Environmental Engineering Reference
In-Depth Information
Table 4.2
Wrought aluminum alloys and their corrosion behavior
Corrosion Resistance
b
Alloy
Typical
class
temper
a
Alloying elements
General
Pitting
Exfoliation
SCC
1xxx
All
Natural impurities
E
E
E
I
in refinery Al
2xxx
T3, T4, T8
Cu
F
P
P
VS
F
P
F
R
3xxx
All
Mn, Mn
Mg
E
E
E
I
4xxx
All
Si
F
G
G
G
5xxx
Most
Mn, Mg, Cr
E
G
G
I R
6xxx
All
Mg, Si
E
G
E
1
7xxx
T6, T73
Zn, Mg, Mn, Cu
F
F
F-P
S-VS
F
F
G
R
a
T3, T4, T6: age hardened; T8, T73: overaged.
b
E, excellent; G, good; F, fair; P, poor; I, immune; R, resistant; S, susceptible; VS, very susceptible.
Source
: Ref. 3.
equilibrium potential of the copper dissolution reaction is far more noble than
that of the cathodic hydrogen evolution reaction. However, the following cathodic
reactions are possible in the presence of oxygen and oxidizing agents, and anodic
dissolution of copper is facilitated (see Table 2.1):
O
2
4H
4e
2H
2
O
(2.7)
Fe
3
e
Fe
2
(2.8)
O
2
2H
2
O
4e
2OH
(2.11)
Copper and its alloys are, in general, resistant to seawater, neutral solutions, and
alkalis with the exception of ammonia which increases general corrosion through
complex formation, as well as SCC in alloys.
Commercially pure copper has traditionally been used for the handling of pota-
ble water and for roofing of churches and monuments. The green copper sulfate
formed on weathering on the roofs is termed ''patina,'' which provides its charac-
teristic aesthetic appearance. Pure copper, however, is soft and is very much
susceptible to impingement attack. Copper alloyed with zinc, tin, nickel, and
aluminum are stronger and are widely used; the nominal compositions of some
of these are given in Table 4.3.
Brass are copper-zinc alloys with zinc content ranging from 10% to 40%.
High brasses (with higher zinc content) are prone to dezincification and SCC in
ammonia. Addition of 1% Sn and a small amount of P, As, or Sb improves the
resistance of brass to dezincification. Bronzes are copper-tin alloys containing
