Environmental Engineering Reference
In-Depth Information
sion are described in subsequent sections. Biologically influenced corrosion has
been dealt with in a separate section.
3.2 GENERAL CORROSION
Corrosive attack in many cases is distributed uniformly or, rather, without any
preference for sites throughout the surface of the exposed metal. A piece of zinc
dissolving in dilute hydrochloric acid (Section 2.1.1), a steel tank or steel roof
rusting in atmosphere are examples of such corrosion. The metal thins down
uniformly and eventually fails if no protective measure is taken.
Atmospheric corrosion is a kind of general corrosion. It takes place under
damp and wet conditions. Relative humidity is an important factor in atmospheric
corrosion. For iron and steels, rust begins to form above a relative humidity of
60%. Even an invisible thin film of moisture formed on the metal surface can
serve as an electrolyte. The aggressiveness of the electrolyte increases with dis-
solved salts in marine atmospheres or dissolved sulfur dioxide (SO
2
) in industrial
atmosphere. Dust particles can be very detrimental as they absorb water and retain
it for a longer time on the metal surface. Chloride-contaminated dust particles
may be instrumental in breaking down the protective surface films on metals like
aluminum and stainless steels and thus initiate corrosion. Temperature is another
important factor. The rate of corrosion increases with the increase of surface
temperature, but at a temperature where the electrolyte starts evaporating the
corrosion rate falls sharply.
The general corrosion in some metals tends to slow down or stop as some
protective film develops on the metal surface after the initial attack. Aluminum
forms an oxide film in normal atmospheric exposures or in any oxidizing medium.
Copper forms a protective surfate ''patina'' when exposed to industrial atmo-
spheres containing SO
2
or a basic copper chloride film in seacoast environments.
Lead also forms a protective sulfate film when exposed to SO
2
-containing atmo-
spheres or waters.
The general corrosion may increase with the passage of stray currents from
the external sources through the structures (stray current corrosion) or in contact
with a nobler metal (galvanic corrosion). These are described in Sections 4.4.3
and 3.3, respectively.
3.2.1 Remedial Measures
The protective measure against general corrosion is often the provision of corro-
sion allowance in wall thickness in the designing of a tank or pipe. All corrosion
rate measurements and corrosion rate expressions are based on the tacit assump-
tion of uniform attack of the metal surface and uniform thinning of the compo-
nent. So a pipe showing a corrosion rate of 20 mils penetration/year (mpy) in a
