Corrosion of Metallic Coatings
281
8.4.1 Mechanism of Control Protection
8.4.1.1 Zinc Coatings
Approximately half of the world’s production of zinc is used to protect steel from
rust. Zinc coatings are probably the most important type of metallic coating for
corrosion protection of steel. The reasons for the wide application include:
1. Prices are relatively low.
2. Due to large reserves, an ample supply of zinc is available.
3. There is great flexibility in application procedures, resulting in many
different qualities with well-controlled layer thickness.
4. Steel provides good cathodic protection.
5. Many special alloy systems have been developed with improved
corrosion-protection properties.
The ability to select a particular alloy or to specify a particular coating
thickness depends on the type of coating process used. Zinc coatings can be
applied in many ways. The six most commonly used procedures follow.
Hot dipping. This is a process in which cleaned steel is immersed in molten zinc or zinc alloy, and a reaction takes place to form a metallurgically
bonded coating.
The coating is integral with the steel because the formation process produces zinc-iron alloy layers overcoated with zinc. Continuity and uniformity are good because any discontinuities are readily visible as “black
spots.”
Coating thicknesses can be varied from approximately 50 to 125 μm on
tube and products. Thicker coatings up to 250 μm can be obtained by grit
blasting before galvanizing. Steel and wire normally receive thicknesses of
10 to 30 μm.
Conventional coatings that are applied to finished articles are not formable. The alloy layer is abrasion resistant but brittle on bending. Special
coatings with little or no alloy layer are readily formed (e.g., on sheet) and
resistance welded.
A chromatic conversion coating over the zinc coating prevents wet storage
stains while phosphate coatings provide a good base (on a new sheet) for paints.
Weathered coatings are often painted after 10 to 30 years for longer service.
Hot dip galvanizing is the most important zinc coating process. All mild
steel and cast iron can be coated by this process. The thickness and structure
of the coating will depend on the alloying elements. Approximately half of
the steel that is coated is in the form of sheet, approximately one quarter is
fabricated work while the remainder is tube or wire. Metallurgically, the processes used for tubes and fabricated work are similar, while the process used
for sheet has small additions to the zinc that reduce the quantity of iron-zinc
in the coating, which provides flexibility.
281
8.4.1 Mechanism of Control Protection
8.4.1.1 Zinc Coatings
Approximately half of the world’s production of zinc is used to protect steel from
rust. Zinc coatings are probably the most important type of metallic coating for
corrosion protection of steel. The reasons for the wide application include:
1. Prices are relatively low.
2. Due to large reserves, an ample supply of zinc is available.
3. There is great flexibility in application procedures, resulting in many
different qualities with well-controlled layer thickness.
4. Steel provides good cathodic protection.
5. Many special alloy systems have been developed with improved
corrosion-protection properties.
The ability to select a particular alloy or to specify a particular coating
thickness depends on the type of coating process used. Zinc coatings can be
applied in many ways. The six most commonly used procedures follow.
Hot dipping. This is a process in which cleaned steel is immersed in molten zinc or zinc alloy, and a reaction takes place to form a metallurgically
bonded coating.
The coating is integral with the steel because the formation process produces zinc-iron alloy layers overcoated with zinc. Continuity and uniformity are good because any discontinuities are readily visible as “black
spots.”
Coating thicknesses can be varied from approximately 50 to 125 μm on
tube and products. Thicker coatings up to 250 μm can be obtained by grit
blasting before galvanizing. Steel and wire normally receive thicknesses of
10 to 30 μm.
Conventional coatings that are applied to finished articles are not formable. The alloy layer is abrasion resistant but brittle on bending. Special
coatings with little or no alloy layer are readily formed (e.g., on sheet) and
resistance welded.
A chromatic conversion coating over the zinc coating prevents wet storage
stains while phosphate coatings provide a good base (on a new sheet) for paints.
Weathered coatings are often painted after 10 to 30 years for longer service.
Hot dip galvanizing is the most important zinc coating process. All mild
steel and cast iron can be coated by this process. The thickness and structure
of the coating will depend on the alloying elements. Approximately half of
the steel that is coated is in the form of sheet, approximately one quarter is
fabricated work while the remainder is tube or wire. Metallurgically, the processes used for tubes and fabricated work are similar, while the process used
for sheet has small additions to the zinc that reduce the quantity of iron-zinc
in the coating, which provides flexibility.