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Fundamentals of Corrosion
Zinc electroplating. This process is sometimes mistakenly referred to as electrogalvanizing. In this process, zinc salt solutions are used in the electrolytic
deposition of a layer of zinc on a cleaned steel surface.
The process provides good adhesion, comparable with other electroplated
coatings. The coating is uniform within the limitations of the “throwing
power” of the bath. Pores are not a problem as exposed steel is protected by
the adjacent zinc.
Coating thicknesses can be varied at will but are usually 2.5 to 15 μm.
Thicker layers are possible but are not usually economical.
Electroplated steel has excellent formability and can be spot welded. Small
components are usually finished before being plated.
Chromate conversion coatings are used to prevent wet storage stain while
phosphate conversion coatings are used as a base for paint.
The process is normally used for simple, fairly small components. It is
suitable for barrel plating or for continuous sheet and wire. No heating
is used in this process, except for hydrogen embrittlement relief on highstrength steels.
Electroplated zinc is very ductile and consequently this process is widely
used for the continuous plating of strip or wire, where severe deformation
may be required.
The coating on steel from this process gives a bright and smooth finish. It
is used for decorative effect to protect delicate objects where rough or uneven
finishes cannot be tolerated (e.g., instrument parts). It is also used for articles
that cannot withstand the temperatures required in other coating processes.
It was previously mentioned that the term “electrogalvanizing” is sometimes used to describe this process. This is misleading because the chief
characteristic of galvanizing is the formation of a metallurgical bond at the
zinc-iron interface. This does not occur in electroplating.
Mechanical coating. This process involves agitating suitably prepared
parts to be coated with a mixture of nonmetallic impactors (e.g., glass
beads), zinc powder, a chemical promoter, and water. All types of steel can
be coated. However, this process is less suitable for parts heavier than 0.5
pound (250 g) because the tumbling process reduces coating thicknesses
at the edges.
The adhesion is good compared to electroplated coatings. Thicknesses
can be varied from 5 μm to more than 70 μm. However, the coating is not
alloyed with steel, nor does it have the hard abrasion resistance iron-zinc
alloy layers of galvanized or sherardized coatings. Conversion coatings
can be applied.
Sherardizing. The articles to be coated are tumbled in a barrel containing zinc dust at a temperature just below the melting point of zinc, usually
around 716°F (380°C). In the case of spring steels, the temperature is somewhat lower. By means of a different process, the zinc bonds to the steel, forming a hard, even coating of zinc-iron compounds. The coating is dull gray in
color and can readily be painted if necessary.
Fundamentals of Corrosion
Zinc electroplating. This process is sometimes mistakenly referred to as electrogalvanizing. In this process, zinc salt solutions are used in the electrolytic
deposition of a layer of zinc on a cleaned steel surface.
The process provides good adhesion, comparable with other electroplated
coatings. The coating is uniform within the limitations of the “throwing
power” of the bath. Pores are not a problem as exposed steel is protected by
the adjacent zinc.
Coating thicknesses can be varied at will but are usually 2.5 to 15 μm.
Thicker layers are possible but are not usually economical.
Electroplated steel has excellent formability and can be spot welded. Small
components are usually finished before being plated.
Chromate conversion coatings are used to prevent wet storage stain while
phosphate conversion coatings are used as a base for paint.
The process is normally used for simple, fairly small components. It is
suitable for barrel plating or for continuous sheet and wire. No heating
is used in this process, except for hydrogen embrittlement relief on highstrength steels.
Electroplated zinc is very ductile and consequently this process is widely
used for the continuous plating of strip or wire, where severe deformation
may be required.
The coating on steel from this process gives a bright and smooth finish. It
is used for decorative effect to protect delicate objects where rough or uneven
finishes cannot be tolerated (e.g., instrument parts). It is also used for articles
that cannot withstand the temperatures required in other coating processes.
It was previously mentioned that the term “electrogalvanizing” is sometimes used to describe this process. This is misleading because the chief
characteristic of galvanizing is the formation of a metallurgical bond at the
zinc-iron interface. This does not occur in electroplating.
Mechanical coating. This process involves agitating suitably prepared
parts to be coated with a mixture of nonmetallic impactors (e.g., glass
beads), zinc powder, a chemical promoter, and water. All types of steel can
be coated. However, this process is less suitable for parts heavier than 0.5
pound (250 g) because the tumbling process reduces coating thicknesses
at the edges.
The adhesion is good compared to electroplated coatings. Thicknesses
can be varied from 5 μm to more than 70 μm. However, the coating is not
alloyed with steel, nor does it have the hard abrasion resistance iron-zinc
alloy layers of galvanized or sherardized coatings. Conversion coatings
can be applied.
Sherardizing. The articles to be coated are tumbled in a barrel containing zinc dust at a temperature just below the melting point of zinc, usually
around 716°F (380°C). In the case of spring steels, the temperature is somewhat lower. By means of a different process, the zinc bonds to the steel, forming a hard, even coating of zinc-iron compounds. The coating is dull gray in
color and can readily be painted if necessary.