What are the consequences of corrosion?
What are the mechanism of electrochemical corrosion (wet corrosion ) ?
1. Plant shutdown due to failure.
2. Replacement of corroded equipment.
3. Preventive maintenance (such as painting).
4. Necessity for over design to allow for corrosion.
5. Loss of useful properties of metal and thus loss of efficiency.
6. Contamination or loss of the product.
7. Decrease in production rate, because efficiency is less and replacement or repair in corroded equipment is time‐consuming.
8. Increase in maintenance and production cost.
Electrochemical or wet corrosion :
- It takes place mostly under wet or moist conditions through the formation of short‐circuited galvanic cells.
- Separate ‘anodic’ and ‘cathodic’ parts are formed, between which current flows through the conducting medium.
- Occurrence of oxidation (corrosion) at anodic area which generates metallic ions.
- M—> M n+ + ne– (oxidation)
- Non‐metallic ions like OH– or O2– are formed at cathodic areas.
- Diffusion of metallic and non‐metallic ions towards each other through conducting medium and formation of corrosion product somewhere between anodic and cathodic areas.
Example : Fe2+ ions originate at anode and OH– ions originate at cathode. Smaller Fe2+ ions diffuse more rapidly than the larger OH– ions, so corrosion occurs at the anode, but rust is deposited near cathode.
- The electrons released at the anode are conducted to the cathode and are responsible for various cathodic reactions such as :
Evolution of hydrogen : This type of corrosion occurs usually in acidic environment.
Example : The anodic reaction is dissolution of iron as ferrous ions with the liberation of Fe2+ electrons.
Fe —> Fe 2 + + 2e– (Oxidation)
These electrons flow through the metal from anode to cathode, where H+ ions (of acidic media) are eliminated by hydrogen gas.
2H + + 2e – —> H2 (Reduction)
Fe + 2H + —> Fe ++ + H2
This type of corrosion causes displacement of H + ion from the acidic solution by metal ions. All metal above hydrogen in the electrochemical series have tendency to get dissolved in acidic solution with simultaneous evolution of hydrogen.
(b) Absorption of oxygen : Rusting of iron in neutral aqueous solution of electrolyte (NaCl) in the presence of atmospheric oxygen is a common example of this type of corrosion. An anodic area of the metal dissolves as ferrous ion with liberation of electrons.
Fe —>Fe 2+ + 2e– (Oxidation)
The liberated electron flows from anodic to cathodic end through iron metal,
O2 + H2O + 2e – —>2OH- (Reduction)
The Fe2+ ions (at anode) and OH- ions (at cathode) diffuse and when they react with each
other ferrous hydroxide is precipitated.
Fe 2 + + 2OH – —> Fe(OH)2
(i) If enough oxygen is present ferrous hydroxide is easily oxidized to ferric hydroxide.
4Fe ( OH ) 2 + O 2 + 2H 2O —>4Fe ( OH)3
The product is formed called yellow rust.
If the supply of oxygen is limited, the corrosion product may be even black anhydrous magnetite Fe3O4 . Fe++ and OH– originate from the anode and cathode respectively, but the formation of rust or deposition of rust occur near to the cathode, because the smaller Fe2+ diffuse more rapidly than the large OH– ions
Difference between chemical corrosion and electrochemical corrosion
- Chemical corrosion occurs in the dry state.
- It involves the direct chemical attack of the metal by the environment.
- It follows absorption mechanism.
- It occurs on both homogeneous and heterogeneous surfaces.
- Corrosion products accumulate at the same spot where corrosion occurs.
- Uniform corrosion takes place.
- It is slow process.
- Electrochemical corrosion occurs the presence of moisture or electrolyte.
- It involves the setting up of a huge number of tiny galvanic cells.
- It follows mechanism electrochemical reactions.
- It occurs only on heterogeneous metal surface.
- Corrosion occurs at the anode but products gather at the cathode.
- Pitting is more frequent especially when the anode area is small.
- It is fast process.