110 Fundamentals of Corrosion
the most important factors affecting the corrosion of zinc in the atmosphere
is the duration and frequency of moisture contact.
Because a thin lm of moisture with a high oxygen concentration promotes
corrosion, the rate of drying time is an important factor. Under normal expo-
sure conditions, the lms dry rapidly, and it is only in sheltered areas that
drying times are so slow that the attack of zinc is accelerated.
The effect of atmospheric humidity on the corrosion of zinc is related to the
conditions that may cause condensation of moisture on the metal surface and
to the frequency and duration of the moisture contact. Moisture is deposited
when the air temperature drops below the dew point. The quantity of dew
deposited is inuenced by the thickness of zinc, and its surface roughness
and cleanliness. When the temperature of the metal surface falls below the
air temperature in a humid atmosphere, moisture condenses on the metal.
If the water evaporates quickly, corrosion is not severe and a protective lm
forms 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 corro-
sion can appear to be severe because the formation of a protective basic zinc
carbonate is prevented.
Atmospheric corrosion of zinc is roughly proportional to the time of wet-
ness in a particular location, provided the nature and quantity of environ-
mental pollution do not change. A high relative humidity, which can cause
condensation, increases corrosion.
Time of wetness is normally secondary to the effect of atmospheric pol-
lutants, primarily sulfur dioxides and other acid forming pollutants, which
have a direct effect and can also make rain more acidic. In less corrosive
areas, time of wetness assumes a greater proportional signicance.
Sulfur dioxide is a primary pollutant leading to the atmospheric corrosion
of zinc. It controls the corrosion rate when the relative humidity is in the
area of 70% or above. Sulfur oxides and other air pollutants are deposited on
zinc surfaces either by dry or wet deposition. Regardless of the method of
deposition, the sulfur dioxide deposited on the zinc surface forms sulfurous
or other strong acids, which react with the protective zinc oxide, hydroxide,
or basic carbonate lm to form zinc sulfate. The lm of protective corro-
sion products is destroyed by the acids, which reforms from the underlying
metal, causing the continuous corrosion by an amount equivalent to the lm
dissolved, and hence to the amount of sulfur dioxide absorbed. Corrosion
rates increase even further when the relative humidity exceeds 85%.
Atmospheric chlorides will lead to the corrosion of zinc, but to a lesser
degree than the corrosion of steel, except in brackish water and owing sea-
water. Any salt deposit should be removed by washing. Chlorides also have
a much smaller effect on the corrosion rate of zinc than sulfur compounds.
The salt content of the atmosphere will usually decrease rapidly inland fur-
ther away from the coast. Corrosion also decreases with distance from the
coast but the change is more gradual and erratic because chloride is not the
primary pollutant affecting zinc corrosion. Chloride is most harmful when