IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)
1. PREFACE 2. CORROSION PHENOMENA 3. CATHODIC PRETECTION 3.1 Cathodic protection with sacrificial anodes 3.2 Criteria for cathodic protection of iron and steel 4. IMPRESSED CURRENT CATHODIC PROTECTION (ICCP) 4.1 Anodes for ICCP system 4.2 ICCP system components 4.3 The design of the ICCP systems 5. CATHODIC PROTECTION OF SHIPS 6. THE ICCP SYSTEM FOR CATHODIC PROTECTION OF OFFSHORE STRUCTURES 7. THE ICCP SYSTEM FOR CATHODIC PROTECTION OF ONSHORE STRUCTURES 8. THE SCOPE OF OUR ACTIVITIES ON ICCP SYSTEMS LIST OF SOME REFERENCES
Steel structures and ships dipped into fresh water are faced with two serious problems: - Corrosion and Fouling (Marine growth)1 Corrosion and Fouling particularly seriously affect commercial and navy ships. Corrosion destroys the very basic structure of the ship – the hull itself, while Fouling, if not properly checked, will progressively decrease the speed of the ship, increase the voyage time, fuel consumption and other expenses. It can also cause overheating and damage to the engines by preventing cooling water to pass through intake pipes obtruded by Fouling. Taken together, Corrosion and Fouling will bring about voyage interruption and ship’s docking causing additional loss to the ship’s owners or, in case of navy, decrease in navy’s defense performance. Proper solution to these problems is achieved by: - few layers of appropriate paint applied on ship’s hull or steel structure (passive protection ) and - installation of an Impressed Current Cathodic Protection (ICCP) system in which Titanium Anodes (MDSA) coated by a mixture of noble and non-noble metal oxides are used.
2. CORROSION PHENOMENA
Corrosion is a process which occurs naturally to metals and alloys immersed or buried in an electrolyte (such as fresh or sea water), as a result of appearance of an electric potential at the metal/electrolyte interface. If two metals, or parts of the same metal with different metal/electrolyte potentials are connected, formation of a corrosion cell, schematically represented in Fig.1., takes place. In such a case the cathodic half-reaction is reduction of oxygen dissolved in water, while the anodic half-reaction is metal dissolution (corrosion).
Fig.1. Shematic representation of a corrosion process
Corrosion cells will also be formed due to the appearance of flaws in the insulating coating, water flow and aeration.
Sticking of sea fruits (algae, shells etc.) to the steel structure and ships and their augmentation by time
3. CATHODIC PROTECTION
Electrochemical methods for protection against corrosion are based on the polarization of metals to the potential at which corrosion process becomes very slow, or practically stopped. Polarization could be realized by impressed current system, or by connecting the metal with the sacrificial anodes (Zn, Al and Mg) and is known under the common name as Cathodic Protection (CP). Only cathodic protection can provide complete protection of metals from all types of corrosion damages (including the most dangerous one – pitting corrosion). 3.1. Cathodic Protection with sacrificial anodes
Sacrificial anodes are mainly made of Zn, Al and Mg alloys. Since they undergo to active anodic dissolution during the process of cathodic protection, their life time is no longer than 23 years and they must be replaced after that period of time. 3.2. Criteria for cathodic protection of iron and steel
Basic criteria for cathodic protection are: protective current density and a protective potential. Correctly calculated and maintained current density and protective potential will guarantee corrosion free life of the ship and steel structure. From this point of view the impressed current system has two major advantages over the system based on the use of sacrificial anodes: 1. an easy measurement and control both parameters (current and potential) during time of exploitation is provided; 2. durability of titanium based anodes in the ICCP system is much longer (over 15 years without replacement, or recoating of anodes). Criteria for cathodic protection of iron and steel, based on the BSI Code of Practice for Cathodic Protection (CP 1021:1973), are given in Table 1.
Table 1: Protective potential measured against various standard electrodes
Reference electrode Copper/copper sulphate Silver/silver chloride/sea water Silver/silver chloride/satur. KCl Zinc/ sea water
Aerobic environment - 0.85 (V) - 0.80 (V) - 0.75 (V) - 0.25 (V)
Anaerobic environment - 0.95 (V) - 0.90 (V) - 0.85 (V) - 0.15 (V)
At potentials more negative than given values, steel structure or ship’s hull is protected from corrosion.
