Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 312

ANSWER:
B . Immunity is the absence of corrosion (as previously defined).
Thus, metal loss is trivial, and biologic responses are suppressed.
Desensitization to elements present in metal alloys is possible, but it is
not called “immunity.” The loss of the fatigue endurance limit may not
be seen under immune conditions.
PROBLEM 12.5
Crevice corrosion (select the phrase that best completes the sentence)
A. Only occurs over small areas and so is not important
B. Is more likely for cobalt-base alloys than for stainless steels
C. Only occurs if an implant has been in place for a substantial period
D. Does not occur if a metal has been passivated
E. May show intense local attack and cause implant failure
ANSWER:
E , since crevice corrosion can produce initiation sites for either single-
cycle or fatigue failure. Stainless steels are generally more prone to crev-
ice corrosion than cobalt-base or titanium-base alloys. Crevice corrosion
is more likely with longer periods of implantation but may occur at any
time. Passivation may be damaged by fretting or reduced by local condi-
tions within a crevice.
PROBLEM 12.6
Passivation of a metal (select the phrase that best completes the sentence)
A. Is a process intended to increase yield strength
B. Reduces the tendency to corrode
C. Is a precautionary measure of little practical use
D. Reduces the hazard of implant site infection
E. Occurs after implantation
ANSWER:
B . Although passivation will occur spontaneously in vivo , factory treat-
ments are more reliable and provide important protection against corro-
sion. However, passivation does not affect yield strength nor does it have
an apparent effect on implant site infectability.
PROBLEM 12.7
Chromium is placed in a saline solution exposed to air and maintained at
+ 0.5 V with respect to an H/H + electrode. If corrosive attack is observed,
the pH of the solution may be
A. 7.0
B. 7.4