Corrosion of Metallic Coatings
277
8.4 Cathodic Control by Sacrificial Metal Coatings
Non-noble metals protect the substrate by means of cathodic control. Cathodic
overpotential of the surface is increased by coating, which makes the corrosion potential more negative than that of the substrate. The coating metals
used for cathodic control protection are zinc, aluminum, manganese, and cadmium, and their alloys, of which the electrode potentials are more negative
than those of iron or steel. Consequently, the coating layers of these metals
act as sacrificial anodes for iron and steel substrates when the substrates are
exposed to the atmosphere. The coating layer provides cathodic protection for
the substrate by galvanic action. These metals are called sacrificial metals.
Sacrificial metal coatings protect iron and steel by two or three protective abilities:
1. Original barrier action of coating metal
2. Secondary barrier action of corrosion product layer
3. Galvanic action of coating layer
The surface oxide film and the electrochemical properties based on the metallography of the coating metal provide the original barrier action.
An air-formed film of Al 2 O 3 , approximately 25-Å thick, forms on aluminum.
This film is chemically inert, and its rapid formation of oxide film by a selfhealing ability leads to satisfactory performance in natural environments.
Zinc, however, does not produce a surface oxide film that is as effective a
barrier as the oxide on aluminum. The original barriers of zinc and zinc alloy
coatings result from the electrochemical properties based on the structure of
the coating layer.
Nonuniformity of the surface condition generally induces the formation
of a corrosion cell. Such nonuniformity results from defects in the surface
oxide film, localized distribution of elements, and the difference in crystal
face or phase. These surface nonuniformities cause the potential difference
between portions of the surface, thereby promoting the formation of a corrosion cell.
Many corrosion cells are formed on the surface, accelerating the corrosion rate, as a sacrificial metal and its alloy-coated materials are exposed
in the natural atmosphere. During this time, corrosion products are gradually formed and converted to a stable layer after a few months of exposure.
Typical corrosion products formed are shown in Table 8.3. Once the stable
layer has formed, the corrosion rate becomes constant. This secondary barrier of corrosion protection regenerates continuously over a long period of
time. In most cases, the service life of a sacrificial metal coating depends on
the secondary barrier action of the corrosion product layer.
277
8.4 Cathodic Control by Sacrificial Metal Coatings
Non-noble metals protect the substrate by means of cathodic control. Cathodic
overpotential of the surface is increased by coating, which makes the corrosion potential more negative than that of the substrate. The coating metals
used for cathodic control protection are zinc, aluminum, manganese, and cadmium, and their alloys, of which the electrode potentials are more negative
than those of iron or steel. Consequently, the coating layers of these metals
act as sacrificial anodes for iron and steel substrates when the substrates are
exposed to the atmosphere. The coating layer provides cathodic protection for
the substrate by galvanic action. These metals are called sacrificial metals.
Sacrificial metal coatings protect iron and steel by two or three protective abilities:
1. Original barrier action of coating metal
2. Secondary barrier action of corrosion product layer
3. Galvanic action of coating layer
The surface oxide film and the electrochemical properties based on the metallography of the coating metal provide the original barrier action.
An air-formed film of Al 2 O 3 , approximately 25-Å thick, forms on aluminum.
This film is chemically inert, and its rapid formation of oxide film by a selfhealing ability leads to satisfactory performance in natural environments.
Zinc, however, does not produce a surface oxide film that is as effective a
barrier as the oxide on aluminum. The original barriers of zinc and zinc alloy
coatings result from the electrochemical properties based on the structure of
the coating layer.
Nonuniformity of the surface condition generally induces the formation
of a corrosion cell. Such nonuniformity results from defects in the surface
oxide film, localized distribution of elements, and the difference in crystal
face or phase. These surface nonuniformities cause the potential difference
between portions of the surface, thereby promoting the formation of a corrosion cell.
Many corrosion cells are formed on the surface, accelerating the corrosion rate, as a sacrificial metal and its alloy-coated materials are exposed
in the natural atmosphere. During this time, corrosion products are gradually formed and converted to a stable layer after a few months of exposure.
Typical corrosion products formed are shown in Table 8.3. Once the stable
layer has formed, the corrosion rate becomes constant. This secondary barrier of corrosion protection regenerates continuously over a long period of
time. In most cases, the service life of a sacrificial metal coating depends on
the secondary barrier action of the corrosion product layer.