Page 300 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

278 CORROSION CONTROL AND PREVENTION

corrosion protection purposes. The zinc coating is smooth enough for painting pur-
poses.
Some coatings referred to as a composite or “piggy bank” coating consisting of a
thin layer of zinc or zinc alloy are applied on steel, and an organic coating is applied
over the zinc or inside surface. These steels have the corrosion protection capability
of electro-galvanized steel and increased perforation corrosion resistance because of
barrier coating.
Use of aluminum in the place of steel has two benefits, such as the lighter nature of
aluminum compared to steel and better corrosion resistance than steel. New designs
and aluminum alloys have resulted in some automobiles made entirely with aluminum
including the frame. Polymers have been used to a greater extent as body panels in
place of steel. Polymer panels are both corrosion and dent resistant. Automobiles
must be designed to use the polymer panels, as the panels do not aid in the structural
rigidity of the automobile.
Stainless steel use has increased over the years. Most of the exhaust system uses
stainless steel or aluminized stainless steel for corrosion resistance. Fuel systems are
made of galvanized or stainless steel. Comparison of a car made in 1978 to one made
in 1996 shows the reduction in the use of regular steel and iron from 68% to 55.5%,
respectively. The use of high-strength steel, stainless steel, plastics, aluminum, and
copper in cars has dramatically increased in the period 1978–1996. These materials
have replaced mild steel for greater strength, weight reduction, and corrosion resis-
tance.
Increased temperature and more aggressive environments present in today’s higher
performance automobiles have led to the use of high-performance alloys for some
critical components in automobiles such as flexible couplings used in exhaust sys-
tems.
To achieve higher engine efficiency and lower emissions, the exhaust system is
operated at higher temperatures leading to increased corrosion rates. At these operat-
ing high temperatures, the stainless steel couplings fail before the stipulated warranty
period of 10 years. High temperatures and high salt concentration along with move-
ment of the flexible couplings have led to failure because of fatigue, corrosion fatigue,
hot salt attack, chloride SCC, pitting, and general corrosion of 316 and 321 stainless
steels.
To overcome this corrosion problem, nickel-based super alloys such as Inconel
alloy 625 and Incoloy alloy 864 have been used. Although these alloys are more costly
than stainless steel, they have shown excellent corrosion resistance in the modern
automotive exhaust environment.
The automotive industry has found several other applications where the additional
cost of specialty may be beneficial. These areas are manifolds and tailpipes, cat-
alytic converters, high-temperature fasteners, exhaust valves, airbag inflators, and
other critical electrical components. The future use of these materials will depend
on the benefits found in service and the changes in automotive technology that affect
the corrosion conditions encountered by automobiles.
Many improvements have been made in the way that vehicles are finished. The
first step in finishing a vehicle is the clear phosphate process. This process involves