zinc phosphate coating. A good understanding and control of parameters of the phos-
phating process has led to improved coatings for better corrosion resistance and paint
adhesion. The systems have been optimized for vehicles with mixed-material bodies
such as aluminum, coated steel, and plastic.
The second step in the finishing process is the application of primer paint. Before
1975, air spray atomizers were used for painting, and this method led to good cov-
erage on the outside, but poor inside coverage, which resulted in corrosion. Later in
1976, PPG industries introduced a cathodic electrodeposition (ELPO) primer process,
which led to every location on the primed part covered with paint. Other advances
in primer technology are using thicker “high-build” primers for increased corrosion
protection and flaw-hiding capabilities.
The third step in the finishing process is body sealing and augmentation coatings.
The body joints and flanges undergo a finishing process involving body sealing and
augmentation coatings. Vehicles have their body joints and exposed flanges sealed to
reduce cosmetic and perforation corrosion.
Sealing is a robotic operation that ensures the quality of the operation. Many new
augmentation coatings have been developed over the past 35 years, and the sealing
process has become a robotic operation to ensure the quality of the sealing job. Several
augmentation coatings have been developed over the past 40 years to increase corro-
sion protection in particular areas of the motor vehicle. Some augmentation coatings
are antichip plastisols and urethane that are applied in the rear of the wheel house
before painting. A second augmentation coating is the use of waxes applied to the
interior body cavities. In early times, the waxes were applied with a handheld airless
probe spray. Now the waxes are applied using automated equipment, and this resulted
in increased rust-through corrosion resistance.
Application of a top coat is the final step in the finishing process. The topcoat is
applied for cosmetic reasons and has little effect on the corrosion performance of the
automobile. Advances over the last 40 years have led to better overall paint system
performance. The robotic processing and control equipment has led to more uniform
paint coverage and superior performance. Simplified vehicle design and optimization
of the painting process resulted in increased finish quality, which in turn increases
corrosion resistance.
Over the past 40 years, automobile engineers have improved the design to reduce
the extent of corrosion. The design improvements consisted of removing crevices
and locations where salt and soil can accumulate. Dissimilar metal contacts were
removed. The number of “nose over” hoods, hood louvers, tuck-under areas, and
other design features that promote chipping and corrosion have been reduced.
These changes as well as material and process changes have resulted in increased
corrosion resistance of North American cars. At the present time, automobiles in high
corrosion areas are driven for 6 or more years with no sign of corrosion in comparison
to duration of 2–3 years in the mid-1970s.
Some field data on corrosion defects such as: (i) perforation; (ii) surface rust;
(iii) blistering, and (iv) other defects are given in Table 4.37.