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? ('click here' if you want to see our answer)
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: