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.

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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

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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,

             

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,

              - annihilation               - 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.