13 Cathodic Protection
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The document discusses cathodic protection and corrosion prevention methods for metal structures. It provides information on types of cathodic protection systems including sacrificial anode and impressed current systems. Key details covered include common materials used for anodes, factors that influence current density requirements, and considerations for protecting different types of structures like ships, pipelines and tanks.
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13 Cathodic Protection
- 1. Organisation Chart Jotun Cathodic Protection Jotun Cathodic Protection Jotun Decorative ( Scandinavia) Jotun Protective Coatings Jotun Decorative Paints Jotun Marine Coatings Jotun Paints Jotun Powder Coatings Jotun Group
- 19. Freely Corroding Steel Cathode 2 e - Anode Cathode Sea water (electrolyte) 2 e - 2 e - 2 e - Steel plate - Fe 2+ ½ O 2 + H 2 O + 2e 2OH - ½ O 2 + H 2 O + 2e 2OH - -
- 20. Potential, mV vs. Cu/CuSO 4 Zn -580 +500 Freely corroding steel -700 -800 +250 Mixed potential ( Protection potential) -900 -1000 -1080 0 Freely corroding Zinc The Principle of Cathodic Protection Potentials vs. different Reference Electrodes
- 21. Cathodic Protection Steel protected by a Sacrificial anode A calcareous deposit is formed on the steel surface 2 e Steel Zinc Zn = Zn + 2 e O 2 2+ - - ½ O 2 + H 2 O + 2e 2OH - -
- 22. Steel Current Source Current O 2 2 e- Permanent anode Cathodic protection Impressed current system A calcareous deposit is formed on the steel surface Reaction at the cathode ½ O 2 + H 2 O + 2e 2OH - - Reaction at the anode 2 Cl ½ Cl 2 + 2e H 2 O 2H + ½ O + 2e - - - +
- 23. Rapid corrosion General corrosion Some corrosion 100% Cathodic protection Overprotection Possible coating damage Corrosion Potentials in Seawater Zinc, Ag/Ag Cl and Cu/CuSO 4 R eference Electrodes Increasing polarisation Ag / Ag Cl Zinc + 0.50 - 0.25 + 0.0 + 0.25 - 0.55 - 1.30 -1.05 - 0.80 Potentials in volt - 0.60 - 1.35 -1.10 - 0.85 Cu / CuSO 4
- 24. Full cathodic protection (Steel surface passivated) Free corrosion of steel Corrosion reduction , % vs Zn 450 400 350 300 250 mV vs Ag/AgC1 -600 -650 -700 -750 -800 mV Reduction of corrosion rate of steel by cathodic protection. Moving seawater Negative polarisation, mV 0 50 87,5 0 50 100 150 200 Actual potential of the steel
- 30. Initial/final design current densities in A/m 2 Tropical (> 20 o C) Sub-Tropical (12-20 o C) Temperate (7 - 12 o C) Arctic (< 7 o C) 0.150 0.090 0.170 0.110 0.200 0.130 0.250 0.170 0.130 0.080 0.150 0.090 0.180 0.110 0.220 0.130 Depth (m) 0 - 30 > 30 Current density requirement acc. to DNV RP B401 (1993)
- 31. Me4an (average) design current densities in A/m 2 0 - 30 > 30 0.070 0.080 0.100 0.120 1) 0.060 0.070 0.080 0.100 1) Effect of any ice scouring are not included Current density requirement acc. to DNV RP B401 (1993) Tropical (> 20 o C) Sub-Tropical (12-20 o C) Temperate (7 - 12 o C) Arctic (< 7 o C) Depth ( m)
