1. DRILL PIPE CORROSION
Corrosion is the destruction of metal by chemical or electrochemical action between
the metal and its environment.
i.e.: Corrosion occurs as a result of interaction between iron steel of drill string and
water base mud.
Four conditions must be met ,however , before wet corrosion:
1. Anode and cathode must exist.
2. The anode and cathode must be immersed in electrolytic medium.
3. A potential difference between anode and cathode exists.
4. There must be a coupling to complete the electrical circuit.
• The anode and cathode exist on the drill pipe itself.
• The drilling mud may serve as electrolytic medium.
• The coupling is creating by the drill pipe steel.
• The potential difference is due to the crystalline structure and different metal used
in the drilling pipe alloy.
Factors affecting corrosion rate:
1. Oxygen:
Oxygen reacts with metal of drill string forming (Fe2O3 & Fe3O4), which accelerates
corrosion on metal.
Oxygen acts to remove protective films on drill string which accelerate corrosion
action and increase pitting deposits ( reddish brown rust of Fe(OH)3 .
Oxygen scavengers, passivating inhibitors and filming inhibitor treatments are used to
mitigate oxygen corrosion attack.
2. H2S:
Fe + H2S FeS + 2H+ .
The increase of H+ atoms in mud will result in retaining acidic medium which will
increase corrosion effect.
H2S, cause severe pitting embattlement and stress cracking also a black sulfide
coating.
• Treat with sulfide scavenger as ZnO. Also Film-forming inhibitors are used.
• Keep pH between 8-9.
3. CO2:
CO2 is an acidic gas, that results in pH reduction and thus increase corrosion effect
and pitting attack.
CO2 + + H2O H2CO3 (Carbonic acid).
H2CO3 + Fe FeCO3 +H2.
i.e.: FeCO3 deposits indicate CO2 attack.
• Increase M.wt to stop gas influx.
• Keep pH between 8-9.
• Add filming amine
2. 4. Bacteria:
• the by-product of bacteria are CO2, H2S and SO4(leads to H2SO4).
• Microiobacids are use to control bacterial effect in drilling environments.
5. Dissolved Salt:
As salt concentration increases, conductivity between charge poles rises, also
electrical resistance of drilling fluid decreases. Also increase the solubility of
corrosive by-products and thus increase corrosion effect.
6. Velocity of Drilling Fluid:
the higher the mud velocity the higher the rate of erosion of film around the drill
string and thus the higher the rate of corrosion (Treat with oil mud , amines).
7. Temperature:
Rule of thumb : Corrosive rate doubles with every 55 ft increase. As the increase of
temperature increases the solubility of corrosive gases(O2, H2S & CO2).
8. Pressure:
the increase of pressure causes an increase in trapping effect of gases in mud such as
O2 and thus causes increase in corrosion effect.
9. pH:
Corrosive is much slower in alkaline medium than in acidic medium.
So corrosive rate decreases as pH increases.
NB: The best medium of pH to minimize corrosion rate is a pH between 8.5-10.
10. Solids:
Increase of abrasive solids in mud accelerate removal of protective film around drill
string due to increase of friction action causing pipe washout.
Also removal of protective film helps corrosive elements attack to drill string steel
and thus accelerate corrosion rate.
See pages 7, 8, 9
