Corrosion of Metallic Coatings
271
layer deposited by electroless plating provides more corrosion resistance
than does an electroplated single layer.
Most electroless plated nickel deposits contain phosphorus, which
enhances corrosion resistance. In the same manner, an electroplated nickel
deposit containing phosphorus will also be more protective.
Satin finish nickel coating. A satin-finish nickel coating consists of nonconductive materials such as aluminum oxide, kaolin, and quartz that are codeposited with chromium on the nickel deposit. Some particles are exposed
on the surface of the chromium deposit, so the deposit has a rough surface.
Because the reflectance of the deposit is decreased to less than half of that of
a level surface, the surface appearance looks like satin.
A satin-finish nickel coating provides good corrosion resistance due to the
discontinuity of the chromium topcoat.
Nickel-iron alloy coating. To reduce production costs of bright nickel, the
nickel-iron alloy coating was developed. The nickel-iron alloy deposits full
brightness, high leveling, and excellent ductility, and exhibits good reception for chromium.
This coating has the disadvantage of forming red rust when immersed in
water; consequently, nickel-iron alloy coating is suitable for use in mild atmospheres only. Typical applications include kitchenware and tubular furniture.
8.3.1.2 Chromium Coatings
In northern parts of the United States immediately after World War II, it was
not unusual for chromium-plated bumpers on the most expensive cars to
show severe signs of rust within a few months of winter exposure. This was
partially the result of trying to extend the short supply of stragic metals by
economizing on the amount used. However, the more basic reason was the
lack of sufficient knowledge of the corrosion process to control the attack by
the atmosphere. Consequently, an aggressive industrial program was undertaken to obtain a better understanding of the corrosion process and ways to
control it.
Chromium-plated parts on automobiles consist of steel substrates with
an intermediate layer of nickel or, in some cases, layered deposits of copper
and nickel. The thin chromium deposit provides a bright appearance and
stain-free surface, while the nickel layer provides corrosion protection to the
steel substrate. With this system, it is essential that the nickel cover the steel
substrate completely because the iron will be the anode and nickel the cathode. Any breaks or pores in the coating will result in the corrosion shown in
Figure 8.8. This figure illustrates the reason for the corrosion of chrome trim
on automobiles after World War II.
The corrosion problem was made worse by the fact that addition agents
used in the plating bath resulted in a bright deposit. Bright deposits contain sulfur, which makes the nickel more active from a corrosion stand-
271
layer deposited by electroless plating provides more corrosion resistance
than does an electroplated single layer.
Most electroless plated nickel deposits contain phosphorus, which
enhances corrosion resistance. In the same manner, an electroplated nickel
deposit containing phosphorus will also be more protective.
Satin finish nickel coating. A satin-finish nickel coating consists of nonconductive materials such as aluminum oxide, kaolin, and quartz that are codeposited with chromium on the nickel deposit. Some particles are exposed
on the surface of the chromium deposit, so the deposit has a rough surface.
Because the reflectance of the deposit is decreased to less than half of that of
a level surface, the surface appearance looks like satin.
A satin-finish nickel coating provides good corrosion resistance due to the
discontinuity of the chromium topcoat.
Nickel-iron alloy coating. To reduce production costs of bright nickel, the
nickel-iron alloy coating was developed. The nickel-iron alloy deposits full
brightness, high leveling, and excellent ductility, and exhibits good reception for chromium.
This coating has the disadvantage of forming red rust when immersed in
water; consequently, nickel-iron alloy coating is suitable for use in mild atmospheres only. Typical applications include kitchenware and tubular furniture.
8.3.1.2 Chromium Coatings
In northern parts of the United States immediately after World War II, it was
not unusual for chromium-plated bumpers on the most expensive cars to
show severe signs of rust within a few months of winter exposure. This was
partially the result of trying to extend the short supply of stragic metals by
economizing on the amount used. However, the more basic reason was the
lack of sufficient knowledge of the corrosion process to control the attack by
the atmosphere. Consequently, an aggressive industrial program was undertaken to obtain a better understanding of the corrosion process and ways to
control it.
Chromium-plated parts on automobiles consist of steel substrates with
an intermediate layer of nickel or, in some cases, layered deposits of copper
and nickel. The thin chromium deposit provides a bright appearance and
stain-free surface, while the nickel layer provides corrosion protection to the
steel substrate. With this system, it is essential that the nickel cover the steel
substrate completely because the iron will be the anode and nickel the cathode. Any breaks or pores in the coating will result in the corrosion shown in
Figure 8.8. This figure illustrates the reason for the corrosion of chrome trim
on automobiles after World War II.
The corrosion problem was made worse by the fact that addition agents
used in the plating bath resulted in a bright deposit. Bright deposits contain sulfur, which makes the nickel more active from a corrosion stand-