Corrosion Pit Shapes

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Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products.

Pits can be either hemispherical or cup-shaped. In some cases they are flat-walled, revealing the crystal structure of the metal, or they may have a completely irregular shape. Pitting corrosion occurs when discrete areas of a material undergo rapid attack while most of the adjacent surface remains virtually unaffected. The following are common pit shapes divided in two groups:

Trough Pits

Narrow, deep:

Shallow, wide:

Elliptical:

Vertical grain attack:

Sideway Pits

Subsurface:

Undercutting:

Horizontal grain attack:

Localized Corrosion of Aluminum

In the pic, A- Uniform pitting, B- Deep pit (anode), C- Inact (canode)

This example of a pitted surface was produced by exposing a specimen of aluminum A92519 to 3.5% NaCl during seven days. The width of the picture is approximately 1 mm.

The corrosion attack of aluminum of the 2000 family is mixed. While most of the surface is uniformly pitted (yellow arrow), as A92024 would do in similar conditions, we can see the formation of deeper pits (red arrow) that are surrounded by unattacked regions (blue arrow).