268
Fundamentals of Corrosion
The corrosion potentials of the nickel deposits depend on the sulfur content. Figure 8.7 shows the effect of sulfur content on the corrosion potential
of a nickel deposit. A single-layer nickel coating must be greater than 30 μm
to ensure the absence of defects.
As the sulfur content increases, the corrosion potential of a nickel deposit
becomes more negative. A bright nickel coating is less protective than a semibright or dull nickel coating. The difference in the potential of bright nickel
and semibright nickel deposits is more than 50 mV.
Use is made of the differences in the potential in the application of multilayer coatings. The more negative bright nickel deposits are used as sacrificial intermediate layers. When bright nickel is used as an intermediate layer,
the corrosion behavior is characterized by sideways diversion. Pitting corrosion is diverted laterally when it reaches the more noble semibright nickel
deposit. Thus, the corrosion behavior of bright nickel prolongs the time for
pitting penetration to reach the base metal.
The most negative of all nickel deposits is trinickel. In this triplex layer
coating system, a coating of trinickel approximately 1 μm thick, containing
0.1 to 0.25% sulfur, is applied between bright nickel and semibright nickel
deposits. The high-sulfur nickel layer dissolves preferentially, even when pitting corrosion reaches the surface of the semibright nickel deposit. Because
the high-sulfur layer reacts with the bright nickel layer, pitting corrosion
does not penetrate the high-sulfur nickel layer in the tunneling form. The
application of a high-sulfur nickel strike definitely improves the protective
ability of a multilayer nickel coating.
In the duplex nickel coating system, the thickness ratio of semibright nickel
deposit to bright nickel deposit is nominally 3:1, and a thickness of 20 to 25
Sulfur Content, wt. %
0.01
0.1
1.0
Bright nickel coating
Semibright
nickel coating
Dull
nickel
coating
0.001
Corrosion Potential of Nickel mV, SCE
–350
–400
–450
–300
FigurE 8.7
Effect of sulfur content on corrosion protection of nickel.
Fundamentals of Corrosion
The corrosion potentials of the nickel deposits depend on the sulfur content. Figure 8.7 shows the effect of sulfur content on the corrosion potential
of a nickel deposit. A single-layer nickel coating must be greater than 30 μm
to ensure the absence of defects.
As the sulfur content increases, the corrosion potential of a nickel deposit
becomes more negative. A bright nickel coating is less protective than a semibright or dull nickel coating. The difference in the potential of bright nickel
and semibright nickel deposits is more than 50 mV.
Use is made of the differences in the potential in the application of multilayer coatings. The more negative bright nickel deposits are used as sacrificial intermediate layers. When bright nickel is used as an intermediate layer,
the corrosion behavior is characterized by sideways diversion. Pitting corrosion is diverted laterally when it reaches the more noble semibright nickel
deposit. Thus, the corrosion behavior of bright nickel prolongs the time for
pitting penetration to reach the base metal.
The most negative of all nickel deposits is trinickel. In this triplex layer
coating system, a coating of trinickel approximately 1 μm thick, containing
0.1 to 0.25% sulfur, is applied between bright nickel and semibright nickel
deposits. The high-sulfur nickel layer dissolves preferentially, even when pitting corrosion reaches the surface of the semibright nickel deposit. Because
the high-sulfur layer reacts with the bright nickel layer, pitting corrosion
does not penetrate the high-sulfur nickel layer in the tunneling form. The
application of a high-sulfur nickel strike definitely improves the protective
ability of a multilayer nickel coating.
In the duplex nickel coating system, the thickness ratio of semibright nickel
deposit to bright nickel deposit is nominally 3:1, and a thickness of 20 to 25
Sulfur Content, wt. %
0.01
0.1
1.0
Bright nickel coating
Semibright
nickel coating
Dull
nickel
coating
0.001
Corrosion Potential of Nickel mV, SCE
–350
–400
–450
–300
FigurE 8.7
Effect of sulfur content on corrosion protection of nickel.