Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 175

present. These include ferrite (16-30% chromium, 0.01-0.2% carbon), aus-
tenite (16-20% chromium, 0.01-0.08% carbon) and martensite (11.5-13.5%
chromium, 0.15-0.2% carbon). Medical grade 316L high grade austenitic
stainless steel is the most commonly accepted steel used for biomedical
devices due to its favourable corrosion resistance, reduced carbon content
and cost (Gotman, 1997; Trethewey & Chamberlain, 1998). It is the most
commonly used implant material in surgery and has been extensively used
for 'fi rst generation' coronary stent production (Gotmann, 1997). This is due
to the fact that 316L stainless steel has been perceived as being well toler-
ated and to possess good mechanical and corrosion properties due to the
fact that it contains 12% nickel, 2% molybdenum and 17% chromium
which develops a passive chromium oxide fi lm, promoting corrosion resis-
tance (Trethewey & Chamberlain, 1998).

Spontaneous oxidation

The initial stage of oxidation is the adsorption of oxygen onto the surface
due to its negative charge (Chilton, 1968), followed by the formation of
oxide nuclei and a continuous oxide fi lm (Uhlig et al. , 1985b) producing a
relatively stable two-dimensional structure of mixed oxygen ion ( − ve)

and metal ions ( ve) (Uhlig et al., 1985a). The second layer of oxygen

is bound less energetically than the fi rst as it is adsorbed without the
dissociation of atoms. Oxygen bonding on transition metals is usually
more stable than on non-transition metals due to the ability of metal
ions to leave their lattice (Uhlig et al. , 1985b). Similarly, different
thicknesses of oxide layers form on the surface for different metals
(Chilton, 1968).
The layer of oxide formed on the surface protects the matrix and pre-
vents further corrosion damage (Chilton, 1968). Stainless steel develops
such a layer based on its chromium content (Trethewey & Chamberlain,
1998; Roubin et al. , 1992). This spontaneously generated layer has typical
polycrystalline oxide (p-oxide) properties and can provide a degree of
protection in aggressive environments. A study by Su (1998) found that an
amorphous oxide fi lm on stainless steel provides a better corrosion resis-
tance. The thin fi lm has a high atomic concentration of oxygen as well as
an abnormally high molybdenum concentration, cited as the most effec-
tive element for reducing pitting corrosion in stainless steel (Su, 1998).
This, combined with the absence of the crystalline defects, such as grain
boundaries and dislocations often found in polycrystalline oxide fi lms, is
thought to reduce the electrochemical breakdown of amorphous oxide
fi lms.
International standards F138-03 and F2257-03 from the American Society
for Testing and Materials (ASTM) stipulate the appropriate composition

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