Page 102 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

80 INTRODUCTION AND FORMS OF CORROSION

1.8.10.14 The Overlapping of Cracking Phenomena There are many
corrosion-based causes of premature fracture of structured materials. The most
common causes of premature fracture of components are given in the literature. CF
cracking occurs only under cyclic or fluctuating operating loads, while SCC and HE
occur under static or slowly rising loads.
It is possible that CF, SCC, and HE could occur at the same time in some service
conditions. The simultaneous operation of SCC and HE can occur in some systems.
The interrelationship among stress corrosion, CF, and HE are discussed in the lit-
erature. The cross-hatched regions represent the most serious practical situations
involving ductile alloy/environment systems. These regions indicate the combination
of any two failure mechanisms. In the center, all three phenomena interact, which is
probably realistic in ductile alloy/aqueous environment systems (4).
For example, in CF and SCC, a surface pit constitutes a stress raisor, in particu-
lar, in the initiation process. CF can assist initiation of the fracture while SCC and/or
HE can assist more or less intensively the crack propagation. In certain conditions,
either the crack initiation or the crack propagation can dominate the morphology and
mechanism of crack propagation. Under fatigue loading, modification of the number
of PSBs in fatigue and of the slip offset height in PSBs had been observed in compar-
ison with air in nickel single crystals in 0.5N H SO and copper crystals according
2 4
to the applied potential. Cyclic softening effects under anodic condition can occur in
austenitic and ferritic stainless steels in NaCl solutions (73, 81). Both the decrease of
K and the acceleration of crack growth rates because of increase of metal strength
ISCC
become apparent under cyclic loading.
K ISCC ≫ ΔK th the threshold intensity range at the corrosion crack growth rate
≤10 −10 m/cycle. The rate da/dN in aqueous solutions is much higher (by a dozen
times) than in ambient air. However, by suitable choice of solution composition, the
CFC growth can be reduced in titanium and magnesium alloys (86, 100).
Cathodic protection generally mitigates CF and SCC but may increase the HE
corrosion of susceptible materials.
The electrode pH and potential at an active crack tip surface may be significantly
different from those on boldly exposed surfaces of a material. Low pH conditions
can lead to local dissolution of the metal and crack tip blunting that reduces the stress
concentration effects. In contrast, low pH conditions favor hydrogen generation and
consequently increase risk of HE corrosion. The reduction in ductility associated with
HE corrosion may produce sharp crack tips, which in turn may increase stress con-
centration effects for any synergistic SCC or CF (4).
In many systems, SCC occurs in a limited range of electrochemical conditions as
shown for potential/pH domains for cracking of mild steel in different environments.
Three main types of SCC as a function of potential have been identified (111):

1. Low-potential regions where significant amounts of hydrogen are produced that
can be absorbed by the material leading to HE.
2. Passive regions close to active–passive transitions such as for Ni–Cr–Fe alloy
600 or steel in caustic solutions (112, 113).