Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 301

FIGUre 12.11 sem image showing pitting corrosion.

Concentration gradients of both Cl − (consumed by complexing) and
metal-bearing ions (produced by the corrosion process) produce poten-
tials that serve to accelerate the rate of attack. A scanning electron
microscope (SEM) image showing clear pitting corrosion in an uniden-
tified material is shown in Figure 12.11.
Pits often occur in large numbers, such as along a scratch, and grow
downward in the direction of the gravitational field. They are a hazard in
highly stressed devices as they constitute points of stress concentration
and may serve as initiation points for fatigue failures. Like the results
of crevice corrosion, they are easy to see. When very small, they may
change the surface finish, often producing a “frosted” or matte appear-
ance. Larger, more developed pits often contain accumulations of col-
ored corrosion products and show up as brown, green, or black spots
against otherwise polished surfaces. However, these products are poorly
adhered to the implant and may be lost during too vigorous cleaning
after retrieval.
Pitting was frequently observed in older stainless steel fracture fixa-
tion hardware, most often on the undersides of screw heads. It also
occurs infrequently on the neck or the underside of the flange of proxi-
mal femoral endoprostheses. Improved alloy “cleanliness,” especially
the use of vacuum melting and remelting, has largely eliminated pitting
in such hardware; however, it is still seen in older hardware and on the
medullary stems of older stainless steel and cobalt-base alloy cast joint
replacement components. There continues to be debate over whether pit-
ting occurs in cobalt-base alloys or whether the attack seen is due to
corrosion-mediated release of impurity inclusions only. Pitting is rare in
titanium-base alloys, but may occur in the presence of very high concen-
trations of halide ions, which is unlikely to occur in vivo .

As discussed in Chapter 7, cast and forged products both possess regions
of continuous structure called grains . Between these grains, especially
in cast devices, there are disordered regions, the grain boundaries ,

Intergranular
corrosion