Types of Wet or Electrochemical Corrosion, Differential aeration corrosion, Galvanic corrosion, Pitting corrosion, Waterline corrosion, Crevice corrosion, Stress corrosion and their mechanisms and suitable examples.

1. Prepared by:
Prof Sarala Prasanna Pattanaik
Assistant Professor
Department of Chemistry

3. Types of Electrochemical Corrosion:
• Galvanic Corrosion
• Differential aeration corrosion
• Waterline corrosion
• Pitting corrosion
• Crevice corrosion
• Stress corrosion.

4. Types of Wet or Electrochemical Corrosion
Wet or Electrochemical Corrosion processes are of the following
types as given below.
 Galvanic or Bimetallic Corrosion
 Concentration Cell or Differential aeration Corrosion
 Waterline Corrosion
 Pitting Corrosion
 Crevice Corrosion
 Stress Corrosion
(a) Season Cracking
(b) Caustic Embrittlement

5. Galvanic or Bimetallic corrosion
Such type of Corrosion occurs when two different or dissimilar metals having different
electrode potentials (Reduction potential) are electrically in contact with each other in
the presence of a conducting medium or when two different metals are electrically
connected with each other in the presence of their respective salt solutions.
The metal placed higher in position in electrochemical series becomes more active metal
and acts as anode while the other metal placed lower in position in electrochemical
series becomes the more noble/passive metal and acts as cathode.
Hence, the more active metal suffers corrosion while the more noble metal is said to be
protected.
Hence, Galvanic couples are required to be avoided in different applications.
Examples:- Nuts and bolts of the same metal is preffered.
A small steel bolt connected to a copper equipment.
A small steel pipe connected to a large copper tank.
Zn suffers corrosion, when electrically coupled with Cu in the presence of an
electrolytic medium.

6. Mechanism of Galvanic corrosion
In Galvanic corrosion, more active metal acts as anode and undergoes oxidation while
more noble/passive metal acts as cathode and undergoes reduction. Hence, the more
active metal suffers corrosion while the more noble metal is said to be protected.
Depending on the nature of the conducting medium or electrolytic medium,
the cathodic reaction on the more noble metal may involve Evolution of
Hydrogen type (H2) reaction or Absorption of Oxygen type of reaction (O2).
In Acidic solution:- Evolution of Hydrogen type (H2) reaction
In neutral or slightly alkaline solution:- Absorption of Oxygen type of reaction (O2)
Example of Galvanic Couples:-
i) Zn (- 0.76V) & Cu (+ 0.34V) → Zn undergoes corrosion (more active metal)
ii) Fe (- 0.44V) & Cu (+ 0.34V) → Fe undergoes corrosion (more active metal)
iii) Ni (- 0.24V) & Cu (+ 0.34V) → Ni undergoes corrosion (more active metal)
iv) Zn (- 0.76V) & Fe (- 0.44V) → Zn undergoes corrosion (more active metal)
v) Zn (- 0.76V) & Al (- 1.66V) → Al undergoes corrosion (more active metal)

7. Image of Galvanic corrosion

8. Minimization of Galvanic corrosion
Galvanic or Bimetallic corrosion can be minimized by adopting the following
methods.
i) By avoiding the use of Galvanic couple in presence of conducting medium.
ii) When the use of Galvanic couple is unavoidable, metals must be as much
close as possible to each other in electrochemical series.
iii) When the direct joining of dissimilar metals is unavoidable, an insulating
material like wood, glass, rubber etc may be used to avoid direct electrical
contact between the metals.
iv) When two dissimilar metals are to be used in contact with an electrolytic
medium, the anodic metal should have as large area as possible and
cathodic metal should have as small area as possible since small anode
and large cathode leads to excessive corrosion.
v) The anodic metal should not be painted or coated when in contact with
cathodic metal since any break in the coating leads to severe localized
galvanic corrosion.

9. Conc. Cell or Differential Aeration Corrosion
• Concentration cell corrosion occurs due to difference in concentration of
the liquid medium around different parts of the metal.
• Differential aeration corrosion is a type of Concentration cell corrosion
which Occurs due to difference in potential between differently aerated
parts of a metal in the presence of an electrolytic medium.
• Part of the metal exposed to higher concentration of air is the more
oxygenated part & acts as Cathode.
• Part of the metal immersed inside the greater depth of the electrolyte is
the poorly oxygenated part & acts as Anode.
• Electron flows from the anode (poor oxygenated part) to the cathode (more
oxygenated part) through the metal while ions migrate to each other
through the electrolytic medium producing the corrosion product.
• At the anodic area, dissolution of metal occurs due to oxidation.
• At the cathodic area, absorption of oxygen type of reduction reaction
occurs producing hydroxide ions.

