Corrosion of Metallic Coatings
273
uniformly over the exterior of the plated material and serve to distribute the
corrosion process over the entire surface. The result has been to extend the
life of chromium-plated steel exposed to outdoor conditions.
Microcracked chromium coatings are produced by first depositing a highstress nickel strike on a sulfur-free nickel layer and then a decorative chromium deposit. The uniform crack network results from the interaction of
the thin chromium layer and the high-stress nickel deposit. The result is a
mirror surface as well as a decorative chromium coating.
Microporous chromium coatings are produced by first electroplating a bright
nickel layer containing suspended nonconductive fine particles. Over this, a
chromium layer is deposited that results in a mirror finish. As the chromium
thickness increases, the number of pores decreases. For a chromium deposit
of 0.25-m thickness, a porosity of more than 10,000 pores/cm 2 are required. A
porosity of 40,000 pores/cm 2 provides the best corrosion resistance.
Hard (engineering) chromium layers are also deposited directly on a variety of metals. The purpose of applying these layers is to obtain a wear-resistant surface with a high hardness or to restore the original dimensions to
a work piece. In addition, the excellent corrosion resistance resulting from
these layers makes them suitable for outdoor applications.
Thick chromium deposits have high residual internal stress and may be
brittle due to the electrodeposition process, in which hydrogen can be incorporated in the deposited layer. Cracks result during plating when the stress
exceeds the tensile strength of the chromium. As the plating continues,
some of the cracks are filled. This led to the development of controlled cracking patterns, which produce wettable surfaces that can spread oil, which is
important for engine cylinders, liners, etc.
Some of the properties of the engineering chromium layers are:
Excellent corrosion resistance
•
Wear resistance
•
Hardness up to 950 HV
•
Controlled porosity is possible
•
8.3.1.3 Tin Coatings (Tinplate)
Tinplate is produced mainly by the electroplating process. Alkaline and acid
baths are used in the production line. The acid baths are classified as either
ferrostan or halogen baths.
A thermal treatment above the melting point of tin follows electrolytic
deposition. The intermetallic compound FeSn 2 forms at the interface between
the iron and tin during this thermal processing. The corrosion behavior of
the tinplate is determined by the quality of the FeSn 2 formed, particularly
when the amount of the free tin is small. The best-performing tinplate is that
in which the FeSn 2 uniformly covers the steel so that the area of iron exposed
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uniformly over the exterior of the plated material and serve to distribute the
corrosion process over the entire surface. The result has been to extend the
life of chromium-plated steel exposed to outdoor conditions.
Microcracked chromium coatings are produced by first depositing a highstress nickel strike on a sulfur-free nickel layer and then a decorative chromium deposit. The uniform crack network results from the interaction of
the thin chromium layer and the high-stress nickel deposit. The result is a
mirror surface as well as a decorative chromium coating.
Microporous chromium coatings are produced by first electroplating a bright
nickel layer containing suspended nonconductive fine particles. Over this, a
chromium layer is deposited that results in a mirror finish. As the chromium
thickness increases, the number of pores decreases. For a chromium deposit
of 0.25-m thickness, a porosity of more than 10,000 pores/cm 2 are required. A
porosity of 40,000 pores/cm 2 provides the best corrosion resistance.
Hard (engineering) chromium layers are also deposited directly on a variety of metals. The purpose of applying these layers is to obtain a wear-resistant surface with a high hardness or to restore the original dimensions to
a work piece. In addition, the excellent corrosion resistance resulting from
these layers makes them suitable for outdoor applications.
Thick chromium deposits have high residual internal stress and may be
brittle due to the electrodeposition process, in which hydrogen can be incorporated in the deposited layer. Cracks result during plating when the stress
exceeds the tensile strength of the chromium. As the plating continues,
some of the cracks are filled. This led to the development of controlled cracking patterns, which produce wettable surfaces that can spread oil, which is
important for engine cylinders, liners, etc.
Some of the properties of the engineering chromium layers are:
Excellent corrosion resistance
•
Wear resistance
•
Hardness up to 950 HV
•
Controlled porosity is possible
•
8.3.1.3 Tin Coatings (Tinplate)
Tinplate is produced mainly by the electroplating process. Alkaline and acid
baths are used in the production line. The acid baths are classified as either
ferrostan or halogen baths.
A thermal treatment above the melting point of tin follows electrolytic
deposition. The intermetallic compound FeSn 2 forms at the interface between
the iron and tin during this thermal processing. The corrosion behavior of
the tinplate is determined by the quality of the FeSn 2 formed, particularly
when the amount of the free tin is small. The best-performing tinplate is that
in which the FeSn 2 uniformly covers the steel so that the area of iron exposed