Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 290

ANSWER:
Silver is more noble than chromium (Table 12.2); thus, the correct
answers are T, T, T, F, F. Chromium is not immune; in this case, it would
form chromium hydroxide, the basis of corrosion protection of many
alloys, and would be said to be passivated. However, its anodic bias
would promote continuing ionic release by surface layer dissolution.

Practical electrochemical (galvanic) series

However, corrosion is dependent on other factors in addition to the differ-
ences in half-cell potentials. These include the rate of approach to equilib-
rium (previously discussed), the physical properties of the reactants and their
corrosion products, and the pH, PO 2 , and local electrical potentials pres-
ent in a particular application. These considerations may be combined and
expressed in a very general way in a practical electrochemical series. Such
a series must be derived from experiments under conditions that match the
application, using a galvanic cell. No such general series exist for generic or
specific biomedical applications, but those obtained in seawater (for marine
applications) are a good predictive guide to behavior in vivo . A short practi-
cal seawater series is given in Table 12.3, together with the relative positions
in the ideal series, where they are known.
Changes in position occur in a practical ranking, relative to those in the
ideal series, reflecting differences in chemical interaction with the particular
environment selected. Thus, platinum is more noble than gold in seawater
because it is less reactive with Cl − ion. Copper and lead react easily with Cl − ,
so they are more basic. Aluminum, magnesium, and titanium react easily
with oxygen and thus are considered basic in pure water. However, titanium

Table 12.3 Practical versus ideal electrochemical

series

Ideal Practical (Seawater)

Noble Gold Platinum

Platinum Gold

Silver Stainless steel (passive)

Copper Titanium

Lead Silver

Nickel Nickel

Iron Stainless steel (nonpassive)

Chromium Copper

Aluminum Lead

Titanium Cast iron

Magnesium Aluminum

Base Magnesium