Page 93 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 71

1.8.6 Some Key Factors of SCC
The stress applied on a metal is nominally static or slowly increasing tensile stress.
The stresses can be applied externally, but residual stresses often cause SCC failures.
Internal stresses in a metal are because of cold work or heat treatment. In general all
manufacturing processes create some internal stresses. Stresses caused by cold work
arise from processes such as lamination, bending, machining, rectification, drawing,
drift, and riveting. Stresses caused by thermal treatments are because of the dilation
and contraction of the metal or indirectly by the modification of the microstructure of
the metal or alloy. Welded steels have residual stresses near the yield point. Corrosion
products can also function as a source of stress and can cause a wedging action.
The manner in which the logarithm of crack growth rate varies as a function of the
crack tip stress intensity magnitude factor is normal. No cracking is observed below
some threshold stress intensity. The stress corrosion cracks initiate when the stress
exceeds a threshold value and propagate when the stress intensity factor exceeds the
threshold value K ISCC (17). The threshold stress intensity level, K ISCC , is determined
by the alloy composition, microstructure, and the environment (composition and tem-
2
perature). The threshold stress intensity K ISCC lies between 10 and 25 MPa/m and
is usually in the order of 60–100% of the yield stress, but much lower values can
1
∘
be observed, such as for 304 stainless steel in boiling magnesium chloride at 154 C.
In some cases, stresses as low as 10% of the elastic limit caused the SCC. Such low
levels must be viewed with prudence as the environmental conditions of a system can
change at the metal/solution interface during service and accidental pit, or a slash can
increase stresses locally and reach the level, K (17).
ISCC
At intermediate stress intensity levels (stage 2) the crack propagation rate levels
off (plateau) V plateau , which is independent of mechanical stress but depends on the
alloy/environment interface and the rate-limiting environmental processes such as the
mass transport of the aggressive species to the crack tip. The plateau in a quenched
∘
and tempered low alloy steel of 1700 MPa yield strength in deaerated water at 100 C
can be higher than that in a similar steel of 760 MPa yield strength by seven orders of
magnitude (91, 92). Stage 3 corresponds to the critical intensity level for mechanical
fracture in an inert environment.
The crack growth rate depends on the strength of the metal in almost all aggres-
sive environments. Doubling the yield strength of martensitic steel (from 800
vs
√
to 1600 MPa) is accompanied by a tenfold (from 70 to 7 MPa m) decrease of the
threshold stress intensity K ISCC corresponding to the onset of stress corrosion crack
growth in alloys in chloride solutions at ambient temperatures (86, 93). Stress sources
likely to promote cracking are weldments and inserts. Welded structures of these
alloys require stress-relief annealing.
In some media, SCC can occur above a certain temperature. Increase in temper-
ature generally lowers the threshold for cracking ( and K ) and increases the
1 ISCC
growth rate of propagation. An example is the SSC of stainless steel in neutral pH
∘
∘
solution above 40 and 80 as opposed to SCC at pH of 1.0 and room temperature
(73). Hydrogen absorption can favor local plasticity very near the crack tip region,
because of enhanced dislocation velocities with hydrogen. Hydrogen penetration can