- 33. Coating Category Depth (m) I II III IV k 1 = 0.10 k 2 0 - 30 0.10 0.03 0.015 0.012 > 30 0.05 0.02 0.012 0.012 k 1 = 0.05 k 2 k 1 = 0.02 k 2 k 1 = 0.02 k 2 where fc = coating break down factor t = coating lifetime k 1 and k 2 = constants dependent on coating properties f c = k 1 + k 2 t Coating Break Down Factor Acc. to DNV RP B401 (1993)
- 40. Lifecycle Cost of CP Systems 13000 DWT Car Carrier 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 USD ICCP Al Zn 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Years Indicates Dry-docking Replacement of an ICCP component Anodes and Reference electrodes can be replaced whilst in service. Normally, this is carried out at the dry-docking
- 41. Life Cycle Cost of CP Systems Panamax Bulkcarrier 0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 USD ICCP Al Zn 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Years Indicates Dry-docking Replacement of an ICCP component Anodes and Reference electrodes can be replaced whilst in service. Normally, this is carried out at the dry-docking
- 43. Tanks: Lloyd's register DNV Segregated ballast 108 100 - 110 Dirty ballast 86 40 - 60 Washed cargo 108 80 - 90 Top wing 120 120 Coated (epoxy) 5 5 - 10 Soft Coats 20 - 40 Current density criteria mA/m 2 Current density Design Criteria
- 45. Tanks Conversion Factors from Volume to Area C.T : W.T : Forepeak : D.B.T : U.W.T : Volume * 0.7 - 0.9 Volume * 1.5 - 2.5 Volume * 1,5 - 2.5 Volume * 0.5 - 0.6 Volume * 1.5 - 2.5 = Area m² = Area m² = Area m² = Area m² = Area m² Examples: Deck head is included Exact calculations must be based on drawings
- 46. Net weight: 6400 m 2 x 120 mA/m 2 x 4 yrs x 50% x 11,2 kg/A.yr ------------------------------------------------------ = 17203 kg 1000 100 17203 kg. No. of anodes : --------------- = 812 pcs : 812 pcs ZTL-230 21,2 kg/pc Gross weight : 812 kg x 23kg/pcs = 18676 kg Check of current requirement: 6400 m 2 x 0,12 A/m 2 = 768 Amp Total current output : 1,3 Amp/pc x 812 pcs = 1055,6 Amp Example: Tanker vessel. Clean Ballast Water. Upper wing tank
- 50. Net weight: 6400 m 2 x 120 mA/m 2 x 4 yrs x 50% x 11,2 kg/A.yr ------------------------------------------------------ = 17203 kg 1000 100 17203 kg. No. of anodes : --------------- = 812 pcs : 812 pcs ZTL-230 21,2 kg/pc Gross weight : 812 kg x 23kg/pcs = 18676 kg Check of current requirement: 6400 m 2 x 0,12 A/m 2 = 768 Amp Total current output : 1,3 Amp/pc x 812 pcs = 1055,6 Amp Example: Tanker vessel. Clean Ballast Water. Double Bottom Tank
- 51. ICCP - Log report Readings Current Output Development With Time 100 - 80 - 60 - 40 - 20 - | | | | | | 1 2 3 4 5 6 System capacity Amp. Years 1. docking 2.. docking 3. docking Grounding (Loss of coating) Example
- 52. Cathodic protection of tanks Current density at different coating breakdown ratio Current density mA/m² 70 60 50 40 30 20 10 2 80 90 Tar Epoxy Epoxy Mastic Upper Wing tank 0 5 10 20 30 40 50 60 70 80 90 100 110 120 100 Cargo/dirty ballast tanks Clean ballast tanks fore-and aft. peak tanks Cargo/clean ballast tank slower wing/double bottom Soft coat / Flow coat Coating breakdown
- 53. Ship hull: Current Density at Different Paint Damage Current density, mA/ m² Paint damage, % 100 45 15 0 20 70 Not applicable: Repaint ICCP is recommended, not sacrificial anodes Sacrificial Anodes and ICCP can be used
