Page 230 - Materials Chemistry, Second Edition

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3.3. Metal Surface Treatments for Corrosion Resistance

As one would expect, there are a number of applications that currently use shape-
memory alloy materials; many more are projected for the future. The earliest
application was for greenhouse window openers, with the metal serving as an
actuator to provide temperature-sensitive ventilation. Some commercial faucets/
showerheads are already equipped with this material that shuts off the water if a
certain temperature is reached, which effectively prevents scalding. An intriguing
future application will be for automobile frames; as we will see later, some plastics
may also be designed with shape memory. Someday soon, your car may reshape
itself in front of your eyes within minutes after an accident!
Since NiTi alloys have been shown to be biocompatible, perhaps the greatest use
for these alloys has been for medical applications. In particular, for minimally
invasive surgery where a metal wire is inserted through tiny incisions, and then
reshaped into the original form (e.g., tweezers, specialized probes, etc.) while inside
the patient due to an increase in temperature! Likewise, metal probes may be bent
into specific shapes for open surgeries and then returned to their original positions
afterward through the heating/sterilization process. Other widespread applications
include nitinol filters that are designed to trap blood clots in arteries, as well as suture
anchors that are inserted directly into bone to facilitate the attachment of soft tissue
such as ligaments and tendons. [20]
Although we have not yet introduced nanotechnology, it is worthwhile to point
out that shape-memory behavior has also been discovered in single-crystalline Cu
[21]
nanowires. This is interesting since this effect does not occur in bulk copper
metal. Due to the extremely high surface/volume ratio of nanowires relative to bulk
structures, these materials show reversible strains of ca. 50% – an order of magni-
tude larger than bulk shape-memory alloys. Further, the martensite–austenite trans-
formation temperature changes dramatically with minute changes in nanowire
diameter. For instance, increasing the diameter from 1.76 to 3.39 nm causes a ca.
800K increase in the transition temperature! This will allow for the design of
nanoscale components of varying sizes that will be functional over an extremely
wide temperature range – not possible with bulk alloy systems. More importantly,
the response time for these materials is also orders of magnitude faster than bulk
alloys due to the extremely small dimensions of the nanoscale. We will see more of
the “nanoworld” in Chapter 6 – this intriguing example was inserted to wet your
appetite a bit.. .

3.3. METAL SURFACE TREATMENTS FOR CORROSION RESISTANCE
The corrosive deterioration of metal surfaces incurs a great cost to the worldwide
economy. Accordingly, there have been many research efforts devoted to under-
standing the surface chemistry behind these reactions. As we have already seen, this
has led to the development of a number of useful alloys that are sufficiently resistant
to corrosion through spontaneous formation of protective oxide layers. However,
for other less resistant metals such as carbon steels, a protective layer must be