Environmental Engineering Reference
In-Depth Information
water application if not protected otherwise because they develop pitting within
months. Austenitic grades are better than other grades in this respect, particularly
the molybdenum-containing varieties (e.g., AISI 316), but they too develop pit-
ting over a period of 1 to 2.5 years. An exception is 29Cr-4Mo grade, which is
comparable to titanium in its performance in seawater. Pitting in solutions con-
taining FeCl
3
, CuCl
2
, or HgCl
2
develops within hours at room temperature, but
this can be inhibited with the addition of NO
3
.
Stainless steels, both austenitic and ferritic, are susceptible to sensitization
and subsequent intergranular corrosion, though their thermal history is different
(Section 3.6).
Austenitic stainless steels are particularly susceptible to SCC in the presence
of Cl
and OH
. Ferritic and duplex stainless steels are resistant to SCC and are
used where it is a problem.
4.1.4 Aluminum and Its Alloys
Aluminum is a reactive metal, but it develops a thin protective oxide layer when
exposed to air or aerated environments that resists further corrosion. The oxide
is stable in the neutral or near-neutral ranges of aqueous solutions but is attacked
by strong acids and alkalis. Aluminum is a soft, light metal that can be rolled
down to very thin sheet or foil. Since aluminum does not rust or lose its luster
on atmospheric exposure and is resistant to mild acidic conditions, the sheets and
foils are used for architectural trims, vessels in food processing and pharmaceuti-
cal units, cans for storage and transportation of food and beaverages, and as a
packaging material.
The strength of aluminum is remarkably increased by alloying additions with
some sacrifice of corrosion resistance and these alloys constitute a wide range
of structural materials. The wrought alloys are designated by four-digit numbers,
i.e., 1000 to 7000 series depending on the alloying elements present (Table 4.2).
The 2000 series age-hardenable alloys containing about 4% copper possess the
highest strength and the lowest corrosion resistance, particularly to pitting and
SCC. The SCC is encountered in sodium chloride solution, seawater, and even
in air and water vapor. The susceptibility SCC can be reduced by overaging.
Alclading is another measure adopted to prevent SCC in these alloys. Aluminum
alloys have extensive applications in aircraft and aerospace vehicles, automobiles,
window and door frames, roofing, cold and hot storage vessels and pipings, food
processing equipment, and household cooking utensils.
4.1.5 Copper and Its Alloys
Copper is the noblest among the common structural metals. It exhibits good resis-
tance to atmospheric corrosion and aqueous environments. In reducing acids cop-
per does not corrode in the absence of oxygen or oxidizing agents because the
