Corrosion can be categorized in some common types

uniform corrosion
pitting corrosion
galvanic corrosion
crevice corrosion
concentration cell corrosion
graphitic corrosion
Uniform or General Corrosion
The metal loss is uniform from the surface.

Often combined with high-velocity fluid erosion, with or without abrasives.

Pitting Corrosion
The metal loss is randomly located on the metal surface.

Often combined with stagnant fluid or in areas with low fluid velocity.

Galvanic Corrosion
Occurs when two metals with different electrode potential is connected in a corrosive electrolytic environment.

The anodic metal develops deep pits and groves in the surface.

Crevice Corrosion
Occurs at places with gaskets, bolts and lap joints where crevice exists.

Crevice corrosion creates pits similar to pitting corrosion.

Concentration Cell Corrosion
Occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies.

Often combined with stagnant fluid or in areas with low fluid velocity.

Graphitic Corrosion
Cast iron loosing iron in salt water or acids.

Leaves the graphite in place, resulting in a soft weak metal.