Cathodic Protection ME 472-061 Corrosion Engineering I ME, KFUPM Dr. Zuhair M. Gasem
Dr. Z. Gasem 2 ME 472-061 KFUPM � 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
Anodic and Cathodic Reactions of Dr. Z. Gasem 3 ME 472-061 Iron in Acids KFUPM Acid Solution + H + H H 2 H 2 Corrosion Cell on a H + + H Metal Surface Fe 2+ H H H H Cathode Anode Fe 2+ + H + H + + H + + H + H H H + H e e Metal Electron Flow
Dr. Z. Gasem 4 Basic Physics of CP of Iron in Acids ME 472-061 KFUPM � Electrons from an external source are forced to flow into the structure to be protected resulting in: Increased cathodic reaction (2H + + 2e - → H 2 ) � � Decreased anodic reaction and hence reduced corrosion rate H + H + Electrolyte H 2 H 2 H 2 H 2 H + H + H + H + H H H H H H H H Fe 2+ H + H + H + H + H + H + H + H + e e e e e e e e e e e e e e e e e Electrons from external source
Dr. Z. Gasem 5 ME 472-061 Polarization Principle of CP of Iron in Acids KFUPM � Before CP: � i anode = i cathode = i corr � E = E corr � After CP: � i corr = i a E CP � i cathode = i c � i app = i c – i a � E = E CP
Dr. Z. Gasem 6 Polarization Principle of CP of Iron in Water ME 472-061 KFUPM � The cathodic reaction for corrosion of steel and iron in aerated-water is usually (O 2 + 2H 2 O+ 2e → 4OH - ) under concentration polarization. � Before CP: � i anode = i cathode = i L � E = E corr E CP � After CP: i corr = i a � i cathode = i c = i L � i app = i c – i a � E = E CP �
Dr. Z. Gasem Cathodic Protection 7 ME 472-061 KFUPM � Summary of cathodic protection: � CP makes the structure’s potential more negative which promotes cathodic reactions and slows anodic reaction Increases i cathode � Decreases i anode � � Need to supply i app = i cathode - i anode � 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 CP of Buried Pipelines 8 ME 472-061 KFUPM � 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 CP of Buried Pipelines 9 ME 472-061 KFUPM � After CP is applied: The structure to be protected becomes the cathode � � The anode is an external electrode: Amore active metal (sacrificial anode) � 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 Sacrificial current in anode electrolyte
Dr. Z. Gasem CP of Buried Pipelines 10 ME 472-061 KFUPM � Sources of current Sacrificial anode system � Impressed current system (note the - polarity from the rectifier) �
Dr. Z. Gasem 11 ME 472-061 Electrochemical reaction in sacrificial Anode CP System KFUPM � Cathodic reactions on the steel structure: In aerated wet soil � O 2 + 2H 2 O+ 4e - ⇒ 4OH - � � In aerated wet acidic soil O 2 + 4H + + 4e - ⇒ 2H 2 O � � In neutral seawater O 2 + 2H 2 O+ 4e - ⇒ 4OH - � In de-aerated soil or water � 2H 2 O+ 2e - ⇒ 2OH - + H 2 � � Anode reactions At active anode in � sacrificial anode CP system (Mg, Al, Zn) M → M + n + ne - �
Dr. Z. Gasem CP of pipelines 12 ME 472-061 KFUPM � Note that cathodic protection current will only protect external surfaces on buried structures, because the anode-electrolyte- cathode 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.
Dr. Z. Gasem Protection Criteria 13 ME 472-061 KFUPM � How 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 Protection Criteria 14 ME 472-061 KFUPM � 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.