- 54. *) Potential in seawater measured versus a copper/coppersulphate reference electrode Galvanic Series in Sea Water Corrosion Potentials vs. 3 Reference Electrodes Graphite + 0.25 + 1.28 + 0.17 Titanium 0 + 1.03 - 0.08 Stainless steel (Passive) - 0.50 + 0.98 - 0.13 Copper-Nickel (90/10) - 0.23 + 0.80 - 0.31 Copper - 0.33 + 0.70 - 0.41 Brass - 0.34 + 0.69 - 0.42 Stainless steel (Corroding) - 0.35 + 0.68 - 0.43 Mild steel - 0.66 + 0.37 - 0.69 Aluminium - 0.80 + 0.23 - 0.88 Zinc - 1.03 0 - 1.11 Magnesium - 1.60 - 0.57 - 1.68 Metal / Alloy Ag / AgCl Zn Cu/Cu SO 4
- 55. Corrosion Secondary structure Pipeline Anode +_ Interference
- 56. 2 Type Anode current density in seawater ( A/m ) Consumption rate ( kg/A year) 500 - 1000 < 1 x 10 500 - 1000 1000 - 5000 6 x 10 1 x 10 160 - 220 160 - 220 0,05 - 0,2 0,03 - 0,06 10 - 40 0,2 - 0,5 10 - 40 0,2 - 0,5 - 7 - 9 -6 -6 -5 2 MIXED METALOXYDE PLATINIUM - Disc - Thread LEAD-SILVER Pb - 6% Sb - 1% Ag Pb - 6% Sb - 1% Ag GRAPHITE IRON-SILISIUM Fe - 14,5% Si - 4,5% Cr SCRAP IRON Impressed current system anodes
- 57. Anode performance data Anode Specific Closed circuit Driving Capacity Consumption types gravity potential vs. Zn voltage (Ah/Kg) rate (Kg/dm ) ( Volt) (Volt) ( Kg/ A*Year) 3 Zinc Aluminium Magnesium 7.13 0 0.23 781 11.2 2.78 -0.02 0.25 2585 3.39 1.84 -0.47 0.7 1200 7.3
- 58. Aluminium Anodes Small vessels This guide is based on a 3 year (36 month) replacement period (dry-docking interval). Vessel Type Surface Anode Type Current Density - mA/m 2 Number of Total Area (% coating breakdown) Anodes Weight Kgs Tug / Small Vessels 500 A - 50 25 (15 %) 32 160 Supply Vessel 2000 A - 80 20 (10 - 15 %) 60 480 Reefer / Container 4000 A -130 15 (5 - 10 %) 58 754 Tanker / Bulker 18000 A - 180 10 (2-5 %) 120 2160 This design is for the hull only and does not allow for the seachests, thruster tunnels etc.. The calculation is the same, but with specific current densities.
- 59. This guide is based on a 3 year (36 month) replacement period (dry-docking interval). Vessel Type Surface Area Anode Type Current Density Number of pieces Total - mA/m 2 Gross (% coating breakdown) Weight - Kgs Tug / Small Vessels 500 Z-85 25 (15 %) 54 459 Supply Vessel 2000 Z-160 20 (10 - 15 %) 92 1472 Reefer / Container 4000 Z-270 15 (5 - 10 %) 80 2160 Tanker / Bulker 18000 Z-200 10 (2-5 %) 316 6320 This design is for the hull only and does not allow for the seachests, thruster tunnels etc.. The calculation is the same, but with specific current Densities Zinc Anodes
- 60. Principle : Effect of using CP Corrosion Curves depend on - Coating condition - CP-design Coating breakdown CP installed CP and coating at newbuilding Time Corrosion
- 61. Steel passivation by sacrificial anodes Paint Steel Rust Without Cathodic Protection Paint Steel With Cathodic Protection Anode Anode current Seawater Seawater Calcareous layer
- 63. Location Current density Seachests Thruster tunnel Propeller nozzle Rudder Rudder flaps Anti-suction tunnels Propeller (Uncoated) Azimuth propeller 40 mA/m 2 150 mA/m 2 150 mA/m 2 100 mA/m 2 150 mA/m 2 100 mA/m 2 500 mA/m 2 150 mA/m 2 CP of ships: Additional Areas Requiring Protection.
