Page 49 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 27

dislocations. An edge dislocation is a region of imperfection that lies along the inter-
nal edge of an incomplete plane of atoms within a crystal. In a screw dislocation,
a portion of the crystal is displaced consisting of continuous ribbon-like structure.
Plane defects are two-dimensional stacking faults and are imperfect regions of a crys-
tal resulting from errors in the positioning of atomic layers. Stacking faults arise
during crystal growth or as a result of plastic deformation, which can occur along
the close-packed planes in a crystal (4).

1.5.1.4 Inclusions Inclusions are three-dimensional defects consisting of soluble
particles of foreign material in the metal. Voids, a three-dimensional defect, are empty
or gas-filled spaces within the metal. Metal oxides, sulfides, and silicates are common
inclusions. For example, manganese sulfide in stainless steel provides a favorable site
for pitting corrosion.

1.5.1.5 Passivation For a passivating metal, the corrosion rate is less dependent
on potential and hence IGC driven by the free energy differences arising from the
disorder in the metal are even less likely. However, the surfaces protected by passive
film, grain boundaries, and defects can cause preferential attack by disrupting the
formation of a protective layer (41). Texture can have an impact on the passivation
characteristics as passivation and repassivation are predominant on densely packed
crystallographic planes because of fewer steps and kinks involved. Thus texture can
have an impact on passivation characteristics. The susceptibility to SCC can also be
impacted by material texture, because the bias in grain orientation will favor align-
ment of stacking faults in different grains along preferred directions with consequent
effects on slip processes and ultimately SCC, depending on the direction of tensile
stresses (41).

1.5.1.6 Breakdown of Passivation and Pitting The local breakdown of passivity
of metals such as stainless steels, nickel, or aluminum occurs preferentially at sites of
local heterogeneities such as inclusions, second-phase precipitates, or dislocations.
The size, shape, distribution as well as the chemical or electrochemical dissolution
behavior of these heterogeneities in a given environment determine to a large extent
whether pit initiation is followed either by repassivation (metastable pitting) or stable
pit growth (29).
Localized corrosion of passivating metals initiates at local heterogeneities, such as
inclusions, second-phase precipitates, grain boundaries, dislocations, flaws, or sites
of mechanical damage. In the case of stainless steels, pit initiation occurs at sites
of MnS inclusions. Rapid quenching or physical vapor deposition are useful in the
exclusion of inclusions and precipitates with the resulting structure either amorphous
or nanocrystalline. Sputter-deposited aluminum alloys containing Cr, Nb, Ta, W, Mo,
or Ti show an increase of 0.2–1 V in pitting potential, and the increase in pitting
resistance has been attributed to reduced pit initiation tendency as well as a more
protective passive film favoring rapid repassivation (37).
With chromium contents of ≥20 wt% and 2–6 wt% of molybdenum the pitting
potential of steels is increased. Molybdenum in superaustenitic stainless steels