112
Fundamentals of Corrosion
or weight loss is observed even after several years of exposure in atmospheric
field testing.
4.7.4.3 Zinc-Iron Alloy Coatings
Compared with pure zinc coatings, the zinc-iron alloy coatings provide
increased corrosion resistance in acidic atmospheres but slightly reduced
corrosion resistance in alkaline atmospheres.
4.7.4.4 Cast Zinc Alloys
All the zinc-rich aluminum (ZA) alloys with an 8 to 29% aluminum content corrode less than 99.99% zinc. ZA-4 (zinc-4% aluminum) castings resist
atmospheric corrosion in much the same manner as pure zinc. One advantage is that more zinc may be lost than with zinc coatings without affecting
the performance.
In general, corrosion resistance is improved by the addition of higher
amounts of aluminum. While ZA-8 (zinc-8% aluminum) and ZA-12 (zinc12% aluminum) exhibit only modest improvements over the standard 4%
alloy, ZA-27 (zinc-27% aluminum) with its significantly larger amount of
aluminum-rich phase is much superior.
In plant atmospheres contaminated with either acidic or alkaline fumes, the
ZA alloys perform better than pure zinc; whereas in alkaline-contaminated
atmospheres, both zinc and the ZA alloys are more suitable than aluminum.
The addition of copper to zinc provides a slight improvement in corrosion resistance.
4.7.5 Copper
For centuries, copper has been used as an architectural material. Copper
roofs are still in existence on many castles and monumental buildings that
are hundreds of years old.
When exposed to the atmosphere over long periods of time, copper forms
a coloration on the surface known as patina, which in reality is a corrosion
product that acts as a protective film against further corrosion. The length
of time required to form the patina depends on the atmosphere because the
color is due to the formation of copper hydroxide compounds. Initially the
patina has a dark color, which gradually turns green. This is also known
as verdigras, which is French for green-gray. In urban or industrial atmospheres, the compound is a mixture of copper/hydroxide/sulfate and in
marine atmospheres a mixture of copper/hydroxide/chloride. It takes
approximately 7 years for these compounds to form. When exposed to clean
or rural atmospheres, tens or hundreds of years might be required to form
the patina, or protective film.
Fundamentals of Corrosion
or weight loss is observed even after several years of exposure in atmospheric
field testing.
4.7.4.3 Zinc-Iron Alloy Coatings
Compared with pure zinc coatings, the zinc-iron alloy coatings provide
increased corrosion resistance in acidic atmospheres but slightly reduced
corrosion resistance in alkaline atmospheres.
4.7.4.4 Cast Zinc Alloys
All the zinc-rich aluminum (ZA) alloys with an 8 to 29% aluminum content corrode less than 99.99% zinc. ZA-4 (zinc-4% aluminum) castings resist
atmospheric corrosion in much the same manner as pure zinc. One advantage is that more zinc may be lost than with zinc coatings without affecting
the performance.
In general, corrosion resistance is improved by the addition of higher
amounts of aluminum. While ZA-8 (zinc-8% aluminum) and ZA-12 (zinc12% aluminum) exhibit only modest improvements over the standard 4%
alloy, ZA-27 (zinc-27% aluminum) with its significantly larger amount of
aluminum-rich phase is much superior.
In plant atmospheres contaminated with either acidic or alkaline fumes, the
ZA alloys perform better than pure zinc; whereas in alkaline-contaminated
atmospheres, both zinc and the ZA alloys are more suitable than aluminum.
The addition of copper to zinc provides a slight improvement in corrosion resistance.
4.7.5 Copper
For centuries, copper has been used as an architectural material. Copper
roofs are still in existence on many castles and monumental buildings that
are hundreds of years old.
When exposed to the atmosphere over long periods of time, copper forms
a coloration on the surface known as patina, which in reality is a corrosion
product that acts as a protective film against further corrosion. The length
of time required to form the patina depends on the atmosphere because the
color is due to the formation of copper hydroxide compounds. Initially the
patina has a dark color, which gradually turns green. This is also known
as verdigras, which is French for green-gray. In urban or industrial atmospheres, the compound is a mixture of copper/hydroxide/sulfate and in
marine atmospheres a mixture of copper/hydroxide/chloride. It takes
approximately 7 years for these compounds to form. When exposed to clean
or rural atmospheres, tens or hundreds of years might be required to form
the patina, or protective film.