Atmospheric Corrosion
95
Dissociative adsorption
of water.
Hydroxylated
metal oxide.
Formation of
aqueous layer.
Formation of
Me x (OH) z
or Me x O y (OH) z .
H
O
H
O
H
O
H
O
H
O
Acidification of
aqueous layer.
Stronger acidification
of aqueous layer.
H 2 SO 3
H 2 SO 3
SO 2
HSO 3
SO 2
SO 4
2–
oxidants
H 2 SO 4
H
+ + HSO 4
–
H
+ + HSO 3
–
H
+ + SO 3
2–
H
+ + SO 4
2–
H 2 SO 4
HSO 4
–
Reversible adsorption
of water.
Electrochemical reactions,
e.g.,
Deposition of atmospheric
species, e.g., SO 2 .
Dissolution of species,
Henry’s law.
Alt: Oxidation of atmospheric species, e.g., by
oxidants or catalytic action
or deposition of oxidized
species.
Me
1
2 O 2 H 2 O + 2e
–
2OH
– .
+
+
Me
n+
ne
–
Process
Result
FigurE 4.1
Schematic illustration of processes occurring in or at the aqueous layer. (Source: From Reference 1.)
TabLE 4.4
Classification of Hard and Soft Acids and Bases
Hard
Intermediate
Soft
Acids:
H + , Na + , Mn 2+ , Al 3+
Cu 3+ , Fe 3+ , T, 4+
Fe 2 , Ni 2+ , Cu 2 +, Zn 2 +Pb 2+
Cu + , Ag +
Bases:
H 2 O, OH – , O 2– , SO 4
2– , NO 3
– , Cu 3
2-–
SO 3
2– , NO 2
-
R 2 S, RSH, RS
Source: From Reference 3.