10. Mechanism of Differential Aeration Corrosion
In case of Iron metal
Reaction at anode:- Fe(s) → Fe+2 + 2e- (Oxidation)
Reaction at cathode:- 1/2O2 + H2O + 2e– → 2OH– (Reduction)
Overall Reaction:- Fe + 1/2O2 + H2O → Fe(OH)2 Or 2Fe + O2 + 2H2O → 2Fe(OH)2
In the presence of excess Oxygen:-
4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O
Similarly in case of Zn metal
Reaction at anode:- Zn(s) → Zn+2 + 2e- (Oxidation)
Reaction at cathode:- 1/2O2 + H2O + 2e– → 2OH– (Reduction)
Overall Reaction:- Zn + 1/2O2 + H2O → Zn(OH)2 Or 2Zn + O2 + 2H2O → 2Zn(OH)2

11. Image of Differential Aeration Corrosion
A metal (say Zn or Fe) partially immersed in a neutral salt solution undergoes
differential aeration corrosion due to potential difference between less oxygenated
part (anode) and more oxygenated part (cathode).

12. Image of Differential Aeration Corrosion
A metal (say Fe) partly covered with few drops of water undergoes differential aeration
corrosion as the portion of the surface covered with water becomes less oxygenated part
(anode) than the uncovered more oxygenated part of the metal (cathode).

13. Image of Differential Aeration Corrosion
A metal (say Fe) partly covered with drops of salt solution undergoes differential aeration
corrosion as the part of the surface covered with salt solution becomes less oxygenated
part (anode) than the uncovered more oxygenated part of the metal (cathode).

14. Image of Differential Aeration Corrosion
A part of metal surface (say Fe) covered with extraneous matter such as dust, dirt, sand,
clay etc undergoes differential aeration corrosion as the part of the surface covered with
the extraneous matter becomes less oxygenated Part (anode) than the uncovered more
oxygenated part of the metal (cathode).

15. Waterline Corrosion
A steel tank containing water kept stagnant for a long time, undergoes differential aeration
corrosion just below the waterline or water level since the concentration of oxygen in the
greater depth becomes less oxygenated Part (anode) than the portion of the tank just
above the waterline and become more oxygenated part of the metal (cathode).

16. Pitting Corrosion
Pitting corrosion is a special type of differential aeration corrosion which involves localized
accelerated attack resulting in the formation of cavities around the metal. Pitting corrosion
arises due to the formation of cracks, pinholes, pits and cavities on the metal surface due
to which small anodic (pit or cavity) and large cathodic areas (rest part) are created.

17. Pitting Corrosion
Pitting corrosion is a special type of differential aeration corrosion which involves localized
accelerated attack resulting in the formation of cavities around the metal. Pitting corrosion
arises due to the formation of cracks, pinholes, pits and cavities on the metal surface due
to which small anodic (pit or cavity) and large cathodic areas (rest part) are created.

18. Crevice Corrosion
Crevice corrosion is a type of differential aeration corrosion which involves localized attack
resulting in the formation of inaccessible areas at the junction of two metals. Crevice area
has a lack of oxygen and acts as anode and becomes prone to corrosion while the well
exposed parts become more oxygenated and act as cathode resulting in intense corrosion.

19. Stress Corrosion
Stress corrosion is a type of electrochemical corrosion which is highly specific
and occurs when a metal is subjected to the combined effect of stress and
exposed to a specific electrolytic medium. Generally, the part of the metal
under stress (such as sharp corners, bends, rivets, pits, joints, crevices etc) acts
as anode while the well metal part acts as cathode.
Pure metals are generally resistant to Stress corrosion but fabricated articles
or fabricated metallic structures like alloys of steel, brass etc undergoes stress
corrosion when exposed to some specific electrolytic medium and under
stress.
Examples:-
Brass alloy (Cu – Zn or Cu – Ni) when exposed to ammonia.
Stainless steel (containing 0.1 – 0.4% carbon) exposed to acid chlorides (HCl).
Mild steel in boilers exposed to caustic alkali (Na2CO3) during softening process of
water by lime – soda process.

