Trethewey and J. Chamberlain, chapter 16. ❑ Handbook of Corrosion
Engineering, P.R.. Roberge. ❑ Cathodic Protection in ARAMCO's Engineering.
Cathodic Protection ME 472-061 Corrosion Engineering I ME, KFUPM Dr. Zuhair M. Gasem
Dr. Z. Gasem ME 472-061 KFUPM
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¾ References:
ASM Handbook, vol 13, pp. 466-477 Corrosion for Science and Engineering, K.R. Trethewey and J. Chamberlain, chapter 16 Handbook of Corrosion Engineering, P.R. Roberge Cathodic Protection in ARAMCO’s Engineering Encyclopedia
Dr. Z. Gasem ME 472-061 KFUPM
Anodic and Cathodic Reactions of Iron in Acids Acid Solution
Corrosion Cell on a Metal Surface
H+
H2 H+
Fe2+
H
H
H+
H+
e
e
Cathode
Anode Fe2+
Metal Electron Flow
H+
H+
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Dr. Z. Gasem ME 472-061 KFUPM
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Basic Physics of CP of Iron in Acids
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Electrons from an external source are forced to flow into the structure to be protected resulting in:
Increased cathodic reaction (2H+ + 2e- → H2) Decreased anodic reaction and hence reduced corrosion rate
Electrolyte
H2
H+
H2
H+
H2
H+
H2 H+
H
H
H
H
H
H H
H+ H+
H+
H+
H+ H+
H+ H+
H Fe2+
e e
e e
e
e
H+ H+
e ee e e e e e e e e
Electrons from external source
Dr. Z. Gasem ME 472-061 KFUPM
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Before CP:
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Polarization Principle of CP of Iron in Acids
ianode = icathode = icorr E = Ecorr
After CP:
icorr = ia icathode = ic iapp = ic – ia E = ECP
ECP
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Dr. Z. Gasem ME 472-061 KFUPM
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The cathodic reaction for corrosion of steel and iron in aerated-water is usually (O2+2H2O+2e→4OH-) under concentration polarization. Before CP:
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Polarization Principle of CP of Iron in Water
ianode = icathode = iL E = Ecorr
After CP:
icorr = ia icathode = ic = iL iapp = ic – ia E = ECP
ECP
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Summary of cathodic protection:
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Cathodic Protection CP makes the structure’s potential more negative which promotes cathodic reactions and slows anodic reaction Increases icathode Decreases ianode Need to supply iapp = icathode - ianode
Where CP is used? CP is often applied to coated structures, with the coat providing the primary form of corrosion protection and the CP system acts as a supporting protection. The main applications of CP include: Buried pipeline Acids storage tanks Offshore steel structures such as platforms and oil rigs Ships Concrete structures exposed to seawater such as bridges
Dr. Z. Gasem ME 472-061 KFUPM
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CP of Buried Pipelines
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Before CP is applied:
Anodes and cathodes are on the same surface of the pipe The soil is the electrolyte Ionic current flow b/w the anode and the cathode in the external surfaces Electrons flow in the metal from anode to cathode
Dr. Z. Gasem ME 472-061 KFUPM
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CP of Buried Pipelines
After CP is applied: The structure to be protected becomes the cathode The anode is an external electrode: Î Amore active metal (sacrificial anode) Î An inert anode with impressed DC current (Impressed current) The soil is the electrolyte Ionic current flow b/w the anode and the cathode in the external surfaces Electrons flow between the anode and cathode through an insulated copper wire. e-
cathode
+ve ions current in electrolyte
Sacrificial anode
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Dr. Z. Gasem ME 472-061 KFUPM
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CP of Buried Pipelines
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Sources of current
Sacrificial anode system Impressed current system (note the - polarity from the rectifier)
Dr. Z. Gasem ME 472-061 KFUPM
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Electrochemical reaction in sacrificial Anode CP System
Cathodic reactions on the steel structure:
In aerated wet soil Î
In aerated wet acidic soil Î
O2+2H2O+4e- ⇒ 4OH-
In de-aerated soil or water Î
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O2+4H+ + 4e- ⇒ 2H2O
In neutral seawater Î
O2+2H2O+4e- ⇒ 4OH-
2H2O+2e- ⇒ 2OH- +H2
Anode reactions
At active anode in sacrificial anode CP system (Mg, Al, Zn) Î
M → M+n + ne-
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Dr. Z. Gasem ME 472-061 KFUPM
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CP of pipelines
Note that cathodic protection current will only protect external surfaces on buried structures, because the anode-electrolytecathode is at external surfaces. Above ground, structures cannot be protected by cathodic protection because the current discharged from the current source can not travel through the atmosphere (no electrolyte). CP is not usually used to protect internal surfaces of pipelines because of difficulty in placing anodes. internal surfaces of pipelines can be protected by: inhibitors, coatings, or by using a corrosion resistant alloy.
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Protection Criteria
Dr. Z. Gasem ME 472-061 KFUPM
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much current is needed to protect the pipeline?
Little current will lead to ineffective protection High current will lead to disbonding of coatings and hydrogen embrittlement (more power consumption and higher cost) Experience show that we should keep the pipeline potential less than a protection potential.
Dr. Z. Gasem ME 472-061 KFUPM
Protection Criteria
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In less corrosive soil, E< -0.850 mV wrt Cu/CuSO4 reference electrode this reference electrode is used because it is less sensitive to temperature variation (0.318 s. SHE) In Saudi’s Aramco, the protection potential for cross-country pipeline is -1.1 V vs Cu/CuSO4 (due to highly corrosive soil) More –ve potential means more current required and more operation cost.
Reference Electrodes
Dr. Z. Gasem ME 472-061 KFUPM
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Cu/CuSO4 in soil Î Î
reference electrodes used in CP
CuSO4 + 2e- ↔ Cu+SO42E vs. SHE 0.318 V
AgCl in seawater Î Î
AgCl + e- ↔ Ag + ClE vs. SHE 0.222 V
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Dr. Z. Gasem ME 472-061 KFUPM
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Potential Protection
Why 20,000
Essentially noncorrosive
10,000 to 20,000
Mildly corrosive
5,000 to 10,000
Moderately corrosive
1,000 to 5,000
Corrosive
