Forms of Corrosion
8 main forms:
- Uniform / General corrosion
- Galvanic corrosion
- Crevice corrosion
- Pitting corrosion
- Inter-granular corrosion
- Erosion corrosion
- Stress corrosion
- Selective leaching
Only the first 3 forms are discussed in s1.
Uniform corrosion
Occurs uniformly over the entire exposed area of the metal. Rust forms all over the surface. Leads to 30% of corrosion failures.
All other forms of corrosion happen on selected areas of a metal.
Galvanic corrosion
When 2 metals are in contact (electrically) and placed in a corrosive environment, only one metal —the element with most negative potential— corrodes. Aka. two metal corrosion. Corrosion happens in the anode near the junction.
Area effect
Smaller the anode, severe will be the corrosion. To reduce the corrosion when 2 metals are in contact, bigger anode is used to reduce the area effect.
Galvanic series
Designed to be an extension of electrochemical series that includes alloys and non-metals. Materials are ordered in ascending order of reactivity. Only a ranking order and not a measure. Materials in the lower position becomes the anode. Specific for a particular environment.
In a single metal
Galvanic corrosion can happen in a single metal. One part of the metal becomes anodic to the rest.
For example, a piece of iron immersed in water. Initially iron goes under uniform corrosion. Once the oxygen in the water is depleted, the iron inside the water will become the anode. Iron just above the water surface becomes the cathode. Severe corrosion occurs just below the water level.
Crevice corrosion
When a crevice is in contact with a liquid, the crevice undergoes corrosion. Corrosion is limited to the crevice.
Crevice becomes anodic becuase of low oxygen supply. Exposed area has high oxygen supply and works as a cathode.
Mechanism of CC
Consider a riveted metal-plate section in contact with domestic water.
Initially it will corrode uniformly. After some time, crevice becomes depleted with oxygen due to restricted convection. Now the crevice becomes the anode. Becasue of area effect, corrosion will be severe.