Page 105 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 83

high-strength steels cannot be used in hydrogen environments. In this regard, H Sis
2
one of the most corrosive environments. At lower yield strengths, the mechanisms
for hydrogen-assisted failures apparently changes, and blistering becomes the more
common failure mode. The threshold stress intensities for high-strength steels
subjected to hydrogen environments are significantly less than those measured under
benign conditions (K ISCC or K ).
HI
In low-strength steels (700 MPa or 100 ksi yield strength or less) hydrogen damage
occurs by loss in tensile ductility or blistering. In the case of loss in tensile ductility,
hydrogen promotes the formation and/or growth of voids by promoting the decohe-
sion of the matrix at carbide particles and inclusion interfaces. At higher hydrogen
fugacities and often in the absence of stress, blistering, or a form of cracking also
associated with inclusions termed as SWC, blister cracking or HIC is observed. This
mode of cracking is not a function of steel strength but depends on steel composition,
processing, and hydrogen environment. In a hydrogen sulfide environment, a crack-
ing morphology has been observed in low-strength steels that combine features of
hydrogen stress cracking and HIC, which has been termed as SOHIC (4).
Localized corrosion and stress corrosion may often be observed. Stress corrosion
cracks usually initiate at pits in many systems. The role of pitting is to disrupt films
that otherwise prevent the ingress of hydrogen (118, 119). Electrochemical polar-
ization technique may be used to distinguish between SCC and HE mechanisms in
high-strength steels in sodium chloride solutions (120).

1.8.10.17 Propagation Models Generally, SCC is considered to be brittle, mean-
ing that it occurs at stresses below yield stress and propagates as essentially an elastic
body even though local plasticity may be required for the cracking process. Thus
linear-elastic fracture mechanics is used in studying SCC. Propagation of the crack is
usually because of periodic microruptures although anodic dissolution is an impor-
tant controlling process. Local attack results in localized hydrogen absorption, which
can cause local or bulk embrittlement and even plasticity. Thus anodic dissolution and
hydrogen can play a role in the system. The local environment leading to crack prop-
agation is not well defined particularly in SCC phenomena. The transport of water to
the crack tip may be an important controlling process. With the exception of the slip
dissolution model, environmentally assisted cracking models are notable for provid-
ing quantitative prediction of the crack propagation rate (73).
Major models of SCC mechanisms are:

Stress Corrosion Cracking Mechanisms
Dissolution Models Mechanical Fracture Models

Film Active path Ductile fracture Brittle fracture
rupture process

Corrosion Plasticity Tarnish Film-induced
tunneling rupture cleavage