36
Fundamentals of Corrosion
3.3 Galvanic Corrosion
This form of corrosion is sometimes referred to as dissimilar metal corrosion, and is found in unusual places, often causing professionals the most
headaches. Galvanic corrosion is often experienced in older homes where
modern copper piping is connected to the older existing carbon steel piping.
The coupling of the carbon steel to the copper causes the carbon steel to corrode. The galvanic series of metals provides details of how galvanic current
will flow between two metals and which metal will corrode when they are in
contact or near each other and an electrolyte is present (e.g., water). Table 3.1
lists the galvanic series.
When two different metallic materials are electrically connected and
placed in a conductive solution (electrolyte), an electric potential exists. This
potential difference will provide a stronger driving force for the dissolution
of the less noble (more electrically negative) material. It will also reduce the
tendency for the more noble metal to dissolve. Notice in Table 3.1 that the
precious metals gold and platinum are at the higher potential (more noble or
cathodic) end of the series (protected end), while zinc and magnesium are at
the lower potential (less noble or anodic) end. It is this principle that forms
the scientific basis for using such materials as zinc to sacrificially protect
the stainless steel drive shaft on a pleasure boat. When placed alone in corrosive media, both members may corrode, but at different rates. The metal
having the higher corrosion rate will become the corroding member in the
couple (anode). The other member becomes the cathode. The cathode may
suffer from hydrogen damage while galvanic corrosion is taking place on
the cathode.
Note that several materials are shown in two places in the galvanic series
being indicated as either active or passive. This is the result of the tendency
of some metals and alloys to form surface films, especially in oxidizing environments. This film shifts the measured potential in the noble direction. In
this state, the material is said to be passive.
The particular way in which a metal will react can be predicted from the
relative positions in the galvanic series. When it is necessary to use dissimilar metals, two materials should be selected that are relatively close in the
galvanic series. The farther apart the metals are in the galvanic series, the
greater the rate of corrosion. The rate of corrosion is also affected by the
relative areas between the anode and cathode. Because the flow of current
is from the anode to the cathode, the combination of a large cathodic area
and a small anodic area is undesirable. Corrosion of the anode can be 100 to
1000 times greater than if the two areas were equal. Ideally, the anodic area
should be larger than the cathodic area.
The passivity of stainless steel is the result of a corrosion-resistant oxide
film on the surface. In most material environments, it will remain in the
passive state and tend to be cathodic to ordinary iron or steel. When
Fundamentals of Corrosion
3.3 Galvanic Corrosion
This form of corrosion is sometimes referred to as dissimilar metal corrosion, and is found in unusual places, often causing professionals the most
headaches. Galvanic corrosion is often experienced in older homes where
modern copper piping is connected to the older existing carbon steel piping.
The coupling of the carbon steel to the copper causes the carbon steel to corrode. The galvanic series of metals provides details of how galvanic current
will flow between two metals and which metal will corrode when they are in
contact or near each other and an electrolyte is present (e.g., water). Table 3.1
lists the galvanic series.
When two different metallic materials are electrically connected and
placed in a conductive solution (electrolyte), an electric potential exists. This
potential difference will provide a stronger driving force for the dissolution
of the less noble (more electrically negative) material. It will also reduce the
tendency for the more noble metal to dissolve. Notice in Table 3.1 that the
precious metals gold and platinum are at the higher potential (more noble or
cathodic) end of the series (protected end), while zinc and magnesium are at
the lower potential (less noble or anodic) end. It is this principle that forms
the scientific basis for using such materials as zinc to sacrificially protect
the stainless steel drive shaft on a pleasure boat. When placed alone in corrosive media, both members may corrode, but at different rates. The metal
having the higher corrosion rate will become the corroding member in the
couple (anode). The other member becomes the cathode. The cathode may
suffer from hydrogen damage while galvanic corrosion is taking place on
the cathode.
Note that several materials are shown in two places in the galvanic series
being indicated as either active or passive. This is the result of the tendency
of some metals and alloys to form surface films, especially in oxidizing environments. This film shifts the measured potential in the noble direction. In
this state, the material is said to be passive.
The particular way in which a metal will react can be predicted from the
relative positions in the galvanic series. When it is necessary to use dissimilar metals, two materials should be selected that are relatively close in the
galvanic series. The farther apart the metals are in the galvanic series, the
greater the rate of corrosion. The rate of corrosion is also affected by the
relative areas between the anode and cathode. Because the flow of current
is from the anode to the cathode, the combination of a large cathodic area
and a small anodic area is undesirable. Corrosion of the anode can be 100 to
1000 times greater than if the two areas were equal. Ideally, the anodic area
should be larger than the cathodic area.
The passivity of stainless steel is the result of a corrosion-resistant oxide
film on the surface. In most material environments, it will remain in the
passive state and tend to be cathodic to ordinary iron or steel. When