270
Fundamentals of Corrosion
25 μm/h. However, a relatively high temperature of 194°F (90°C) is required.
Because hydrogen ions are formed in the reduction reaction,
Ni
2H PO 2H O Ni
H
H
2
2
–
2
2
2
2
2
+
−
+
+
→ +
+
a high buffering capacity of the solution is necessary to ensure a steady-state
process. For this reason, acetate, citrate, propionate, glycolate, lactate, or aminoacetate is added to the solutions. These substances, along with buffering, may
form complexes with nickel ions. Binding Ni 2+ ions into a complex is required
in alkaline solutions (here, ammonia and pyrophosphate may be added in
addition to citrate and aminoacetate). In addition, such binding is desirable
in acid solutions because free nickel ions form a compound with the reaction
product (phosphate) that precipitates and prevents further use of the solution.
When hypophosphite is used as the reducing agent, phosphorus will be
present in the coating. Its amount, in the range of 2 to 15 mass %, depends
on pH, buffering capacity, ligands, and other parameters of electroless
solutions.
Borohydride and its derivatives can also be used as reducing agents. When
borohydride is used in the reduction, temperatures of 140°F to 194°F (60°C to
90°C) are required. The use of dimethylaminoborane (DMAB) enables the
deposition of Ni-B coatings with a small amount of boron (0.5 to 1.0 mass %
at temperatures in the range of 86°F to 140°F (30°C to 60°C). Both neutral and
alkaline solutions may be used.
Depending on exposure conditions, certain minimum coating thicknesses
to control porosity are recommended for the coating to maintain its appearance and have a satisfactory life:
Exposure
Minimum Coating Thickness
Indoor exposures
0.3–0.5 / 0.008–0.013 mm
Outdoor exposures
0.5–1.5 / 0.013–0.04 mm
Chemical industry
1–10 / 0.025–0.25 mm
For applications near the seacoast, thicknesses in the area of 1.5 mil (0.04
mm) should be considered. This also applies to automobile bumpers and
applications in industrial atmospheres.
Nickel is sensitive to attack by industrial atmospheres and forms a film of
basic nickel sulfate that causes the surface to “fog” or lose its brightness. To
overcome this fogging, a thin coating of chromium (0.01 to 0.03 mil/0.003
to 0.007 mm) is electrodeposited over the nickel. This finish is applied to all
materials for which continued brightness is desired.
Single-layer coatings of nickel exhibit less corrosion resistance than multilayer coatings due to their discontinuities. The electroless plating process
produces a coating with fewer discontinuous deposits. Therefore, the single
Fundamentals of Corrosion
25 μm/h. However, a relatively high temperature of 194°F (90°C) is required.
Because hydrogen ions are formed in the reduction reaction,
Ni
2H PO 2H O Ni
H
H
2
2
–
2
2
2
2
2
+
−
+
+
→ +
+
a high buffering capacity of the solution is necessary to ensure a steady-state
process. For this reason, acetate, citrate, propionate, glycolate, lactate, or aminoacetate is added to the solutions. These substances, along with buffering, may
form complexes with nickel ions. Binding Ni 2+ ions into a complex is required
in alkaline solutions (here, ammonia and pyrophosphate may be added in
addition to citrate and aminoacetate). In addition, such binding is desirable
in acid solutions because free nickel ions form a compound with the reaction
product (phosphate) that precipitates and prevents further use of the solution.
When hypophosphite is used as the reducing agent, phosphorus will be
present in the coating. Its amount, in the range of 2 to 15 mass %, depends
on pH, buffering capacity, ligands, and other parameters of electroless
solutions.
Borohydride and its derivatives can also be used as reducing agents. When
borohydride is used in the reduction, temperatures of 140°F to 194°F (60°C to
90°C) are required. The use of dimethylaminoborane (DMAB) enables the
deposition of Ni-B coatings with a small amount of boron (0.5 to 1.0 mass %
at temperatures in the range of 86°F to 140°F (30°C to 60°C). Both neutral and
alkaline solutions may be used.
Depending on exposure conditions, certain minimum coating thicknesses
to control porosity are recommended for the coating to maintain its appearance and have a satisfactory life:
Exposure
Minimum Coating Thickness
Indoor exposures
0.3–0.5 / 0.008–0.013 mm
Outdoor exposures
0.5–1.5 / 0.013–0.04 mm
Chemical industry
1–10 / 0.025–0.25 mm
For applications near the seacoast, thicknesses in the area of 1.5 mil (0.04
mm) should be considered. This also applies to automobile bumpers and
applications in industrial atmospheres.
Nickel is sensitive to attack by industrial atmospheres and forms a film of
basic nickel sulfate that causes the surface to “fog” or lose its brightness. To
overcome this fogging, a thin coating of chromium (0.01 to 0.03 mil/0.003
to 0.007 mm) is electrodeposited over the nickel. This finish is applied to all
materials for which continued brightness is desired.
Single-layer coatings of nickel exhibit less corrosion resistance than multilayer coatings due to their discontinuities. The electroless plating process
produces a coating with fewer discontinuous deposits. Therefore, the single