290
Fundamentals of Corrosion
Cadmium coatings should not be allowed to come into contact with food
products because cadmium salts are toxic. This coating is commonly used on
nuts and bolts but, because of its toxicity, usage is declining.
8.4.1.9 Manganese Coatings
Manganese is very active, having an electrode potential more negative than
zinc (Mn: −1.5 V, Zn: –1.03 V, SCE). In a natural atmosphere, a dense corrosion layer builds on the surface of manganese during a very short time
period. However, defects in the coating accelerate the anodic dissolution of
manganese, thereby shortening the life of the coating. Therefore, manganese
is combined with zinc to form a duplex Mn-Zn alloy coating. The types of
corrosion products found on these coatings are shown in Table 8.3. The compound γ-Mn 2 O is effective for the formation of a barrier. The more γ-Mn 2 O 3
in the corrosion products, the denser the layer on the Mn-Zn coating.
Manganese is so negative in electrochemical potential, and active, that its
alloy and duplex coatings provide galvanic protection. Mn-Zn alloy coatings exhibit high corrosion resistance, and the corrosion potential of manganese is more negative than that of zinc; therefore, this alloy coating provides
cathodic protection to a steel substrate. The structure of the Mn-Zn alloy
is composed of the single phase of ε in the manganese content range <20%
and ε and γ phases in the range above 20%. As the manganese content in the
deposit increases, so does the percentage of γ-Mn.
8.5 Mixed Control Protection
The term “conversion coating” is used to describe coatings in which the
substrate metal provides ions that become part of the protective coating.
The coating layers are composed of inorganic compounds that are chemically inert. These inert compounds on the surface reduce both anodic and
cathodic areas and delay the transit of reactive species to the base metal.
This results in increases in the slope of the anodic and cathodic polarization
curves, thereby decreasing the rate of corrosion of the substrate.
Conversion layers are used for various reasons, including:
1. To improve the adherence of the organic layers
2. To obtain electrically insulating barrier layers
3. To provide a uniform, grease-free surface
4. To provide active corrosion inhibition by reducing the rate of the
oxygen reduction reaction, or by passivating the metallic substrate
Fundamentals of Corrosion
Cadmium coatings should not be allowed to come into contact with food
products because cadmium salts are toxic. This coating is commonly used on
nuts and bolts but, because of its toxicity, usage is declining.
8.4.1.9 Manganese Coatings
Manganese is very active, having an electrode potential more negative than
zinc (Mn: −1.5 V, Zn: –1.03 V, SCE). In a natural atmosphere, a dense corrosion layer builds on the surface of manganese during a very short time
period. However, defects in the coating accelerate the anodic dissolution of
manganese, thereby shortening the life of the coating. Therefore, manganese
is combined with zinc to form a duplex Mn-Zn alloy coating. The types of
corrosion products found on these coatings are shown in Table 8.3. The compound γ-Mn 2 O is effective for the formation of a barrier. The more γ-Mn 2 O 3
in the corrosion products, the denser the layer on the Mn-Zn coating.
Manganese is so negative in electrochemical potential, and active, that its
alloy and duplex coatings provide galvanic protection. Mn-Zn alloy coatings exhibit high corrosion resistance, and the corrosion potential of manganese is more negative than that of zinc; therefore, this alloy coating provides
cathodic protection to a steel substrate. The structure of the Mn-Zn alloy
is composed of the single phase of ε in the manganese content range <20%
and ε and γ phases in the range above 20%. As the manganese content in the
deposit increases, so does the percentage of γ-Mn.
8.5 Mixed Control Protection
The term “conversion coating” is used to describe coatings in which the
substrate metal provides ions that become part of the protective coating.
The coating layers are composed of inorganic compounds that are chemically inert. These inert compounds on the surface reduce both anodic and
cathodic areas and delay the transit of reactive species to the base metal.
This results in increases in the slope of the anodic and cathodic polarization
curves, thereby decreasing the rate of corrosion of the substrate.
Conversion layers are used for various reasons, including:
1. To improve the adherence of the organic layers
2. To obtain electrically insulating barrier layers
3. To provide a uniform, grease-free surface
4. To provide active corrosion inhibition by reducing the rate of the
oxygen reduction reaction, or by passivating the metallic substrate