Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 288

Table 12.2 electrochemical series (ideal)

E 0 increasingly negative (noble) Gold Au/Au 3+

Platinum Pt/Pt 2+

Silver Ag/Ag +

Copper Cu/Cu 2+

E 0 = 0 Hydrogen H/H +

Iron Fe/Fe 3+

Lead Pb/Pb 2+

Molybdenum Mo/Mo 3+

Nickel Ni/Ni 2+

Vanadium V/V 5+

Cobalt Co/Co 2+

Iron Fe/Fe 2+

Tantalum Ta/Ta 3+

Zinc Zn/Zn 2+

Chromium Cr/Cr 3+

Manganese Mn/Mn 2+

Vanadium V/V 2+

Aluminum Al/Al 3+

Titanium Ti/Ti 2+

E 0 increasingly positive (base) Magnesium Mg/Mg 2+

The following observations can be made in such an arrangement:

1. There is a difference in electrical potential between the two metals
( electrodes ), inferred by observing the passage of an electron cur-
rent through the wire.

2. The metal that is base with respect to the other, that is, below it
in the electrochemical series, is negatively charged, is said to be
anodic ,* and undergoes an oxidation process leading to either the
release of metal or the formation of a passive layer.

3. The metal that is noble with respect to the other, that is, above it
in the electrochemical series, is positively charged, is said to be
cathodic , and is immune. Such immunity, imposed by electrical
coupling to another electrode, is called cathodic protection.

* There is often confusion about the sign of the potential of the cathode or anode. In a
formal way, the cathode is always the electrode at which reduction takes place. If the
reduction is driven spontaneously by the chemical energetics of the experiment, as in
this case, the cathode is positive with respect to the anode. If the cathode is polarized
by an external power source, as in faradic stimulation of bone growth (see Chapter 5),
it is negative with respect to the anode.