1. Corrosion Objectives Corrosion process Environmental factors
Common forms of corrosion
Methods of corrosion control and prevention
Dr Suban K Sahoo
MSc, NET (JRF), PhD

2. Metal Corrosion
“The destruction of a material by chemical or electrochemical reaction to its environment”
typically a transfer of electrons from one metal to another through an Oxidation-Reduction Reaction.
Slow process but spontaneous

3. Oxidation - Reduction Anodic metal gives up electrons (oxidation)
Cathodic metal accepts electrons (reduction)
Or gases accept electrons (reduction)
-- in an acid solution
-- in a neutral or base solution

4. Corrosion may be…
dry/chemical corrosion…direct action of gases such as oxygen, halogen, H2S, H2SO4, liq. metal etc..generally in the absence of moisture
Metal oxide formed on the metal surface decide the further action: stable, unstable, volatile, porous etc
Pilling-Bedworth rule: An oxide is protective or non-protective porous, if the volume of the oxide is at-least as great as the volume of the metal from which it is formed’
Li, K, Na, Mg etc formed oxide volume less than metal volume but Al formed higher oxide volume

5. Wet/Electrochemical corrosion
Corrosion involves : (i) presence of a conducting medium, (ii) the formation of anodic and cathodic areas or parts may be contact with each other, (iii) corrosion occurs at anodic areas
Rust

6. Galvanic corrosion….
Occurs when two dissimilar metals (exa. Zn and Cu) are connected and exposed to an electrolyte: the metal higher in electrochemical series undergoes corrosion
Concentration cell corrosion
This type of corrosion is due to electrochemical attack on the metal surface, exposed to an electrolyte of varying concentrations or of varying aeration.

7. STANDARD EMF SERIES • EMF series • Metal with smaller • Ex: Cd-Ni cell
V corrodes.
• Ex: Cd-Ni cell V o o
metal
metal
metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K V
+0.340 o
DV =
0.153V 5

8. Acceleration of Corrosion
Physical Characteristics
exposed area (less, increases corrosion rate)
time of exposure (more time, more corrosion)
Environmental Characteristics
acidic environment
sulfur gas environment
temperature (high temps, more corrosion)
moisture (oxygenated moisture)

9. Passivation or Passivity
A protective film in oxidizing atmospheres
chromium,nickel, titanium, aluminum
Metal oxide layer adheres to parent metal
barrier against further damage
self-healing if scratched
Sensitive to environmental conditions
passivated metal may have high corrosion rates

10. FORMS OF CORROSION • Stress corrosion • Uniform Attack
work together
at crack tips.
• Uniform Attack
Oxidation & reduction
occur uniformly over
surface.
• Erosion-corrosion
Break down of passivating
layer by erosion (pipe
elbows).
• Selective Leaching
Preferred corrosion of
one element/constituent
(e.g., Zn from brass (Cu-Zn)).
• Pitting
Downward propagation
of small pits & holes.
Localized accelerated attack
• Intergranular
Corrosion along
grain boundaries,
often where special
phases exist.
• Galvanic
Dissimilar metals are
physically joined. The
more anodic one
corrodes. Zn & Mg
very anodic.
• Crevice Between two
pieces of the same metal.
Waterline and microbiological corrosion 9

11. Corrosion of Metals in Concrete Reinforcing Steel & Prestressing Steel
Concrete is Normally Highly Alkaline
Protects Steel from Rusting if Properly Embedded
If Corrosion Occurs, the Reaction Products are greater in Volume Than the Original Steel
Corrosion Initiation and Rate Depends On
Amount of Concrete Cover, Quality of Concrete
Details of Construction, & Exposure to Chlorides

12. Avoiding Corrosive Situations
Choose couple metals close on the galvanic series
Use large anode, and small cathode areas
Electrically insulate dissimilar metals
Connect a more anodic metal to the system
Avoid turbulent flow and impingements in pipe systems

13. Prevention of Corrosion
Basic goal   protect the metal  avoid localized corrosion
When possible chose a nobler metal
Avoid electrical / physical contact between metals with very different electrode potentials (avoid formation of a galvanic couple)
If dissimilar metals are in contact make sure that the anodic metal has a larger surface area / volume
In case of microstructural level galvanic couple, try to use a course microstructure (where possible) to reduce number of galvanic cells formed
Modify the base metal by alloying
Protect the surface by various means
Modify the fluid in contact with the metal  Remove a cathodic reactant (e.g. water)  Add inhibitors which from a protective layer
Cathodic protection  Use a sacrificial anode (as a coating or in electrical contact)  Use an external DC source in connection with a inert/expendable electrode