86 Fundamentals of Corrosion
rates. When pockets are present, the time of wetness increases, which leads
to corrosive conditions. The design should make maximum use of exposure
to the weather. Sheltering from rain should be avoided.
4.2.6 Wind Velocity
Wind speed and type of wind ow have pronounced effects on the atmo-
spheric corrosion rate. This is illustrated by the dry deposition velocity,
which is dened as the ratio of deposition rate of any gaseous compound
and the concentration of that compound in the atmosphere.
4.2.7 Nature of Corrosion Products
The atmospheric corrosion rate is inuenced by many parameters, one of
the more important being the formation and protective ability of the corro-
sion products formed. The composition of the corrosion products depends
on the participating dissolved metal ions and the access to anions solved in
the aqueous layer. The eventual thickening of the lm of corrosion products
can be described in a sequence of consecutive steps — dissolution → coordi-
nation → reprecipitation — where the dissolution step is acid dependent, the
coordination is based on the hard and soft acid–base (HSAB) principle, and
reprecipitation depends on the activities of the species involved.
Depending on the rate of crystallation and the rate of formation, the cor-
rosion products may be amorphous or crystalline. If the former is rate deter-
mining, one expects amorphous phases to form. From colloid chemistry it is
known that aging, or slow growth, of amorphous phases can result in a tran-
sition from the amorphous to the crystalline state, a process that may occur
through slow transformation in the solid state or through the dissolution–
reprecipitation process.
The corrosion products formed are the composition of the metal or alloy
and the contaminants present in the atmosphere. For example, carbon steel
does not have the ability to form its own protective coating except in a dry,
clean atmosphere. Under this condition, a thick oxide lm will form that pre-
vents further oxidation. Initiation of corrosion is the result of the presence of
solid particles on the surface. This settled airborne dust promotes corrosion
by absorbing SO
2
and water vapor from the air. Even greater corrosive effects
result when particles of hygroscopic salts, such as chlorides or sulfates, settle
on the surface and form a corrosive electrolyte.
When small amounts of copper, chromium, nickel, phosphorus, silicon,
manganese, or various combinations thereof are added to conventional car-
bon steel, low-alloy carbon steel results. These steels are known as weathering
steels. The corrosion resistance of these steels depends on climatic condi-
tions, pollution levels, the degree of sheltering from the atmosphere, and the
specic composition of the steel. Upon exposure to most atmospheres, the
corrosion rate becomes stabilized within 3 to 5 yr. A dark brown to violet