Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 292

(a) (b)

2.0
1.6
1.2
0.8
0.4

Corrosion Corrosion

Saliva

Interstitial
Intracellular

E (V)

Gastric

0

Passivation

Passivation

-0.4
-0.8
-1.2
-1.6

Bile
Urine

Immunity Immunity

pH 0246810

12 14

pH 0246810

12 14

NoCl - 0.1 M Cl -

FIGUre 12.4 Pourbaix (ph-potential) diagrams for chromium in
water.

ions in solution is 10 −6 M. They define three areas that correspond to the
three fundamental generic processes of corrosion previously discussed.

Corrosion . In this region, a variety of reactions either oxidize chro-
mium (3.9 < pH < 15.2) or promote the solubility of the passive
coating (3.9 ≥ pH ≥ 15.2). Throughout this region, the concentra-
tion of chromium-bearing ions is greater than 10 −6 M. Note that
the valence of the dissolved species may vary within this corrosion
region. Under acidic conditions (pH < 7), the predominant valence
is +3, whereas under alkaline conditions (pH > 7), the predomi-
nant valence is +6. (This may have a dramatic effect on the bio-
logic effects of corrosion products; see Chapter 14.)
Passivation. In this region, the dominant reactions lead to the forma-
tion of oxides and hydroxides of chromium. Since these products
are largely insoluble at pH values above 4, they cling to the metal,
reducing and eventually preventing further reaction of the base
metal with the solution. This renders the chromium passive; it has
a similar beneficial effect on any alloy containing chromium and
is the basis of the excellent corrosion resistance of stainless steels
and chromium-containing cobalt-base alloys. Throughout this
region, the solubility of the passive layer is low enough that the
concentration of chromium-containing ions in solution is less than
10 −6 M.
Immunity . In this region, the dominant reaction is ionization.
However, throughout this part of the diagram, the resulting equi-
librium concentration of chromium in solution is less than 10 −6 M.
The metal remains bright; conditions do not favor formation of a
passive layer. If a passive layer is preformed by chemical means,
it will disassociate in this region; however, the rate may be slow
enough near the passive boundary, as is the case for chromium, to
render the passive layer metastable and thus effectively to expand