- 68. Location of Pitguard Anodes Web Frame Web Frame
- 70. Slipring Arrangement Shaft Silver Graphite Brush Steel Slipring Earth to Hull mV meter Silver Inlay
Editor's Notes
- As anodes, aluminium or zinc is used. They are technically equal, but aluminium has some advantages compared to zinc: The consumption rate for zinc is 11,2 kg/A/year, while for aluminium it is only 3,37. This means that for the same protection the requirement for aluminium is only 1/3 of the weight of zinc. The marketprice for zinc and aluminium varies from day to day, but as a rough rule of the thumb the total price for a aluminium system is approximately 1/2 of the zinc price With less weight and therefore less number of anodes, the installation cost for aluminium is lower than for zinc.
- We have now given you a comprehensive set of tools you can use to fight the competition. There is, however, one product line no other paint company has, which put Jotun in a unique position. It has briefly been discussed earlier, but we have decided to include some of the products in this presentation. We are talking about cathodic protection, of course. So far this is probably the most underrated productline in Jotuns’ assortment. We believe that the reason why cathodic protection has not yet been the success we expected, are the following. To the owners this is a simple commodity product. They need it, but prefer not to spend time on it. For many of our salespeople, unfortunately, cathodic protection is still considered some kind of a complicated technology you need a specialist for. It is easy to see that when this salesman and the customer meet, they are on different planets talking about the same subject. There is, however, no doubt that we can use cathodic protection as an added value and help Jotun secure more paint business. We will go through 3 products which all can give you the upper hand:- READ OVERHEAD
- Being fitted externally onto the hull, it increases the frictional resistance. The faster the speed, the more severe impact. A reefer vessel, is a typical example of a vessel most severly affected. To avoid this, and have the same protection, ICCP systems are recommended. These are, however, mostly installed at newbuilding stage and seldom during a drydocking. In an ICCP system the anodes are recessed into the hull. For a vessel of approx 40,000dwt a typical requirement for sacrificial anodes can be 1,6 tons. The impact the weight has on the vessel increases with increasing size. Being nominal compared to the ship, you should keep it in the back of your mind. Our toughest competitor in many cases, apart from ourselves, is often the yard. They have their own arrangements, and are able to supply a package of anodes and installation. In many cases, when the yard is not getting this order, increase the price of the installation to such an extent that it is cheaper for the owner to order everything from the yard. This is rubbish, my friends, and you should tell that to the owner if you are faced with the same scenario. Jotun supply top quality anodes, and our customers can rely on it. What the yards sometimes supply is something completely different.
- Being fitted externally onto the hull, it increases the frictional resistance. The faster the speed, the more severe impact. A reefer vessel, is a typical example of a vessel most severly affected. To avoid this, and have the same protection, ICCP systems are recommended. These are, however, mostly installed at newbuilding stage and seldom during a drydocking. In an ICCP system the anodes are recessed into the hull. For a vessel of approx 40,000dwt a typical requirement for sacrificial anodes can be 1,6 tons. The impact the weight has on the vessel increases with increasing size. Being nominal compared to the ship, you should keep it in the back of your mind. Our toughest competitor in many cases, apart from ourselves, is often the yard. They have their own arrangements, and are able to supply a package of anodes and installation. In many cases, when the yard is not getting this order, increase the price of the installation to such an extent that it is cheaper for the owner to order everything from the yard. This is rubbish, my friends, and you should tell that to the owner if you are faced with the same scenario. Jotun supply top quality anodes, and our customers can rely on it. What the yards sometimes supply is something completely different.
- The most important use of the slipring is to prevent spark corrosion of the main engine bearings, which occurs when earthing happens across those bearings in the engine. If spark corrosion occurs, it will require out of service repairs at a very high cost. Of course, as you would expect use of the slipring extends the service life of the propeller even though it may not totally stop the corrosion. An average slipring system cost USD2,700, but the savings for the owner can be tremendous. Instead of giving discount on the paint, why not give him a slipring system instead f.o.c.?
- A slipring system is designed to earth the propeller and its shaft to the vessels hull. It is easily installed at any time and should be used in all cases regardless if cathodic protection is in use or not