340 Fundamentals of Corrosion
3. After the carbon bed, pass the treated air through a nal lter to
remove the micrometer-sized particulates.
Many older control rooms and motor control centers cannot be maintained
under positive pressure because of air leakage and the inability to limit
access. Under these conditions, an installation similar to the aforementioned
can be incorporated on a recirculating and make-up air system.
When these systems are designed, care must be taken in the selection of
the construction materials for the various components and to make sure that
they are compatible with the corrodents to be encountered. Installations of
this type greatly reduce maintenance costs and downtime on control equip-
ment and electrical components.
11.3.1.5 Protection of Carbon Steel
Carbon steel is the most widely used material of construction. It is the pri-
mary choice for bridges, towers, various types of structures, and other types
of outdoor construction. In nature, iron is found in the form of various oxides,
which are used as ores for rening to make steel. Steels in atmospheric service
therefore tend to return to their oxide form by the process known as rusting.
The rusting of steel is the most common form of corrosion that the general
public sees, but it is very complex, having over a dozen variables determin-
ing the rate of corrosion. Water is the most common corrosive solvent.
Atmospheric corrosion of steel is a function of location. In country air, the
products of corrosion are either oxides or carbonates; in an industrial atmo-
sphere, sulfuric acid is present, producing iron sulfate; and near the ocean,
some salt is in the air, producing iron chlorides. Corrosion takes place more
rapidly in industrial areas because of the presence of the acid, and it is higher
both near cities and near the ocean because of the higher electrical conductiv-
ity of the rain and the tendency to form soluble chlorides or sulfates, which
cause the removal of protective scale.
Except in the most innocuous atmospheres, iron and steel must be protected
from corrosion by one of several means. Temporary rust preventatives, gal-
vanizing, painting, or other protective coatings may be employed, depend-
ing on the severity of the atmosphere.
Weathering steels, such as ASTM standards A-242 and A-588, are low-
alloy steels of relatively high strength that form self-protecting rust
lms when exposed to many industrial as well as uncontaminated atmo-
spheres. The steels contain small amounts of copper, nickel, phosphorus,
silicon, manganese, or various combinations thereof that impart corro-
sion resistance.
The corrosion resistance of these steels depends on the climatic conditions,
pollution levels, degree of sheltering from the atmosphere, and specic com-
position of the steel.