Two dislocations in a FCC material can react with each other to form

Report
Mech 473 Lectures
Professor Rodney Herring
Corrosion
Some materials form an oxide scale.
Other materials form pits when they oxide.
Both oxides are detrimental but the consequences for the two types
of corrosion are different.
How does this corrosion affect the performance of the material?
•
scale oxide reduces the thickness of the material. A reduce
thickness can cause an overload of the material, which can
lead to failure.
•
pits form surface cracks. The cracks can reach a critical crack
size leading to failure as seen in our fracture lecture.
Corrosion and Crystal Structure
BCC materials such as plain carbon steels form a scale on their
surface.
FCC materials such as stainless steels and aluminum form pits if
and when they corrode.
Why might there be this difference in oxide structure that depends
on crystal structure?
Corrosion and Dislocations
Dislocations form grain boundaries as a network and act as pipes
for diffusion, which is required for corrosion.
•
Dislocations in BCC materials do not react with each other to
form extended networks so they remain localized, i.e., those
dislocations at the surface tend not to extend deep inside the
material. Thus corrosion remains at the surface to form a
scale.
•
Dislocations in FCC materials can react with each other to
form extended networks that can extend deep into the
material from the surface. Thus corrosion can penetrate the
material to form pits.
Corrosion and Dislocations
Two dislocations in a FCC material can react with each other to
form a third dislocation such as,
1 10111  0 1 1111  101111
FCC Dislocation Interactions
Surface
As seen in this
figure, two
dislocations can
interact to form a
third dislocation,
which can extend
the dislocation
network below the
surface of the
material allowing a
pathway for
corrosion products.
Depth
Corrosion and Dislocations
Two dislocations in a BCC material cannot react with each other to
form a third dislocation. For example,
1 1 1011  1 1 1011  000111 - annihilation
1 1 1011  1 1 1011  022111 - too high of energy
to form
Corrosion
When we discuss steels in this course, we’ll add some further
details of corrosion such as the role of alloying additions.
Especially Chromium is effective in “healing” a surface or crack,
IF the steel has a sufficient Cr concentration.

similar documents