Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 304

erosion and
fretting corrosion

In its pure form, erosion is a rare form of corrosion. It is an acceleration
of attack caused by relative motion between the surrounding electrolyte
and the metallic surface. It is not a unique process; it simply serves to
increase the rate of attack by several other mechanisms. This occurs
since many corrosion processes are self-limiting, that is, inhibited by
their reaction products. Flowing solutions will sweep away these prod-
ucts as well as provide new amounts of dissolved reactants, such as Cl −
and oxygen (Figure 12.14). In extreme cases, the solution may actually
erode the passive layer in regions of passivity. The continual reformation
of this layer and its removal by continued flow processes can produce
progressive attack on the metal surface in regions of the Pourbaix dia-
gram where passivation is normally expected.
Fretting corrosion is closely related to erosion corrosion. Fretting is
the physical removal of metal, by wear, owing to the relative motion of
surfaces. Such a condition may occur between screw head and plate in the
case of unstable internal fixation of fractures. However, if the surfaces
are passivated and in a corrosive environment, such as in vivo , the result
will be continued accelerated corrosion as the passive layer is physically
removed and reformed. The damage seen in Figure 12.9 is probably
partially due to fretting and possibly to fretting corrosion, owing to the
intrinsic relative motion in such modular designs. Paradoxically, serum
proteins, which increase uniform attack rates, appear to reduce fretting
corrosion significantly, at least for stainless steels, in vitro .
The physical damage caused by erosion and fretting corrosion resem-
bles pitting, except that the defects are elongated in the direction of flow
or relative movement and are generally larger and less symmetric than
in true pitting. In true erosion corrosion, especially if the flow pattern is
stable, it may be visualized by the resulting etched surface pattern.

Stress corrosion The final form of corrosion to be considered results from tensile stresses

that increase the chemical activity of metals. Thus, a straight fracture
plate, when flexed, will experience a tensile stress on its convex surface

(O - )

(M + )

Flow

e -

(O 2 )

FIGUre 12.14 erosion corrosion.