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8
Corrosion of Metallic Coatings
Metallic coatings on a metal substrate are applied for several purposes,
including improved appearance, improved wear resistance, influence on the
solderability of materials, and corrosion resistance. The selection of the type
of metal coating and the application method both depend on the properties required for the application. This chapter is primarily concerned with
the application of metallic coatings for the purpose of corrosion protection.
Because metallic corrosion almost always proceeds electrochemically in the
material environment, the electrochemical principle of corrosion protection
by coatings is discussed.
Commonly used corrosion-resistant coatings are classified into five categories on the basis of electrochemical principles:
1. Eclipse Molding Framework (EMF) control protection
2. Cathodic control protection
3. Anodic control protection
4. Mixed control protection
5. Resistance control protection
Theories of these protective abilities can be explained by the mechanisms of
the corrosion cell.
8.1 Corrosion Cell
In a natural environment, the corrosion of most metals occurs and proceeds
electrochemically. That is, the corrosion cell is formed on a metal surface
in an atmosphere in which oxygen and water are present, as shown in
Figure 8.1. The electrochemical reactions in the corrosion cell are written in
the general form:
Anodic reaction: M → M n+ + ne − (M = metal)
Cathodic reaction:
in acidic solution: 2H + + 2e − → H 2
in neutral and alkaline solutions: O 2 + 2H 2 O + 4e − → 4OH −
8
Corrosion of Metallic Coatings
Metallic coatings on a metal substrate are applied for several purposes,
including improved appearance, improved wear resistance, influence on the
solderability of materials, and corrosion resistance. The selection of the type
of metal coating and the application method both depend on the properties required for the application. This chapter is primarily concerned with
the application of metallic coatings for the purpose of corrosion protection.
Because metallic corrosion almost always proceeds electrochemically in the
material environment, the electrochemical principle of corrosion protection
by coatings is discussed.
Commonly used corrosion-resistant coatings are classified into five categories on the basis of electrochemical principles:
1. Eclipse Molding Framework (EMF) control protection
2. Cathodic control protection
3. Anodic control protection
4. Mixed control protection
5. Resistance control protection
Theories of these protective abilities can be explained by the mechanisms of
the corrosion cell.
8.1 Corrosion Cell
In a natural environment, the corrosion of most metals occurs and proceeds
electrochemically. That is, the corrosion cell is formed on a metal surface
in an atmosphere in which oxygen and water are present, as shown in
Figure 8.1. The electrochemical reactions in the corrosion cell are written in
the general form:
Anodic reaction: M → M n+ + ne − (M = metal)
Cathodic reaction:
in acidic solution: 2H + + 2e − → H 2
in neutral and alkaline solutions: O 2 + 2H 2 O + 4e − → 4OH −