Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 286

except in a very general way: it can be assumed that the further away
from equilibrium conditions a reaction is (on the basis of the deviation of
actual reactant/product ratios from those known to exist at equilibrium),
the more rapidly the reaction will proceed. Beyond this, there are no
simple general rules, and much of corrosion research has been directed
toward determining reaction rates under various conditions.

Consequences
of corrosion
reactions

All corrosion reactions will result in attack on the bulk metal and an
increase in the concentration of metallic or metal-containing ions in
solution. If any reaction is possible, then there will always be some con-
centration of products in a solution containing the reactants. Thus, if a
cobalt-base alloy is placed in physiological saline solution, after the pas-
sage of some time, Co 2+ will always be found in solution, no matter what
the physiochemical conditions are. However, the concentration may or
may not be sufficient to be of clinical consequence. It is customary in
engineering practice to group the equilibrium conditions resulting from
corrosion reactions into three generic terms:

Corrosion * is the condition when the sum of the equilibrium con-
centrations of all ions containing a particular metallic element is
equal to or greater than 10 −6 M. †
Passivation is the condition when the sum of the equilibrium concentra-
tions of all ions containing a particular metallic element is less than
10 −6 M and there is an oxide or hydroxide layer on the surface of the
bulk material. This surface layer is called the passivation layer.
Immunity is the condition when the sum of the equilibrium concen-
trations of all ions containing a particular metallic element is less
than 10 −6 M in the absence of an oxide or hydroxide layer on the
surface of the bulk material.

Half-cell potentials

Although reaction kinetics are hard to predict, examination of reaction
energetics can provide reasonable assurance as to whether a particular
reaction can occur at all and, in the case of several possible reactions,
which one is favored.
For any chemical reaction, such as those given in Table 12.1, the
difference in energy level between the products and the reactants may
be expressed as an electrical potential. This potential can be formally
derived from this relationship:

Δ F = − n ∙ E 0 ∙ ℱ

* This is a narrower definition of the word corrosion than previously given. Both are
correct; the latter is used frequently to define the area of a Pourbaix diagram that is
associated with neither passive nor immune conditions.
† M is the abbreviation for molar, the situation in which one gram-atom/liter equivalent
of an element is in solution. Thus, a 10 −6 M solution of Co 2+ contains 58.9 μg of Co 2+ per
liter, since the atomic weight of cobalt is 58.9.