7.4.1.6 Zinc Dust Painting
Zinc dust paints may be used alone for protection or as aprimer followed by
conventional topc oats. More details will be found in Chapter8.
7.4.1.7 Corrosion of Zinc Coatings
In general, zinc coatings corrode in asimilar manner as solidzinc. However,
there are some differences. For example, the iron–zinc alloy present in most
galvanized coatings has ah igher corrosionr esistance than solid zinc in
neutral andacid solutions. At points where the zinc coating is defective, the
bare steel is cathodically protected undermostconditions.
The corrosion of zinc coatings in air is an approximate straight line
relationship betweenw eight loss and time. Because the protective film on
zinc increases with time in rural and marine atmospheres of some types,
undert hese conditions, the life of the zinc may increase more than in
proportion to thickness. However, this does not always happen.
Zincc oatingsa re primarily used to protect ferrousp arts against
atmospheric corrosion. These coatings have good resistance to abrasion by
solid pollutants in the atmosphere.G eneralp oints to consider are:
1. Corrosion increases with time of wetness.
2. The corrosion rate increases with an increase in the amount of
sulfur compounds in the atmosphere.C hlorides and nitrogen
oxides usually have alessereffect but are often very significant in
combination with sulfates.
Zinc coatings resist atmospheric corrosion by forming protective films
consisting of basic salts, notably carbonate. The most widely accepted
formula is 3Zn(OH) 2 $ 2ZnCO 3 .E nvironmental conditions that prevent the
formation of such films, or conditions that lead to the formation of soluble
films, may cause rapid attacko nt he zinc.
The duration and frequency of moisture contact is one such factor.
Another factor is the rate of drying because athin film of moisture with high
oxygenc oncentration promotes reaction. For normal exposure conditions,
the films dry quite rapidly.Itisonly in sheltered areas that drying times are
slow so that the attacko nz inc is accelerated significantly.
The effect of atmospheric humidity on the corrosion of az inc coating is
related to the conditions that may cause condensation of moisture on the
metal surfacea nd to the frequency and duration of the moisture contact. If
the air temperature drops below the dew point,moisture will be deposited.
The thickness of the piece, its surfacer oughness, and its cleanliness also
influence the amount of dew deposited. Lowering the temperature of ametal
surfaceb elow the air temperature in ah umida tmosphere willc ause
moisture to condense on the metal.Ifthe water evaporates quickly,corrosion
is usually not severe, and aprotective film is formed on the surface. If water
from rain or snow remains in contactw ith zincw hena ccess to air is
Coatings
443
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
Zinc dust paints may be used alone for protection or as aprimer followed by
conventional topc oats. More details will be found in Chapter8.
7.4.1.7 Corrosion of Zinc Coatings
In general, zinc coatings corrode in asimilar manner as solidzinc. However,
there are some differences. For example, the iron–zinc alloy present in most
galvanized coatings has ah igher corrosionr esistance than solid zinc in
neutral andacid solutions. At points where the zinc coating is defective, the
bare steel is cathodically protected undermostconditions.
The corrosion of zinc coatings in air is an approximate straight line
relationship betweenw eight loss and time. Because the protective film on
zinc increases with time in rural and marine atmospheres of some types,
undert hese conditions, the life of the zinc may increase more than in
proportion to thickness. However, this does not always happen.
Zincc oatingsa re primarily used to protect ferrousp arts against
atmospheric corrosion. These coatings have good resistance to abrasion by
solid pollutants in the atmosphere.G eneralp oints to consider are:
1. Corrosion increases with time of wetness.
2. The corrosion rate increases with an increase in the amount of
sulfur compounds in the atmosphere.C hlorides and nitrogen
oxides usually have alessereffect but are often very significant in
combination with sulfates.
Zinc coatings resist atmospheric corrosion by forming protective films
consisting of basic salts, notably carbonate. The most widely accepted
formula is 3Zn(OH) 2 $ 2ZnCO 3 .E nvironmental conditions that prevent the
formation of such films, or conditions that lead to the formation of soluble
films, may cause rapid attacko nt he zinc.
The duration and frequency of moisture contact is one such factor.
Another factor is the rate of drying because athin film of moisture with high
oxygenc oncentration promotes reaction. For normal exposure conditions,
the films dry quite rapidly.Itisonly in sheltered areas that drying times are
slow so that the attacko nz inc is accelerated significantly.
The effect of atmospheric humidity on the corrosion of az inc coating is
related to the conditions that may cause condensation of moisture on the
metal surfacea nd to the frequency and duration of the moisture contact. If
the air temperature drops below the dew point,moisture will be deposited.
The thickness of the piece, its surfacer oughness, and its cleanliness also
influence the amount of dew deposited. Lowering the temperature of ametal
surfaceb elow the air temperature in ah umida tmosphere willc ause
moisture to condense on the metal.Ifthe water evaporates quickly,corrosion
is usually not severe, and aprotective film is formed on the surface. If water
from rain or snow remains in contactw ith zincw hena ccess to air is
Coatings
443
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485