Biomaterials - Introduction to Biomedical Engineering - page 243

TABLE 5.3 Electromotive Force Series: Standard Reduction Potentials
(E 0 V) in Aqueous Solution at 25 C

K þ þ e K ¼ -2.93 Active (more anodic)

Na þ þ e Na ¼ -2.71

Al þ3 3e Al ¼ þ -1.66

Ti þ2 2e

Ti ¼ þ -1.63

Zn þ2 2e Zn ¼ þ -0.76

Cr þ3 3e Cr ¼ þ -0.74

Fe þ2 2e Fe ¼ þ -0.44

Co þ2 2e Co ¼ þ -0.28

Ni þ2 2e

Ni ¼ þ -0.25

Sn þ2 2e Sn ¼ þ -0.14

2H þ þ 2e H 2 ¼ 0.000

Cu þ2 2e Cu ¼ þ þ 0.34

Ag þ þ e Ag ¼ þ 0.80

Pt ¼ Pt þ2

2e

þ þ 1.20

Au þ3 3e

Au ¼ þ þ 1.50 Noble (more cathodic)

potential in the positive or noble direction. As the Fe þ2 concentration increases, the poten-
tial difference between the iron and copper will become less as the iron becomes more
cathodic. Similarly, the concentration of oxygen at the cathode will affect the EMF of the
cell. Increasing O 2 will make it more noble, while decreasing O 2 will make it more anodic.
In fact, crevice corrosion is initiated by changes in oxygen concentration, as is discussed in a
following paragraph.
Galvanic cells occur not only with different alloys but also with differences within an alloy.
Carbides, grain boundaries, and different phases within an alloy also present differences in
EMF and thus the possibility for localized galvanic cells. Cold working also increases the free
energy of metal and thus its susceptibility to corrosion. Bending a plate or pounding on a nail
head causes localized cold working and makes that area anodic to the rest of the piece.
Galvanic corrosion can also be utilized to prevent corrosion by cathodically polarizing
the part to be protected. Steel ships are protected from rusting by the attachment of blocks
of zinc. The zinc blocks (“zincs”) serve as a sacrificial anode and protect the steel hull. Metal
pumps and other metallic components on ships are also protected with zincs. A power sup-
ply can be attached to a part, such as in a steel underground pipeline, to make the pipe
cathodic to a replaceable anode. This protects the pipeline.
Electrode size also has an effect on galvanic reaction rates. The classic example is the
difference between galvanized and tin-plated iron. As Table 5.3 shows, zinc is anodic to
iron. Thus, galvanization results in coating the iron with an anodic material. When the zinc