Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 293

the borders of the passive region. Such a process is performed on
all chromium-bearing alloy implants during manufacture (see
Chapter 7). The resulting chromium oxide-hydroxide passivation
layer is extremely thin and colorless and cannot be seen with the
naked eye but can be detected instrumentally.
Two additional features of Figure 12.4a are of interest. The upper
and lower diagonal dashed lines define the interactions of oxygen and
hydrogen, respectively, with water under standard conditions. Between
the lines, water is stable; at potentials above the upper line, gaseous oxy-
gen is released, whereas at those below the lower line, gaseous hydrogen
is released. These are the familiar anodic and cathodic products of elec-
trolysis; the line separation is constant and equal to 1.228 V, correspond-
ing to the lowest possible inter-electrode electrolysis voltage for water.
On the Pourbaix diagram, dominant reactions that depend on pH pro-
duce vertical traces, whereas those that depend on potential (electron
availability) produce horizontal traces. Reactions that depend on both
pH and potential, such as the equilibrium between oxygen and water,
produce diagonal lines that pass from upper left to lower right. Reactions
independent of pH and potential do not produce regions of dominance
in such a diagram.
In biologic environments, where pH is controlled by buffering, the
local oxygen or hydrogen partial pressure can then define an effective
local potential. Thus, tissues that are perfused with arterial blood and
are maintained at pH 7.37 have an equivalent potential of 0.782 V at
37°C. This characteristic of biologic environments makes it possible
to use Pourbaix diagrams directly in predicting the dominant corro-
sion process (corrosion, passivation, or immunity) for any metal after
implantation.
Practical aspects of Pourbaix diagrams
Most environments contain other chemical species in addition to H + ,
OH − , and metal ions released from the bulk. Figure 12.4b is the Pourbaix
diagram for chromium in water with a chloride concentration of 0.1 M,
to better simulate the typical in vivo conditions. The principal difference
from Figure 12.4a is a radical shrinkage of the passivation region, owing
to Cl − forming complexes with ions in solution and increasing the solu-
bility of the passive layer.
It is possible to locate conditions present in various body fluids on
this diagram through knowledge of their pH and gaseous exposure. In
the absence of an applied (external) voltage, the pH and pO 2 at a par-
ticular point define an “equivalent” potential. If an electrode possessing
this potential is placed at this location, there will be no effect on local
chemical reactions. Thus, knowing any two of the triad (pH, pO 2 , and
electric potential) permits certain knowledge of the third. As a result,
fluids exposed directly to air, such as saliva, or present in the upper gas-
trointestinal (GI) tract may be located near the oxygen equilibrium line