Page 96 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

74 INTRODUCTION AND FORMS OF CORROSION

1.8.10 Electrode Potential and its Effect on Crack Growth
In SCC systems, variation of material composition or microstructure, or modifica-
tion of environmental composition or redox potential may change the sensitivity to
cracking. The crack growth rate is frequently determined by the electrode potential
whereby an insignificant change with all other conditions (specimen, pH, solution,
temperature, frequency, stress ratio) being constant can lead to the acceleration of
fatigue crack growth by a dozen times. This can cause the rapid growth of the crack
that was “static” at open-circuit potential (K < K ISCC ), retard, or stop the crack growth
at K > K ISCC (98, 100).

1.8.10.1 Inhibitors The SCC of an alloy/solution system is generally associated
with the presence of a specific corrosive environment that, mostly, would attack the
alloy superficially in the absence of stress. Nitrates and hydroxides that generally
act as anodic inhibitors for carbon steels also cause cracking of mild steels. Failure
of storage steel reservoirs containing anhydrous ammonia generally occurs in
the presence of air containing CO that forms the inhibitor (NH ) CO , which
4 2
3
2
causes cracking. Current density differences slow in polarization curves at various
potentials between fast and slow sweep rates for mild steel immersed in hydroxide,
carbonate–bicarbonate, and nitrate solutions have been studied. The results obtained
from controlled potential slow-strain-rate tests show that cracking is at a maximum
between −500 and −600 mV.
A differential aeration galvanic cell consists of differences in oxygen concentra-
tion, leading to pitting and cracking in distilled water (96).

1.8.10.2 Alloy/Liquid Interface The following table 1.2 lists some alloy–environ-
ment combinations that result in SCC. This table serves as a general guide (9).
SCC is often encountered in hot gaseous atmospheres on materials under creep or
fatigue conditions. SCC susceptibility of titanium alloys in moist chlorine, dry HCl,
and dry hydrogen has been observed. The titanium alloy (Ti–8Al–1Mo–1V) exhibits
∘
cracking in moist chlorine gas at 288 C. Binary titanium alloys also exhibit cracking
∘
in moist chlorine at 427 C (96).
Hot salt SCC and hot gaseous SCC of titanium alloys have been reported to occur.
Irradiation-assisted SCC has been reported to occur in nuclear reactors (96).

1.8.10.3 HE and Hydrogen Stress Cracking The interaction between hydrogen
and metals results in solid solutions of hydrogen in metals, gaseous products formed
by the reactions between hydrogen and elements present in the alloy and metal
hydrides. Depending on the type of metal/hydrogen interaction, hydrogen damage
occurs as follows (9, 91).
HE is the loss of ductility of materials containing hydrogen, which occurs in
high-strength steels, primarily quenched and tempered and precipitation-hardened
steels, with tensile strengths greater than about 1034 MPa. There are two types of
HE (9, 91).