Biomedical Engineering Reference
In-Depth Information
The significance of the Pourbaix diagram is as follows. Different parts of the body have different pH
values and oxygen concentrations. Consequently, a metal which performs well (is immune or passive) in
one part of the body may suffer an unacceptable amount of corrosion in another part. Moreover, pH can
change dramatically in tissue that has been injured or infected. In particular, normal tissue fluid has a pH
of about 7.4, but in a wound it can be as low as 3.5, and in an infected wound the pH can increase to 9.0.
Pourbaix diagrams are useful, but do not tell the whole story; there are some limitations. Diagrams
are made considering equilibrium among metal, water, and reaction products. The presence of other
ions, for example, chloride, may result in very much different behavior, and large molecules in the body
may also change the situation. Prediction of passivity may in some cases be optimistic, since reaction
rates are not considered.
1.7.3 Rate of Corrosion and Polarization Curves
The regions in the Pourbaix diagram specify whether corrosion will take place, but they do not deter-
mine the rate. The rate, expressed as an electric current density (current per unit area), depends on
electrode potential as shown in the polarization curves in Figure 1.13. From such curves, it is possible to
calculate the number of ions per unit time liberated into the tissue, as well as the depth of metal removed
by corrosion in a given time. An alternative experiment is one in which the weight loss of a specimen of
metal due to corrosion is measured as a function of time.
The rate of corrosion also depends on the presence of synergistic factors, such as those of mechanical
origin (uneven distribution of mechanical stress). The stressed alloy failures occur due to the propaga-
tion of cracks in corrosive environments. For example, in corrosion fatigue (stress corrosion cracking),
repetitive deformation of a metal in a corrosive environment results in acceleration of both the cor-
rosion and the fatigue microdamage. Since the body environment involves both repeated mechanical
loading and a chemically aggressive environment, fatigue testing of implant materials should always be
performed under physiological environmental conditions, under Ringer's solution at body temperature.
In
fretting corrosion
, rubbing of one part on another disrupts the passivation layer, resulting in acceler-
ated corrosion. In
pitting
, the corrosion rate is accelerated in a local region. Stainless steel is vulnerable
to pitting. Localized corrosion can occur if there is inhomogeneity in the metal or in the environment.
Grain boundaries
in the metal may be susceptible to the initiation of corrosion, as a result of their higher
Active
Base metal substrate
0.2
Amalgam
0
3
16 Stainless steel
0.2
0.4
Co
-
Cr alloy
0.6
Dental gold
0.8
Ti alloy
1.0
Noble
10
-5
10
-4
10
-3
Current density (mA/cm
2
)
10
-2
10
-1
10
0
FIGURE 1.13
Potential-current density curves for some biomaterials. (Adapted from Greener, E.H., Harcourt,
J.K., and Lautenschlager, E.P. 1972.
Materials Science in Dentistry
. Baltimore, MD: Williams & Wilkins.)
