The loss of metal coating resulting from corrosion determines the service
life of the coating. The degree of loss is dependent on the time of wetness
on the metal surfacea nd the typeo fc oncentration of pollutantsi nt he
atmosphere. Table 7.9 shows the average corrosion lossesofzinc, aluminum,
and 55% Al–Znc oatings in variousl ocationsa nd atmospheres. The losses
were calculated from the mean values of timeo fw etness and the average
corrosion rate during wet duration. The timeo fw etness of walls is 40% of
that of roofs. Coating metalsa nd coating thicknesses can be decided from
Table 7.9 since the corrosion losses of zinc, aluminum, and Al–Zn alloya re
proportional to exposure time.
As can be seen from the table, aG 90 sheet that has a1mil zinc coating,
cannot be used for aroof havingadurability of ten years in any atmosphere
except in ar ural area. Were this sheet to be used in an urban, marine, or
industrial atmosphere, it would have to be painted for protection.
TABLE 7.8
Corrosion Potentials of Sacrificial Metals in a
3% NaCl Solution
Metal
CorrosionPotential (V,SCE)
Mn
K 1.50
Zn
K 1.03
Al
K 0.79
Cd
K 0.70
Steel
K 0.61
O 2
Electrolyte
Zn
Steel
Steel
e
OH
−
Zn (OH) 2
FIGURE 7.15
Schematic illustration of galvanic action of sacrificialm etal coating.
Corrosion of Linings and Coatings
438
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
life of the coating. The degree of loss is dependent on the time of wetness
on the metal surfacea nd the typeo fc oncentration of pollutantsi nt he
atmosphere. Table 7.9 shows the average corrosion lossesofzinc, aluminum,
and 55% Al–Znc oatings in variousl ocationsa nd atmospheres. The losses
were calculated from the mean values of timeo fw etness and the average
corrosion rate during wet duration. The timeo fw etness of walls is 40% of
that of roofs. Coating metalsa nd coating thicknesses can be decided from
Table 7.9 since the corrosion losses of zinc, aluminum, and Al–Zn alloya re
proportional to exposure time.
As can be seen from the table, aG 90 sheet that has a1mil zinc coating,
cannot be used for aroof havingadurability of ten years in any atmosphere
except in ar ural area. Were this sheet to be used in an urban, marine, or
industrial atmosphere, it would have to be painted for protection.
TABLE 7.8
Corrosion Potentials of Sacrificial Metals in a
3% NaCl Solution
Metal
CorrosionPotential (V,SCE)
Mn
K 1.50
Zn
K 1.03
Al
K 0.79
Cd
K 0.70
Steel
K 0.61
O 2
Electrolyte
Zn
Steel
Steel
e
OH
−
Zn (OH) 2
FIGURE 7.15
Schematic illustration of galvanic action of sacrificialm etal coating.
Corrosion of Linings and Coatings
438
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485