20. Types of Stress Corrosion
Stress corrosion can be of two types.
i) Season Cracking – Occurs in Brass exposed to ammonia.
ii) Caustic Embrittlement – Occurs in mild Steel in water softening boilers
exposed to caustic alkali like (Na2CO3) used in Lime – Soda process.
Season Cracking:-
This type of corrosion generally refers to the corrosion of copper alloys such as
Brass (Cu – Zn) when exposed to the action of ammonia. When brass is exposed to
ammonia solution, both copper and zinc form complexes by losing electrons in the
solution due to which dissolution of brass occurs at the boundaries and forms
cracks for stress corrosion. Season cracking is characterized by deep brittle cracks
which penetrate into the affected components. If the concentration of ammonia is
very high, then attack is much more severe.
Reactions:-
Zn → Zn+2 + 2e- & Cu → Cu+2 + 2e-
Zn+2 + 4NH3 → [Zn(NH3)4]+2 & Cu+2 + 4NH3 → [Cu(NH3)4]+2

21. Images of Season Cracking Corrosion

22. Caustic Embrittlement
Caustic embrittlement is the phenomenon during which the boiler material becomes
brittle due to the accumulation of caustic substances. This type of boiler corrosion is
caused by the use of highly alkaline water in the high pressure boiler and also due to
stress. Water softened by lime soda process may contain NaOH which is formed by the
hydrolysis of Na2CO3.
This type of corrosion generally Occurs in mild Steel material of water softening boilers
exposed to caustic alkali (Na2CO3) used in lime – soda process at high temperature and
pressure. When water of high alkalinity attack the mild steel near the stressed parts like
bends, sharp corners, joints, rivets, hair – cracks etc., iron of the boiler material suffers
corrosion due to the formation of Fe3O4.
Water containing NaOH flows through the small pores in the stressed areas like bends,
joints, sharp corners, hair-cracks etc by capillary action and when water in these places
evaporates at high temperature, the concentration of NaOH increases as compared to
the unstressed parts and it corrodes the iron of the surrounding area by forming Sodium
ferroate (Na2FeO2). This causes embrittlement of boiler materials particularly at stressed
parts causing even failure of the boiler.

23. Reactions during Caustic Embrittlement
At high temperature and pressure in steel boiler plants
Na2CO3 + H2O → 2NaOH + CO2
Fe + 2NaOH → Na2FeO2 + H2
(Sodium ferroate)
3Na2FeO2 + 4H2O → Fe3O4 + 6NaOH + H2
or 6Na2FeO2 + 6H2O + O2 → 2Fe3O4 + 12NaOH
(Sodium ferroate)
Concentration Cell Presentation of Caustic embrittlement:-
Caustic embrittlement arises due to the setting up of a concentration cell. With the Iron
surrounded by dil. NaOH acting as the Cathode, while the iron surrounded by conc. NaOH
acting as the anode. The iron in the anodic part gets dissolved or corroded.
Fe (under stress)│Concentrated NaOH ‖ Dilute NaOH│Fe (stress free)
(inaccessible area) (main body)
Anode Cathode
Prevention of Caustic embrittlement:-
1. By using Na2SO4 and Na3PO4 in place of Na2CO3 that blocks the minute hair cracks.
2. By maintaining uniform flow rate of boiler water by avoiding inaccessible areas.
3. Using tannin or lignin as additive in boiler water which blocks the hair cracks.

24. Images of Caustic Embrittlement

25. Text books references
1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006
Dhanpat Rai Publishing Company, NewDelhi.
2. Dara S.S. & Umare S.S. A Text Book of Engineering Chemistry(12th
Edition ) 2008 S.Chand Publishing Company, New Delhi
3. Chawla Shashi: A text book of Engineering Chemistry (3rd Edition)
2010 Dhanpat Rai Publishing Company, New Delhi.
4. Palanna O G : A text book of Engineering Chemistry(4th Reprint)
2012 McGraw Hill, New Delhi
5. Sharma BK, Industrial Chemistry (16th Edition), 2014, Krishna
Prakashan Media (P) ltd. Meerut.