Page 52 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

30 INTRODUCTION AND FORMS OF CORROSION

The second model is an extension of the surface diffusion model in the sense
importance is given to the atomic placement of atoms in the randomly packed alloy.
The model considers that a continuous connected cluster of the less noble atoms must
exist to maintain the selective dissolution process for more than a few monolayers of
the alloy. This percolating cluster of atoms provides a continuous pathway for the
corrosion process as well as a pathway for the electrolyte to penetrate the solid. This
mechanism is expected to depend on a sharp critical composition of the less noble
element below which dealloying does not occur (44).

1.5.1.13 IGC and Exfoliation IGC consists of preferential attack of either grain
boundaries or areas adjacent to grain boundaries in a sample material exposed to a
corrosive environment, but with little corrosion of grains themselves. This dissolution
is caused by potential differences between the grain boundary region and any pre-
cipitates, intermetallic phases, or impurities that form at the grain boundaries. IGC
susceptibility depends on the corrosive solution and on the extent of intergranular
precipitation, which in turn is a function of alloy composition, fabrication, and heat
treatment (41).
Steel phases have an influence on the corrosion rate. Ferrite has a weak resis-
tance to pitting. Martensite can increase the fragilization of steel. Intermetallic phases
such as Fe Mo in high Ni bearing alloys can influence the corrosion resistance. The
2
CuAl in series 2000 aluminum alloys is more noble than the matrix with corrosion
2
around the precipitate. The majority of IGC processes occur in austenitic stainless
steels and aluminum alloys and to a lesser extent in some ferritic stainless steels and
nickel-based alloys (46).
Impurities that segregate at grain boundaries may promote galvanic action in a cor-
rosive environment by acting either as anodic or cathodic sites. In 2000-series (2xxx)
aluminum alloys, the copper-depleted (anodic) band on either side of the grain bound-
ary is dissolved while the grain boundary is cathodic because of CuAl precipitates.
2
In the 5000-series (5xxx) aluminum alloys, intermetallic precipitates such as Mg Al
2 3
(anodic) are attacked when they form a continuous phase in the grain boundary. In
chloride solutions the galvanic couples between the precipitates and the alloy matrix
can lead to severe intergranular attack. The actual susceptibility to intergranular attack
and the extent of corrosion depends on the corrosive environment and the extent of
intergranular precipitation, which is a function of alloy composition, fabrication, and
heat treatment parameters (46).
Precipitates that form on the exposure of metals to high temperature (during pro-
duction, fabrication, and welding) often nucleate and grow preferentially at grain
boundaries. If these precipitates are rich in alloying elements that are essential for
corrosion resistance, the regions adjacent to the grain boundaries are depleted of these
elements. The metal/alloy is thus sensitized and is susceptible to intergranular attack
in a corrosive medium. An example is AISI type 304 steel corrosion because of pre-
cipitation of chromium (46).
∘
At temperatures above 1035 C, chromium carbides are completely soluble in
austenitic stainless steels. However, on slow cooling of these steels or reheated in the
∘
range 425–815 C, chromium carbides are precipitated at the grain boundaries. The