7.5Z inc–5% Aluminum Hot-DipC oatings
This zinc alloy coating is known as Galfan. Galfan coatings have acorrosion
resistance up to three times that of galvanized steel. The maind ifference
between these two coatings lies in the degree of cathodic protection they
afford. This increase in corrosion protection is evident in both ar elatively
mild urban industrial atmosphere as can be seen in Table7 .11. The latter is
particularly significant, because, unlike galvanizing, the corrosion rate
appearst ob es lowing after about foury ears, and conventional galvanized
steel would show rusti nF ive years. See Figure 7.16.T he slower rate of
corrosion also means that the zinc–5% aluminum coatings provide full
cathodic protection to cut edges over al onger period. Refer to Table 7.12.
Because Galfan can be formed with much smaller crackst han can be
obtainedi nc onventionalg alvanized coatings,i tp rovidese xcellent
protection at panel bulges. Thisr educed cracking means that less zinc is
exposed to the environment that increases the relative performance factor
compared with galvanized steel.
7.6Z inc–55%Aluminum Hot-DipC oatings
These coatings areknown as Galvalume and consist of zinc–55%aluminum–
1.5% silicon. This alloy is sold unders uch tradenames as Zaluite,A luzene,
Alugalva, Algafort, Aluzink, and Zincalume. Galvalume exhibits superior
corrosion resistance over galvanized coatings in rural, industrial, marine,
and severe marine environments. However,t his alloy has limited cathodic
protection and less resistance to somea lkaline conditions and is subject to
weathering discolorationa nd wets torage staining.T he latter two
disadvantages can be overcome by chromate passivation that also improves
its atmospheric corrosion resistance.
TABLE 7.11
Five-Year Outdoor ExposureR esults of Galfan Coating
Thickness Loss ( m m)
Atmosphere
GalvanizedG alfan
Ratio of
Improvement
Industrial
15.0
5.2
2.9
Severem arine
O 20.0
9.5
O 2.1
Marine
12.5
7.5
1.7
Rural
10.5
3.0
3.5
Source:F rom F.C. Porter.1 994. Corrosion Resistance of Zinc and Zinc Alloys,
New York: Marcel Dekker.
Corrosion of Linings and Coatings
448
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
This zinc alloy coating is known as Galfan. Galfan coatings have acorrosion
resistance up to three times that of galvanized steel. The maind ifference
between these two coatings lies in the degree of cathodic protection they
afford. This increase in corrosion protection is evident in both ar elatively
mild urban industrial atmosphere as can be seen in Table7 .11. The latter is
particularly significant, because, unlike galvanizing, the corrosion rate
appearst ob es lowing after about foury ears, and conventional galvanized
steel would show rusti nF ive years. See Figure 7.16.T he slower rate of
corrosion also means that the zinc–5% aluminum coatings provide full
cathodic protection to cut edges over al onger period. Refer to Table 7.12.
Because Galfan can be formed with much smaller crackst han can be
obtainedi nc onventionalg alvanized coatings,i tp rovidese xcellent
protection at panel bulges. Thisr educed cracking means that less zinc is
exposed to the environment that increases the relative performance factor
compared with galvanized steel.
7.6Z inc–55%Aluminum Hot-DipC oatings
These coatings areknown as Galvalume and consist of zinc–55%aluminum–
1.5% silicon. This alloy is sold unders uch tradenames as Zaluite,A luzene,
Alugalva, Algafort, Aluzink, and Zincalume. Galvalume exhibits superior
corrosion resistance over galvanized coatings in rural, industrial, marine,
and severe marine environments. However,t his alloy has limited cathodic
protection and less resistance to somea lkaline conditions and is subject to
weathering discolorationa nd wets torage staining.T he latter two
disadvantages can be overcome by chromate passivation that also improves
its atmospheric corrosion resistance.
TABLE 7.11
Five-Year Outdoor ExposureR esults of Galfan Coating
Thickness Loss ( m m)
Atmosphere
GalvanizedG alfan
Ratio of
Improvement
Industrial
15.0
5.2
2.9
Severem arine
O 20.0
9.5
O 2.1
Marine
12.5
7.5
1.7
Rural
10.5
3.0
3.5
Source:F rom F.C. Porter.1 994. Corrosion Resistance of Zinc and Zinc Alloys,
New York: Marcel Dekker.
Corrosion of Linings and Coatings
448
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485