236
Fundamentals of Corrosion
1. Pretreatment (degreasing and phosphating) by dipping or spraying
2. Primer coating by cathodic electrodeposition
3. Sealing by blow gun or undercoating by air spray
4. Surface conditioning by flatting
5. Intermediate spray coating by auto-electrostatic powder process or
spraying
6. Surface conditioning by flatting and wax injection
7. Top paint coating by auto-electrostatic powder process, as by spray
The paint system for auto bodies is composed of a combination of various
types of paint and effective coating processes, thereby providing optimal
corrosion protection and decorative appearance.
The paint system for a steel structure is selected based on the required service life and environmental conditions under which the steel structure must
exist. Structures in mild environments are commonly coated with general
coating systems. Those in severe environments are treated with heavy-duty
coating systems. Typical paint coating systems for steel structures are given
in Table 7.11.
Anodic reactions
Deposition of film
2H 2 O
4H
+
O 2
4e
–
+
+
Anodic reactions
Deposition of film
2H 2 O
4H
+
O 2
4e
–
+
+
Cathodic reactions
Cathodic reaction
2H 2 O
4OH
–
2H
+
H 2
2e
–
O 2 4e
– +
+
+
2H 2
H 2
2e
–
+
(–)
(+)
(–)
(+)
O 2
H 2
OH
–
Deposition
of coating
film
Deposition
of coating
film
Anodic
Cathodic
H +
H 2 O
H 2 O
O 2
H 2
OH
–
H
+
(a)
(b)
FigurE 7.4
(a) Anodic and (b) cathodic electrodeposition of paints.