284 Fundamentals of Corrosion
Zinc coatings resist atmospheric corrosion by forming protective lms con-
sisting of basic salts, notably carbonate. The most widely accepted formula
is 3Zn(OH)
2
2ZnCO
3
. Environmental conditions that prevent the formation
of such lms, or conditions that lead to the formation of soluble lms, may
cause rapid attack on the zinc.
The duration and frequency of moisture contact is one such factor. Another
factor is the rate of drying because a thin lm of moisture with high oxygen
concentration promotes reaction. For normal exposure conditions, the lms
dry quite rapidly. It is only in sheltered areas that drying times are slow, so
that the attack on zinc is accelerated signicantly.
The effect of atmospheric humidity on the corrosion of zinc is related to
the conditions that may cause condensation of moisture on the metal sur-
face, and to the frequency and duration of the moisture contact. If the air
temperature drops below the dew point, moisture will deposit. The thick-
ness of the piece, its surface roughness, and its cleanliness will also inuence
the amount of dew deposited. Lowering the temperature of a metal surface
below the air temperature in a humid atmosphere will cause moisture to
condense onto the metal. If the water evaporates quickly, corrosion is usually
not severe and a protective lm is formed on the surface. If water from rain
or snow remains in contact with zinc when access to air is restricted and the
humidity is high, the resulting corrosion can appear to be severe (wet storage
stain, known as “white rust”) because the formation of a protective basic zinc
carbonate is prevented.
In areas having atmospheric pollutants, particularly sulfur oxides and
other acid-forming pollutants, time of wetness becomes of secondary impor-
tance. These pollutants can also make rain more acidic. However, in less cor-
rosive areas, time of wetness assumes a greater proportional signicance.
In the atmospheric corrosion of zinc, the most important atmospheric con-
taminant to consider is sulfur dioxide (SO
2
). At relative humidities of about
70% or above, it usually controls the corrosion rate.
Sulfur oxides and other corrosive species react with the zinc coating
in two ways: (1) dry deposition and (2) wet deposition. SO
2
can deposit
on the dry surface of galvanized steel panels until a monolayer of SO
2
is
formed. In either case, the SO
2
that deposits on the surface of the zinc forms
a sulfurous or other strong acid, which reacts with the lm of zinc oxide,
hydroxide, or basic carbonate to form zinc sulfate. The conversion of SO
2
to sulfur-based acids may be catalyzed by nitrogen compounds in the air
(i.e., NO
x
compounds). This factor may affect corrosion rates in practice.
The acids partially destroy the lm of corrosion products, which will then
reform from the underlying metal, thereby causing continuous corrosion
by an amount equivalent to the lm dissolved, and hence the amount of
SO
2
absorbed.
Chlorine compounds have less effect than sulfur compounds in determining
the corrosion rate of zinc. Chloride is most harmful when combined with acid
due to sulfur gases. This is prevalent on the coast in highly industrial areas.