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Fundamentals of Corrosion
8.2 EMF Control Protection by Noble Metals
Noble coating metals that provide corrosion protection by means of EMF
control include copper, silver, platinum, gold, and their alloys. The standard
single potentials of these metals are more noble than those of hydrogen (refer
to Table 8.2). Therefore, the oxidizer in corrosion cells formed on these metals
in a natural environment, containing no other oxidizers, is dissolved oxygen.
Consequently, the electromotive force that causes corrosion is so small that
coating with noble metals is an effective means of providing corrosion protection. With the exception of copper, the other members of this group are
precious metals and are used primarily for electrical conduction and decorative appearance.
8.2.1 Mechanism of Control Protection
8.2.1.1 Copper Coatings
Although copper is soft, it has many engineering applications in addition
to its decorative function. One such application is the corrosion protection
of steel. It can be used as an alternative to nickel to prevent fretting and
scaling corrosion. Copper can be deposited electrochemically from various
aqueous solutions. The properties of the deposit will depend on the chosen
bath and the applied procedures. The hardness of the layers varies from 40
to 160 HV.
Because copper is very noble, it causes extreme galvanically induced local
corrosion of steel and aluminum substrates. Because of this, extreme care
must be taken to produce well-adhering nonporous layers.
The corrosion protection provided by copper coating is twofold, consisting of an original barrier action of the coating layer and a secondary barrier
action of corrosion products. The low EMF of copper is responsible for the
formation of the original barrier action. The electrochemical reactions in the
corrosion cell on copper are as follows:
Anodic reaction:
Cu → Cu + + e −
Cu → Cu 2+ + 2e −
Cathodic reaction:
O 2 + H 2 O → 4e − + 4OH −
Chloride ions in a natural environment stabilize cuprous ions. Cupric ions
are more stable. Because the EMF of corrosion on copper is less than that on
iron, the reactivity of a steel surface decreases by coating it with copper.
Over a period of time, corrosion products gradually build up a secondary
layer against corrosion. Initially, a cuprous layer is formed, followed by the
copper surface covered with basic copper salts. Pollutants in the atmosphere
determine the formation of basic copper salts as follows:
Fundamentals of Corrosion
8.2 EMF Control Protection by Noble Metals
Noble coating metals that provide corrosion protection by means of EMF
control include copper, silver, platinum, gold, and their alloys. The standard
single potentials of these metals are more noble than those of hydrogen (refer
to Table 8.2). Therefore, the oxidizer in corrosion cells formed on these metals
in a natural environment, containing no other oxidizers, is dissolved oxygen.
Consequently, the electromotive force that causes corrosion is so small that
coating with noble metals is an effective means of providing corrosion protection. With the exception of copper, the other members of this group are
precious metals and are used primarily for electrical conduction and decorative appearance.
8.2.1 Mechanism of Control Protection
8.2.1.1 Copper Coatings
Although copper is soft, it has many engineering applications in addition
to its decorative function. One such application is the corrosion protection
of steel. It can be used as an alternative to nickel to prevent fretting and
scaling corrosion. Copper can be deposited electrochemically from various
aqueous solutions. The properties of the deposit will depend on the chosen
bath and the applied procedures. The hardness of the layers varies from 40
to 160 HV.
Because copper is very noble, it causes extreme galvanically induced local
corrosion of steel and aluminum substrates. Because of this, extreme care
must be taken to produce well-adhering nonporous layers.
The corrosion protection provided by copper coating is twofold, consisting of an original barrier action of the coating layer and a secondary barrier
action of corrosion products. The low EMF of copper is responsible for the
formation of the original barrier action. The electrochemical reactions in the
corrosion cell on copper are as follows:
Anodic reaction:
Cu → Cu + + e −
Cu → Cu 2+ + 2e −
Cathodic reaction:
O 2 + H 2 O → 4e − + 4OH −
Chloride ions in a natural environment stabilize cuprous ions. Cupric ions
are more stable. Because the EMF of corrosion on copper is less than that on
iron, the reactivity of a steel surface decreases by coating it with copper.
Over a period of time, corrosion products gradually build up a secondary
layer against corrosion. Initially, a cuprous layer is formed, followed by the
copper surface covered with basic copper salts. Pollutants in the atmosphere
determine the formation of basic copper salts as follows: