Corrosion of Metallic Coatings
269
μm is required to provide high corrosion resistance. The properties required
for a semibright nickel deposit are as follows:
1. The deposit contains little sulfur.
2. Internal stress must be slight.
3. Surface appearance is semibright and extremely level.
For a trinickel (high-sulfur) strike, the following properties are required:
1. The deposit contains a stable 0.1 to 0.25% sulfur.
2. The deposit provides good adhesion for semibright nickel deposits.
Nickel coatings can be applied by electrodeposition or electrolessly from
an aqueous solution without the use of an external applied current.
Depending on the production facilities and the electrolyte composition,
electrodeposited nickel can be relatively hard (120 to 400 HV). Despite competition from hard chromium and electroless nickel, electrodeposited nickel
is still being used as an engineering coating because of its relatively low
price. Some of its properties are:
1. Good general corrosion resistance
2. Good protection from fretting corrosion
3. Good machineability
4. The ability of layers of 50 to 75 μm to prevent scaling at high
temperatures
5. Mechanical properties, including the internal stress and hardness,
that are variable and can be fixed by selecting the manufacturing
parameters
6. Excellent combination with chromium layers
7. A certain porosity
8. A tendency for layer thicknesses below 10 to 20 μm on steel to give
corrosion spots due to porosity
The electrodeposition can be either directly on steel or over an intermediate
coating of copper. Copper is used as an underlayment to facilitate buffing,
because it is softer than steel, and to increase the required coating thickness
with a material less expensive than nickel.
The most popular electroless nickel plating process is the one in which
hypophosphite is used as the reducer. Autocatalytic nickel ion reduction by
hypophosphite takes place in both acid and alkaline solutions. In a stable
solution with high coating quality, the deposition rate may be as high as 20 to
269
μm is required to provide high corrosion resistance. The properties required
for a semibright nickel deposit are as follows:
1. The deposit contains little sulfur.
2. Internal stress must be slight.
3. Surface appearance is semibright and extremely level.
For a trinickel (high-sulfur) strike, the following properties are required:
1. The deposit contains a stable 0.1 to 0.25% sulfur.
2. The deposit provides good adhesion for semibright nickel deposits.
Nickel coatings can be applied by electrodeposition or electrolessly from
an aqueous solution without the use of an external applied current.
Depending on the production facilities and the electrolyte composition,
electrodeposited nickel can be relatively hard (120 to 400 HV). Despite competition from hard chromium and electroless nickel, electrodeposited nickel
is still being used as an engineering coating because of its relatively low
price. Some of its properties are:
1. Good general corrosion resistance
2. Good protection from fretting corrosion
3. Good machineability
4. The ability of layers of 50 to 75 μm to prevent scaling at high
temperatures
5. Mechanical properties, including the internal stress and hardness,
that are variable and can be fixed by selecting the manufacturing
parameters
6. Excellent combination with chromium layers
7. A certain porosity
8. A tendency for layer thicknesses below 10 to 20 μm on steel to give
corrosion spots due to porosity
The electrodeposition can be either directly on steel or over an intermediate
coating of copper. Copper is used as an underlayment to facilitate buffing,
because it is softer than steel, and to increase the required coating thickness
with a material less expensive than nickel.
The most popular electroless nickel plating process is the one in which
hypophosphite is used as the reducer. Autocatalytic nickel ion reduction by
hypophosphite takes place in both acid and alkaline solutions. In a stable
solution with high coating quality, the deposition rate may be as high as 20 to