Forms of Corrosion

8 forms:

1. Uniform / General corrosion
2. Galvanic corrosion
3. Crevice corrosion
4. Pitting corrosion
5. Inter-granular corrosion
6. Erosion corrosion
7. Stress corrosion
8. Selective leaching

Uniform corrosion

Occurs uniformly over the entire exposed area of the metal. Rust forms all over the surface. Leads to 30% of corrosion failures.

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.

Galvanic corrosion can happen even inside 1 metal. In such cases, one part of the metal becomes anodic to the rest. For example, consider a piece of iron, immersed in water. Initially iron goes under uniform corrosion. Once the oxygen in the water is depleted, iron just above the water surface starts to react as a cathode. Severe corrosion occurs just below the water level.

Corrosion will be severe at the junction.

Area effect

Smaller the anode, severe will be the corrosion.

Galvanic series

Designed to be an extension of electrochemical series that includes alloys and non-metals. Materials are ordered in ascending order of reactivity. Materials in the lower position becomes the anode. Specific for a particular environment.

Crevice corrosion

If a crevice (a crack forming a narrow opening) is in contact with a liquid, the crevice undergoes corrosion with virtually no attack in the other region.

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. As the anode area is much smaller compared to cathode, corrosion will be severe.

If the water contained salt(s), metal chloride(s) will be formed. All metal chlorides other than NaCl, KCl hydrolize in water. And produces acid. Acids increase the rate of corrosion.