restricted and the humidity is high, the resulting corrosioncan appear to be
severe (wet-storage stain known as “whiterust”) because the formationofa
protective basic zinc carbonate is prevented.
In areas havinga tmospheric pollutants, particularly sulfur oxides and
othera cid forming pollutants, time of wetness becomeso fs econdary
importance. These pollutants can also makerain more acid. However,inless
corrosive areas, time of wetness assumes agreater proportional significance.
In the atmospheric corrosion of zinc, the most important atmospheric
contaminent to be considered is sulfur dioxide. At relative humidities of
about 70% or above, it usually controls the corrosion rate.
Sulfur oxides and other corrosive speciesr eact with the zinc coating in
two ways: dry deposition and wetd eposition. Sulfur dioxide can deposit
on ad ry surfaceo fg alvanized steel panels until am onolayer of SO 2 is
formed. In eithercase,the sulfur dioxide that deposits on the surface of the
zinc formsasulfurous or other strong acidt hat reacts with the film of zinc
oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of
sulfur dioxidet os ulfur-baseda cids mayb ec atalyzedb yn itrogen
compounds in the air (NO x compounds). Thisf actor may affect corrosion
rates in practice.T he acids partially destroy the film of corrosion products
thatw illt henr eformf romt he underlying metal, thereby,c ausing
continuous corrosion by an amount equivalent to the film dissolved,
hencet he amount of SO 2 absorbed.
Chloridec ompoundsh avel esse ffectt hans ulfurc ompounds in
determiningt he corrosionr ate of zinc. Chloride is most harmful when
combined with acidity because of sulfur gases. This is prevalent on the coast
in highly industrial areas.
Atmospheric chloridesw ill lead to the corrosiono fz inc, but to al esser
degree than the corrosion of steel, except in brackish water and flowing
seawater.Any salt deposit should be removed by washing.The salt content
of the atmosphere will usuallydecrease rapidly inland farther 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 mosth armful when combined with
acidity resulting from sulfur gases.
Other pollutantsa lso have an effect on the corrosiono fg alvanized
surfaces. Deposits of soot or dust can be detrimental because they have the
potential to increase the risk of condensation onto the surfaceand hold more
water in position. This is prevalent on upward-facing surfaces. Soot (carbon)
absorbs large quantities of sulfur that arer eleased by rainwater.
In rural areas, overmanuring of agricultural land tends to increase the
ammonia content of the air.The presence of normal atmospheric quantities
of ammonia does not accelerate zinc corrosion, and petrochemical plants
where ammonium salts arep resents hown oa ccelerated attack on
galvanized steel. However,a mmonia will reactw ith atmospheric sulfur
oxides, producing ammonium sulfate that accelerates paintfilm corrosion as
wella sz inc corrosion. When ammonium reacts with NO
K
x compounds in
Corrosion of Linings and Coatings
444
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
severe (wet-storage stain known as “whiterust”) because the formationofa
protective basic zinc carbonate is prevented.
In areas havinga tmospheric pollutants, particularly sulfur oxides and
othera cid forming pollutants, time of wetness becomeso fs econdary
importance. These pollutants can also makerain more acid. However,inless
corrosive areas, time of wetness assumes agreater proportional significance.
In the atmospheric corrosion of zinc, the most important atmospheric
contaminent to be considered is sulfur dioxide. At relative humidities of
about 70% or above, it usually controls the corrosion rate.
Sulfur oxides and other corrosive speciesr eact with the zinc coating in
two ways: dry deposition and wetd eposition. Sulfur dioxide can deposit
on ad ry surfaceo fg alvanized steel panels until am onolayer of SO 2 is
formed. In eithercase,the sulfur dioxide that deposits on the surface of the
zinc formsasulfurous or other strong acidt hat reacts with the film of zinc
oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of
sulfur dioxidet os ulfur-baseda cids mayb ec atalyzedb yn itrogen
compounds in the air (NO x compounds). Thisf actor may affect corrosion
rates in practice.T he acids partially destroy the film of corrosion products
thatw illt henr eformf romt he underlying metal, thereby,c ausing
continuous corrosion by an amount equivalent to the film dissolved,
hencet he amount of SO 2 absorbed.
Chloridec ompoundsh avel esse ffectt hans ulfurc ompounds in
determiningt he corrosionr ate of zinc. Chloride is most harmful when
combined with acidity because of sulfur gases. This is prevalent on the coast
in highly industrial areas.
Atmospheric chloridesw ill lead to the corrosiono fz inc, but to al esser
degree than the corrosion of steel, except in brackish water and flowing
seawater.Any salt deposit should be removed by washing.The salt content
of the atmosphere will usuallydecrease rapidly inland farther 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 mosth armful when combined with
acidity resulting from sulfur gases.
Other pollutantsa lso have an effect on the corrosiono fg alvanized
surfaces. Deposits of soot or dust can be detrimental because they have the
potential to increase the risk of condensation onto the surfaceand hold more
water in position. This is prevalent on upward-facing surfaces. Soot (carbon)
absorbs large quantities of sulfur that arer eleased by rainwater.
In rural areas, overmanuring of agricultural land tends to increase the
ammonia content of the air.The presence of normal atmospheric quantities
of ammonia does not accelerate zinc corrosion, and petrochemical plants
where ammonium salts arep resents hown oa ccelerated attack on
galvanized steel. However,a mmonia will reactw ith atmospheric sulfur
oxides, producing ammonium sulfate that accelerates paintfilm corrosion as
wella sz inc corrosion. When ammonium reacts with NO
K
x compounds in
Corrosion of Linings and Coatings
444
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485