Page 83 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 61

until the area of the transverse section of the metal is reduced sufficiently, such that
the ultimate stress limit is reached and then a fragile and brutal rupture occurs.

1.7.32 Important Factors of CF
CF is not specific as most materials suffer degradation because of the fatigue prop-
erties in aqueous media. For example, steel undergoes CF in fresh waters, seawater,
products of combustion condensates, and other chemical environments. In the case
of materials, the fatigue strength (or fatigue life at a given maximum stress value)
generally decreases in an aggressive environment. The effect varies widely depend-
ing on the particular metal-environment combination. The environment may affect
the probability of fatigue crack initiation, fatigue crack growth rate, or both (80).
CF depends strongly on the combined interactions of the mechanical (loading),
metallurgical, and environmental variables (9).

1.7.33 Stresses

The main mechanical properties of importance are (9): maximum stress or stress
intensify factor max or max , cyclic stress or stress intensity range, Δ or Δ ,stress
ratio R, cyclic loading frequency, cyclic load waveform (constant-amplitude loading),
load interactions in variable-amplitude loading, state of stress, residual stress, crack
size and shape, and their relation to component size geometry (9).
The greater the applied stress at each cycle, the shorter the time to failure. Mechan-
ical damage is more likely when load and frequency are high and corrosion damage
is likely at intermediate load and frequency. In these conditions, cracking may be
transgranular or intergranular, and the morphology may be similar to SCC morphol-
ogy. Cyclic stress has negligible influence on the resistance to fatigue while it has a
distinct influence on CF. The corrosion influence in fatigue corrosion is influenced
significantly by the frequency of the cyclic stress. The corrosion influence in fatigue
corrosion is more pronounced at lower frequencies as the contact between the metal
and corrosive agent is of longer duration.
Ultra-high-strength steels are very sensitive to the environment, such as distilled
water, and are characterized by high growth rates, which depend on the stress inten-
sity range Δ to a reduced power. Time-dependent CF crack growth occurs mainly
above the threshold stress intensity for static load cracking and modeled by linear
superposition of SCC and inert environment fatigue rates. The CF behavior may be
described by taking into account the level of mechanical loading, the frequency, and
the shape of the cycles. It is preferable to express CF as a function of crack growth
rate rather than frequently used cycle-dependent crack growth rate (73).
Cyclic load frequency is the most important factor that influences CF in most
material environment and stress intensity conditions. The dominance of frequency
is related directly to the time dependence mass transport and chemical reaction steps
involved for brittle cracking.