Environmental Engineering Reference
In-Depth Information
fact that the corrosive environment gets more time to interact with the metal. A
pit is expected to provide aggressive corrosive conditions congenial to the pro-
cess, and very often the corrosion fatigue crack initiates at the base of a pit.
Remedial Measures
Lowering of Stress.
Lowering of operating stresses by lowering the mean stress
or the amplitude of the cyclic stress leads to an increase of corrosion fatigue life.
This can be accomplished by changes in design. Points of stress concentration,
like sharp fillets, may be suitably modified.
Control of Environment.
Corrosion fatigue is reduced if the corrosive factors
are controlled. Deaeration of saline solution has been reported [53] to restore the
normal fatigue limit in air for steels. Addition of inhibitors is also effective.
Use of Coatings.
Organic coatings, like paints, act as a physical barrier between
the metal and the environment and lower the incidence of corrosion fatigue. In-
hibitors can be incorporated in such coatings. Coatings of zinc or cadmium on
steel provide cathodic protection to the base metal. Noble metal coatings on steel
act as barriers, but they should be sufficiently dense and thick. Breakage and
discontinuity not only increases corrosion at those points but provides a ready-
made site for corrosion crack initiation.
Polarization.
Cathodic protection by impressed current reduces corrosion and,
subsequently, corrosion fatigue. For steels, cathodic polarization to
0.49 V
(SHE) in salt water provides protection. In high-strength steels, however, cathodic
polarization may lead to hydrogen-induced cracking. In stainless steels, protec-
tion can be achieved by producing passivity by anodic polarization.
Shot Peening.
Shot peening introduces compressive stresses to the surface and
is effective in combating fatigue in air. It is marginally effective for corrosion
fatigue, as severely aggressive environments tend to remove the compressive
layer by dissolution.
Practical Examples
Vibrating parts or metal structures that have been designed to operate safely in
air below the fatigue limit often fail due to corrosion fatigue. Examples include
wire ropes, springs, marine propellers, oil-well sucker rods, and heat ex-
changers.
Corrosion fatigue failure has been reported [54] in a hollow, spindled alloy
steel aircraft shaft that in operation was subject to static radial, cyclic torsional,
and cyclic bending stresses. The inner surface of the hollow shaft was continu-
ously exposed to hydraulic oil at temperatures of 0-80
C. The failure inves-
tigation revealed that fatigue cracks had originated from corrosion pits. The hy-
draulic oil was originally noncorrosive, but water contamination made it corro-
sive.
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