Corrosion of Metallic Coatings
287
corrosion resistance over galvanized coatings in rural, industrial, marine,
and severe marine environments. However, this alloy has limited cathodic
protection and less resistance to alkaline conditions, and is subject to weathering discoloration and wet storage staining. The latter two disadvantages
can be overcome by chromate passivation, which also improves its atmospheric corrosion resistance.
Initially, a high corrosion loss is observed for Galvalume sheet as the zincrich portion of the coating corrodes and provides sacrificial protection at cut
edges. This takes place in all environments, whereas aluminum provides
adequate galvanic protection only in marine chloride environments. After
approximately 3 years, the corrosion–time curves take on a more gradual
slope, reflecting a change from active, zinc-like behavior to passive, aluminum-like behavior as the interdentric regions fill with corrosion products. It
has been predicted that Galvalume sheets should outlast galvanized sheets
of equivalent thickness by at least two to four times over a wide range of
environments. Figure 8.15 compares the performance of galvanized sheet
and Galvalume sheet.
Galvalume sheets provide excellent cut-edge protection in very aggressive
conditions, where the surface does not remain too passive. However, it does
not offer as good a protection on the thicker sheets in mild rural conditions,
where zinc-5% aluminum coatings provide good general corrosion resistance and when sheared edges are exposed or localized damage to the coating occurs during fabrication or service, the galvanic protection is retained
for a longer period.
8.4.1.5 Zinc-15% Aluminum Thermal Spray
Zinc-15% aluminum coatings are available as thermally sprayed coatings.
These coatings have a two-phase structure consisting of a zinc-rich and an
aluminum-rich phase. The oxidation products formed are encapsulated in
the porous layer formed by the latter and do not build up a continuous
TabLE 8.7
Comparison of Cathodic Protection for Galfan
and Galvanized Coatings
Environment
Amount (mm) of Bare Edges
Exposed after 3 Years
(coating recession from edge)
Galvanized
Galfan
Severe marine
1.6
0.1
Marine
0.5
0.06
Industrial
0.5
0.05
Rural
0.1
0
287
corrosion resistance over galvanized coatings in rural, industrial, marine,
and severe marine environments. However, this alloy has limited cathodic
protection and less resistance to alkaline conditions, and is subject to weathering discoloration and wet storage staining. The latter two disadvantages
can be overcome by chromate passivation, which also improves its atmospheric corrosion resistance.
Initially, a high corrosion loss is observed for Galvalume sheet as the zincrich portion of the coating corrodes and provides sacrificial protection at cut
edges. This takes place in all environments, whereas aluminum provides
adequate galvanic protection only in marine chloride environments. After
approximately 3 years, the corrosion–time curves take on a more gradual
slope, reflecting a change from active, zinc-like behavior to passive, aluminum-like behavior as the interdentric regions fill with corrosion products. It
has been predicted that Galvalume sheets should outlast galvanized sheets
of equivalent thickness by at least two to four times over a wide range of
environments. Figure 8.15 compares the performance of galvanized sheet
and Galvalume sheet.
Galvalume sheets provide excellent cut-edge protection in very aggressive
conditions, where the surface does not remain too passive. However, it does
not offer as good a protection on the thicker sheets in mild rural conditions,
where zinc-5% aluminum coatings provide good general corrosion resistance and when sheared edges are exposed or localized damage to the coating occurs during fabrication or service, the galvanic protection is retained
for a longer period.
8.4.1.5 Zinc-15% Aluminum Thermal Spray
Zinc-15% aluminum coatings are available as thermally sprayed coatings.
These coatings have a two-phase structure consisting of a zinc-rich and an
aluminum-rich phase. The oxidation products formed are encapsulated in
the porous layer formed by the latter and do not build up a continuous
TabLE 8.7
Comparison of Cathodic Protection for Galfan
and Galvanized Coatings
Environment
Amount (mm) of Bare Edges
Exposed after 3 Years
(coating recession from edge)
Galvanized
Galfan
Severe marine
1.6
0.1
Marine
0.5
0.06
Industrial
0.5
0.05
Rural
0.1
0