Dr. Z. Gasem Reference Electrodes 15 ME 472-061 KFUPM � Common reference electrodes used in CP � Cu/CuSO4 in soil � CuSO 4 + 2e - ↔ Cu+ SO 4 2- � E vs. SHE 0.318 V � AgCl in seawater � AgCl + e - ↔ Ag + Cl - � E vs. SHE 0.222 V
Dr. Z. Gasem Potential Protection 16 ME 472-061 KFUPM � Why < -0.850 mV vs. Cu/CuSO4? � From Pourbaix diagram, Fe is stable below -0.6 V vs SHE � Cu/CuSO4 is more + ve than SHE by 0.318 V � Hence, Fe is stable and corrosion is minimum if potential is (-0.6-0.318= -0.918 V vs Cu/CuSO4)
Dr. Z. Gasem 17 ME 472-061 Potential and Corrosion of Buried Steel KFUPM Potential (V vs. Cu/CuSO4) Corrosion condition -0.5 to -0.6 Intense corrosion -0.6 to -0.7 Corrosion -0.7 to -0.8 Slow corrosion -0.8 to -0.9 Cathodic protection -0.9 to -1.1 Overprotection Severe overprotection -1.1 to -1.4 (disbonding of coatings, hydrogen blistering, HE)
18 NACE Standards for CP Dr. Z. Gasem ME 472-061 KFUPM
Saudi Aramco’s Potential Dr. Z. Gasem 19 ME 472-061 Requirements KFUPM Structure Minimum Required Potentials Buried Cross-Country Pipelines -1.10 volts versus CuSO 4 electrode. Buried Plant Piping, Tank -1.00 volt versus CuSO 4 electrode. Bottom Externals, -850 mV versus CuSO4 electrode. Isolated Buried Casings Water Tank Interiors -0.90 volts vs. AgCl electrode Marine structures -0.90 volt or more negative versus AgCl electrode
Dr. Z. Gasem Soil Corrosivity 20 ME 472-061 KFUPM � Soil is composed mainly of mineral particles (mainly SiO 2 ). � Soil is composed of a mixture of: � Fine sand (0.02-0.2 mm) � Coarse sand (0.2-2 mm) � Slit (0.002-0.02 mm) � Clay (< 0.002 mm) � The soil particles are covered with thin surface film of moisture with dissolved salts and gases. � The total volume of soil consists of solid particles and pores filled with moisture and air. � Soils with a high proportion of sand have very limited storage capacity for water whereas clays are excellent in retaining water � Air in the pores contains 10-20 times as much CO 2 as atmospheric air. � Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be most corrosive.
Dr. Z. Gasem Soil Corrosivity 21 ME 472-061 KFUPM � Variables affecting soil corrosivity: � Water is the electrolyte for electrochemical corrosion reactions � Oxygen: the oxygen concentration decreases with increasing depth of soil � pH: soils usually have a pH range of 5-8 Soil acidity is produced by decomposition of acidic plants, � industrial wastes, and acid rain Alkaline soils tend to have high sodium, potassium, � magnesium and calcium contents which form calcareous deposits on buried structures with protective properties against corrosion. � Chloride level: harmful for metals � sulfate level: harmful for concreter
Dr. Z. Gasem Soil Corrosivity 22 ME 472-061 KFUPM Soil Corrosivit � For CP design against resistivity y Rating corrosion, the most important (ohm cm) property of a soil in determining its corrosivity is its >20,000 Essentially Dry sand electrical conductivity. non- corrosive � The table shows soil corrosion severity ratings. 10,000 to Mildly 20,000 corrosive � Soil corrosion causes corrosion in underground petroleum 5,000 to Moderately storage tanks, pipelines, and 10,000 corrosive water distribution systems. 1,000 to Corrosive Clay with � Soil resistivity is measured by 5,000 saline Wenner 4-pin method <1,000 Very water corrosive (sabkha)
Dr. Z. Gasem Soil Corrosivity 23 ME 472-061 KFUPM Seawater Progressively Less Corrosive Resistivity 10,000 � Electrolyte Resistivity (ohm-cm) Mildly Corrosive � Seawater (Gulf) 16 � Raw water 200-2000 � Drinking water 2000-5000 2,000 Moderately Corrosive 1,000 Corrosive 500 Very Corrosive Ohm-cm
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