Page 91 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 69

an apparent threshold Δ at the lower end (region I) with an intermediate region II
th
that depends on some power of or Δ of the order of 2–10. The following power-law
relationship
da n
= c(ΔK)
dN
where c and N are constants for a particular material and stress ratio have been
advanced. In an aggressive environment, the CFC curve can be different from the
pure fatigue curve, depending on the sensitivity of the material to the given environ-
ment and the occurrence of various static stress fracture pathways. The environmental
effects are quite strong above the threshold for sec (K ISCC ) and negligible below this
level (K ISCC is the stress intensity threshold for plane-strain environment assisted
cracking). Other loading factors, such as frequency stress ratio and stress waveform
can markedly affect crack growth rates in aggressive media.
CF tests may be carried out in an apparatus designed by the Continental Oil Com-
pany. The apparatus consists of a Monel tank with four samples subjected to cyclic
bending. The first step consists of determining the displacement caused by the applied
load. The exact stresses are determined by strain gauges. The electrolyte is deaerated
with 3% sodium chloride. Polished or sand blasted samples are used, and the behavior
of the alloy in CF may be studied at the free corrosion potential under different per-
centages of stress amplitude of the elastic limit. From potentiokinetic curves, I = f(E):
the protection or pitting potential applied during the stress test. Each test can have four
samples and the difference between results (85) for similar tests does not exceed 15%.
Environmentally assisted cracking (EAC) occurs not only in metals but also in
glasses (Plexiglas), ceramics, and polymers. Structural failures because of EAC can
be sudden and unpredictable, occurring after a few hours of exposure or after months
or years of satisfactory service (86, 87). The two types of environmentally assisted
cracking (EAC) are mechanically assisted cracking and EIC.
The total cost of material fracture is about 4% of gross domestic product in the
United States and Europe (88, 89). Fracture modes included in the cost estimates were
stress-induced failures (tension, compression, flexure, and shear), overload, defor-
mation, and time-dependent modes, such as fatigue, creep, SCC, and embrittlement.
The environmentally assisted corrosion problem is very much involved in the main-
tenance of the safety and reliability of potentially dangerous engineering systems,
such as nuclear power plants, fossil fuel power plants, oil and gas pipelines, oil pro-
duction platforms, aircraft and aerospace technologies, chemical plants, and so on.
Losses because of environmentally assisted cracking (EAC) of materials amount to
many billions of dollars annually and is on the increase globally (87).

1.8.4 SCC
The necessary conditions for the occurrence of SCC are: (i) a crack-promoting envi-
ronment; (ii) the susceptibility of the material to SCC; (iii) exceeding threshold value
with regard to tensile stresses. One of the distinguishing features of SCC is that stress
corrosion faces suffer very low corrosion even in solutions that damage free surfaces.