4. IMPRESED CURRENT CATHODIC PROTECTION (ICCP)
4.1 Anodes for ICCP system Our ICCP system is based on the use of two types of anode: platinized titanium anodes (Pt-Ti) for application in sea and fresh water and titanium anodes (MDSA) coated with the mixture of noble and non-noble metal oxides, specially designed for application in sea water. It should be emphasized here that we are the owners of the technology of production of both types of anode and that we can also be suppliers of anodes for the ICCP systems. Characteristics of both types of anode are given in Table 2. Table 2: Wear rate and typical current density of anodes Type of anode environment Typical current Wear rate (µg A-1h-1) density (A m ²) Fresh water 100 approx. 5 Pt - Ti Brackish water 300 approx. 10-15 Sea water 700 approx. 1 MDSA Sea water 500 < 0.1
Apart of a very low wear arte, the main advantage of MDSA anode over Pt-Ti anode for application in sea water is the simultaneous cathodic protection and antifouling. Namely, during the process of cathodic protection chloride ions oxidation takes place on the MDSA anodes producing hypochlorite and providing antifouling in the vicinity of anodes. Current efficiency is 95%. At the world fair for invention Brussels Eureka 1990 our experts dr. N. Krstajic and dr. V. Jovic were awarded gold medals for this invention.
4.2. ICCP system components Our ICCP system generally comprises a power unit (incorporating the controller), the reference electrodes and anodes, while for application on ships shaft earthing assembly and rudder bonding are also provided. Location and number of anodes and reference electrodes are the most important factors for an optimal distribution of a protective potential and successful antifouling. Power units ensure reliable trouble free operation by incorporating solid state circuitry to transform and rectify the AC supply into controlled, low voltage DC output. Their operation can be fully automatic with built in manual override or not, depending on the type of structure to be protected.
4.3. The design of the ICCP systems The procedure of designing the ICCP systems for almost all types of steel structures in fresh and sea water (ships, docks, offshore structures, etc.) is completely computerized.
5. CATHODIC PROTECTION OF SHIPS
Our ICCP system for application on ships is schematically represented in Fig. 2., for both, sea water (MDSA anodes – simultaneous cathodic protection and antifouling) and fresh water operation (Pt-Ti anodes). All anodes are supplied with non-conductive backing shields to ensure uniform distribution of a protective current and at the same time to prevent over-protection of the area immediately adjacent to the anodes. Anodes and Zn reference electrodes are also supplied with watertight cofferdams to allow cable access into the hull. All cable entries involve double gland sealing, as recommended by the classification societies.
Fig.2. Schematic representation of the ICCP system on the ships.
The dimensions of standard Pt-Ti and MDSA anodes and corresponding backing shields for the ICCP system on the ships in fresh and sea water are given in Table 3. All anodes are made of 2 mm thick Ti plates.
Table 3: Anode and shield dimensions for the ICCP system on ships
I/A 10 20 30 40
Sea water Plate dim./mm 250 x 800 320 x 125 400 x 150 444 x 180
Shield dim./mm 2500 x 8000 3200 x 1250 4000 x 1500 4440 x 1800
I/A 0.2 0.4 0.6 0.8
Fresh water Plate dim./mm 250 x 800 320 x 125 400 x 150 444 x 180
Shield dim./mm 2500 x 8000 3200 x 1250 4440 x 1800 4440 x 1800
To design and provide a formal quotation for a CP system for ship, following information will be required: 1. Length between perpendiculars; 2. Maximum design draught; 3. Breadth; 4. Block coefficient of displacement; 5. Number and dimensions of rudder; 6. Number and diameter of propeller; 7. Intermediate shaft diameter; 8. Ship’s AC supply; 9. General arrangement drawing of ship; 10. Sea or fresh water.
6. THE ICCP SYSTEM FOR CATHODIC PROTECTION OF OFFSHORE STRUCTURES
The same ICCP system can be applied on any type of offshore steel struckture or docks in sea and fresh water. Ins uch a case MDSA and Ti-Pt anodes of different shape, schematically presented in Fig. 3., are used. Dimensions of these anodes are given in Table 4.
