Page 94 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

72 INTRODUCTION AND FORMS OF CORROSION

be accelerated very near the crack tip region by stress-assisted diffusion and disloca-
tion transport (94).

1.8.7 Material Properties in SCC

The susceptibility to SCC is affected by: (i) the chemical composition; (ii) prefer-
ential orientation of the grains; (iii) the composition and distribution of intergranular
precipitates; (iv) the interaction of dislocations; (v) the progression of the phase trans-
formation; and (vi) cold work (17).
The carbon content and its distribution in the steel matrix are the most important
factors controlling SCC. The threshold stress for cracking was found to depend on the
carbon content of the steel. The carbon in the steel affects the mechanical properties
of the steel in a favorable way, but the influence of carbon on the microstructure
is important and depends on the distribution of the different phases. For example,
carbon particles at ferrite grain boundary regions have been observed in the case of
intergranular cracking of carbon steels with >0.1% C. The other alloying elements
have either harmful or beneficial influence on SCC resistance, depending on their
effect on the segregation of the carbon particles (cementite) at grain joints. These
elements tend to segregate at the grain joints, but their influence is much weaker than
carbon because of their low concentrations.
Increasing zinc content of brasses increases the cracking rate in ammonia solutions
while low amounts of tin, lead, or arsenic improve the resistance (95). The addition
of molybdenum to austenitic steels increases their resistance to cracking in chloride
solutions and decreases it in caustic media. This susceptibility depends on the nature
of the environment. For example, a network of intergranular, coherent, chromium-rich
carbide precipitates increases the resistance to intergranular cracking in hot caustic
solutions, but the related chromium depletion of grain boundaries (sensitization) pro-
motes intergranular cracking in hot, aerated “pure” water or in polythionates (73).

1.8.8 Potential–pH Diagram and SCC
Critical potentials for SCC of a metal/solution system can be related to its E–pH
diagram as these diagrams describe the conditions at which film formation and metal
oxidation occur. The potential–pH (E–pH) diagram of carbon steel shows that SCC is
associated with potentials and pH values at which phosphate, carbonate, or magnetite
−
films are stable, while Fe 2+ and HFeO are metastable. A comparable diagram exists
2
for a 70 Cu–30 Zn brass in a variety of solutions (96).
The effect of factors such as pH, oxygen concentration, and temperature can be
related to their effect on the E–pH diagram. Change in pH and/or potential corre-
sponds to a region of stability because of the oxide film to a region of active general
corrosion or a zone of severe cracking susceptibility for a specific ion. An increase in
oxygen concentration shifts the potential to more noble or positive potentials. Hydro-
gen evolution or reduction of iron in water may be a dashed line. This shows that
hydrogen evolution becomes less endothermic with a shift to lower pH or more acidic