280
Fundamentals of Corrosion
the corrosion losses of zinc, aluminum, and Al-Zn alloy are proportional to
exposure time.
As Table 8.5 reveals, a G90 sheet, which has a 1-mil zinc coating, cannot
be used for a roof having a durability of 10 years in any atmosphere except
in a rural area. Were this sheet to be used in an urban, marine, or industrial
atmosphere, it would have to be painted for protection.
Aluminum and 55% Al-Zn alloy provide galvanic protection for steel substrates. In rural and industrial atmospheres, an aluminum coating does not
act as a sacrificial anode. However, in a chloride atmosphere such as a marine
area, it does act as a sacrificial anode.
The choice as to which sacrificial metal coating to use will be based on the
environment to which it will be exposed and the service life required. The service life required will also determine the coating thickness to be applied, which
will, in turn, influence the coating process to be used. Sacrificial metal coatings
have been used successfully for roofs, walls, ducts, shutters, doors, and window
frames in the housing industry, and on structural materials such as transmission
towers, structural members of bridges, antennae, chimney structures, grandstands, steel frames, high-strength steel bolts, guardrails, corrugated steel pipe,
stadium seats, bridge I-beams, footway bridges, road bridges, and fencing.
TabLE 8.5
Average Corrosion Losses of Sacrificial Metal Coatings for 10 Years
Location
Atmosphere
Average Corrosion Loss (mil/10 yr) a
Zinc
55% AlZn
Aluminum
Roof Wall Roof Wall Roof Wall
Inland
Rural
0.42
0.17
0.15
0.06
0.06
0.02
Urban
1.48
0.59
Industrial
1.40
0.56
0.25
0.06
0.06
0.02
Severe industrial
1.59
0.64
Inland shore of lake
or marsh
Rural
0.59
0.24
0.20
0.08
0.07
0.03
Urban
1.97
0.79
Industrial
1.40
0.56
0.20
0.08
0.08
0.03
Severe industrial
2.12
0.85
Coast
Rural
0.74
0.23
0.25
0.10
0.08
0.04
Urban
2.47
0.99
Industrial
1.75
0.70
0.25
0.10
0.10
0.04
Severe industrial
2.65
1.06
Seashore
Severe industrial
2.06
0.82
0.46
0.18
0.19
0.07
a 1 mil = 25.4 μm.
Fundamentals of Corrosion
the corrosion losses of zinc, aluminum, and Al-Zn alloy are proportional to
exposure time.
As Table 8.5 reveals, a G90 sheet, which has a 1-mil zinc coating, cannot
be used for a roof having a durability of 10 years in any atmosphere except
in a rural area. Were this sheet to be used in an urban, marine, or industrial
atmosphere, it would have to be painted for protection.
Aluminum and 55% Al-Zn alloy provide galvanic protection for steel substrates. In rural and industrial atmospheres, an aluminum coating does not
act as a sacrificial anode. However, in a chloride atmosphere such as a marine
area, it does act as a sacrificial anode.
The choice as to which sacrificial metal coating to use will be based on the
environment to which it will be exposed and the service life required. The service life required will also determine the coating thickness to be applied, which
will, in turn, influence the coating process to be used. Sacrificial metal coatings
have been used successfully for roofs, walls, ducts, shutters, doors, and window
frames in the housing industry, and on structural materials such as transmission
towers, structural members of bridges, antennae, chimney structures, grandstands, steel frames, high-strength steel bolts, guardrails, corrugated steel pipe,
stadium seats, bridge I-beams, footway bridges, road bridges, and fencing.
TabLE 8.5
Average Corrosion Losses of Sacrificial Metal Coatings for 10 Years
Location
Atmosphere
Average Corrosion Loss (mil/10 yr) a
Zinc
55% AlZn
Aluminum
Roof Wall Roof Wall Roof Wall
Inland
Rural
0.42
0.17
0.15
0.06
0.06
0.02
Urban
1.48
0.59
Industrial
1.40
0.56
0.25
0.06
0.06
0.02
Severe industrial
1.59
0.64
Inland shore of lake
or marsh
Rural
0.59
0.24
0.20
0.08
0.07
0.03
Urban
1.97
0.79
Industrial
1.40
0.56
0.20
0.08
0.08
0.03
Severe industrial
2.12
0.85
Coast
Rural
0.74
0.23
0.25
0.10
0.08
0.04
Urban
2.47
0.99
Industrial
1.75
0.70
0.25
0.10
0.10
0.04
Severe industrial
2.65
1.06
Seashore
Severe industrial
2.06
0.82
0.46
0.18
0.19
0.07
a 1 mil = 25.4 μm.