Table 4: Anode dimensions for the ICCP system of offshore structures
I/A 10 20 30 40
Sea water Anode dimensions/mm φ 38 x 168 φ 38 x 335 φ 38 x 503 φ 38 x 670
I/A 0.2 0.4 0.6 0.8
Fresh water Anode dimensions/mm φ 38 x 168 φ 38 x 335 φ 38 x 503 φ 38 x 670
For design a ICCP system and provide a formal quotation, following data will be required:
Fig. 3. Schematic representation of anodes for the ICCP system of offshore steel structures
1. Customer preference in system design (if any); 2. Estimated surface area to be protected; 3. The configuration of all structural steelworks with particular reference to highly stressed areas (such as nodes); 4. Details of pipelines, piles, conductor tubes, pipeline rises, wells and all other steelworks above and below seabed level; 5. Details of steelwork conduits, grouting tubes, pipe guides, flotation chambers and launch pads; 6. Details of internal flooded areas, including fresh water, drill water, ballast and fuel tanks; 7. Details of chains, propulsion units, fenders and anchor wires; 8. Nature of any coatings to be applied and extent of application; 9. An estimate of coating performance and period of time for which cathodic protection is required (for existing installations, age and condition of structure steelworks and coating); 10. Water resistivity and temperature range; 11. Location of structure, water depth, wave hights, weather cycles, tidal flow, nature of sea bed and probability of scour; 12. Location of electrically hazardous areas and availability of electrical power supplies; 13. Design limitations, especially with respect to weight loading of structure, particularly during float-out; 14. Details of existing or proposed adjacent or linked structures and type of cathodic protection installed or planned.
7. THE ICCP SYSTEM FOR CATHODIC PROTECTION OF ONSHORE STRUCTURES
The same ICCP system can be applied on any type of onshore steel structure. For this purpose we usually use Fe-Si anodes. To design a ICCP system and provide a formal quotation, following data will be required: 1. Geographical location of job site and climatic conditions - if considering solar powered system, give latitude and longitude; 2. An estimate of all submerged, wetted and/or buried areas of the structure to be protected; 3. The configuration, layout and dimensions of all structural steelworks to ensure adequate distribution of anodes; 4. Nature, types, thickness and extent of coating applied to steelworks; 5. Estimate of percentage of mechanical breakdown of coatings during and after installation (in case of existing installations, age and condition of structure steelworks and coating); 6. Period of time for which cathodic protection is required; 7. Location of electrically hazardous areas and availability of electrical power supplies; 8. Customer preference in system design (if any); 9. Design limitations in terms of size and weight of equipment, due to fabricators' or site contractors' lifting capabilities; 10. Special easement problems and/or requirements. Anode ground beds can be located up to 200 m from structure; 11. Degree of specialist involvment, e.g. supervision only, yurnkey project, etc.; 12. In case of pipelines:material, length, wall thickness and size, together with information regarding electrical continuity of pipe; 13. For pipeline running offshore, weight and thickness of the concreteweight coating, and depth immersed in sea-bed; 14. Data on weather cycles, wave action, location, water depth, tidal flow, nature of seabed, and whether scour may occur; 15. For buried installations, nature and variation of soil, presence of anaerobic areas; 16. Water or soilresistivity and temperature range. In the case of a cross country pipeline, measurements at 1-2 km intervals are required; 17. Details of any foreign structures, pipelines or services close to the protected structure which may be affected by interference; 18. Details of any ancillary steelworks attached to the structure, or branch lines, valves, washouts, earthing, etc., which must be protected, or isolated from the system; 13.Size, thickness and pressure rating of all main and branch lines, for determing details of insulated joints.
8. THE SCOPE OF OUR ACTIVITIES ON ICCP SYSTEMS
1. 2. 3. 4. 5. 6. 7. Collection of all relevant details for design (already mentioned above); Preliminary measurements of the corrosion parameters (if necessary); Calculation and detailed design of the ICCP system components; Manufacturing and delivery of anodes, reference electrodes and rectifiers; Supervision of project realization; Adjucement and putting into operation; Guarantee.
LIST OF REFERENCES
(A) OFFSHORE 1. Temporary (for 8 months in 1990.) cathodic and fouling protection of a navy ship in shipyard of Tivat (MDSA anodes). The results of this project are published in International Progress, 39, no.419 (1992.), pp. 279-285. 2. Cathodic protection of two docks in the shipyard of Tivat (MDSA anodes) March 1991. 3. Cathodic protection of the River Fire fighting Vessel in the river Sava in Belgrade (Ti-Pt anodes), October 1991. 4. Cathodic protection of crane vassel (MDSA anodes), shipyard Belgrade, July 1992. 5. Cathodic protection of steel structure of 6 gates in port Bar (MDSA anodes), April 1995. (B) ONSHORE Design and cathodioc protection of 45 different pipe lines all over Yugoslavia.