Sewer Explosion due to Corrosion

An aluminum specimen exposed to a 3,5% NaCl solution for seven days one can clearly (?) see different types of attack on the same surface.

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

In this micrograph of an aluminum specimen exposed to a 3,5% NaCl solution for seven days one can clearly (?) see different types of attack on the same surface. The width of the picture is approximately 1 mm. 

Can you see at least three regions of attack in this picture? 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