Corrosion of Metallic Coatings
267
8.3 Anodic Control Protection by Noble Metals
Coating metals that provide protection by means of anodic control include
nickel, chromium, tin, lead, and their alloys.
They protect the substrate metal as a result of their resistance to corrosion
insofar as they form a well-adhering and nonporous barrier layer. However,
when the coating is damaged, galvanically induced corrosion will lead to
severe attack. This corrosion process is extremely fast for coated systems due
to the high current density effect as a result of the large ratio between the
surface areas and the cathodic outer surface and the anodic effect, as shown
in Figure 8.6. To compensate for these defects in the coating, multilayer
coating systems have been developed. The corrosion resistance of a single
noble layer metal coating results from the original barrier action of the noble
metal, the surface of the noble metal being passivated. With the exception of
lead, a secondary barrier of corrosion products is formed. Noble metals do
not provide cathodic protection for steel substrates because their corrosion
potential is more noble than those of iron and steel in a natural environment
(see Table 8.1). In multilayer coating systems, a small difference in potential
between coating layers results in galvanic action in coating layers.
8.3.1 Mechanisms of Control Protection
8.3.1.1 Nickel Coatings
There are three types of nickel coatings: bright, semibright, and dull bright.
The difference between the coatings is in the quantity of sulfur contained in
them, as shown:
Nickel Coating
Sulfur Content
(%)
Bright nickel deposits
>0.04
Semibright nickel deposits
<0.005
Dull bright nickel deposits
<0.001
Substrate
e –
e
–
Metallic film
Me
n+
FigurE 8.6
Dissolution of substrate metal in coating defect.
267
8.3 Anodic Control Protection by Noble Metals
Coating metals that provide protection by means of anodic control include
nickel, chromium, tin, lead, and their alloys.
They protect the substrate metal as a result of their resistance to corrosion
insofar as they form a well-adhering and nonporous barrier layer. However,
when the coating is damaged, galvanically induced corrosion will lead to
severe attack. This corrosion process is extremely fast for coated systems due
to the high current density effect as a result of the large ratio between the
surface areas and the cathodic outer surface and the anodic effect, as shown
in Figure 8.6. To compensate for these defects in the coating, multilayer
coating systems have been developed. The corrosion resistance of a single
noble layer metal coating results from the original barrier action of the noble
metal, the surface of the noble metal being passivated. With the exception of
lead, a secondary barrier of corrosion products is formed. Noble metals do
not provide cathodic protection for steel substrates because their corrosion
potential is more noble than those of iron and steel in a natural environment
(see Table 8.1). In multilayer coating systems, a small difference in potential
between coating layers results in galvanic action in coating layers.
8.3.1 Mechanisms of Control Protection
8.3.1.1 Nickel Coatings
There are three types of nickel coatings: bright, semibright, and dull bright.
The difference between the coatings is in the quantity of sulfur contained in
them, as shown:
Nickel Coating
Sulfur Content
(%)
Bright nickel deposits
>0.04
Semibright nickel deposits
<0.005
Dull bright nickel deposits
<0.001
Substrate
e –
e
–
Metallic film
Me
n+
FigurE 8.6
Dissolution of substrate metal in coating defect.