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Cathodic protection
- 1. Cathodic protection Cathodic protection is oneof the methods of protecting pipelines, ships, platforms and other steel structure24-08-2018 1
- 2. Cathodic protection Corrosion occursas a result of an electrochemical reaction driven by a potential difference between two electrodes, anode and cathode, connected by an electronic path and immersed in the same electrolyte.24-08-2018 2
- 3. High pH (>9) Preventing Corrosion Salt 20/08/2018 NM 3
- 4. Cathodic protection There arevarious time-proven methods for preventing and controlling corrosion. One of these is cathodic protection. The function of cathodic protection is to reduce the potential difference between anodes and cathodes to a neglected value. This reduction is due to the polarization of cathodes to the potential of most active anodes. In this way, corrosion current is mitigated according to Ohm’s law.24-08-2018 4
- 5. Cathodic protection There arevarious time-proven methods for preventing and controlling corrosion. One of these is cathodic protection. Corrosion occurs at anodic areas. If all anodic area can be converted to cathodic areas the structure would become cathode and corrosion stops as there is no corrosion at regions where current enters from the environment to metal (cathodic areas). 24-08-2018 5
- 6. Cathodic Protection Criteria Inorder to achieve adequate Cathodic Proctection (CP), the protected structure must be polarized to a certain value. The polarized potential is measured with respect to a certain reference electrode. A copper/copper sulfate reference electrode (CSE) is the most common electrode used in soil and freshwater. There are two types of criteria for assessing cathodic protection. Either one may be used24-08-2018 6
- 7. Types of CathodicProtection There are two types of cathodic protection: – Galvanic anode cathodic protection – Impressed current cathodic protection 24-08-2018 7
- 8. Galvanic anode cathodic protection Inthis type of CP, protection is achieved by connecting the protected structure to a sacrificial anode, which is placed close to the protected structure. – Sacrificial anodes are made from active metals such as zinc, aluminium, or magnesium, which are considered the most active metals according to the galvanic series. – CP current is created by the potential difference24-08-2018 8
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- 10. Preventing the Corrosionof Iron (cathodic protection/sacrificial anode) Sacrificial anode 20/08/2018 NM 10
- 11. Impressed current cathodicprotection (ICCP) 24-08-2018 11 In this type of CP, protection is achieved by connecting the protected structure to an anode bed through a transformer rectifier (TR). The anode bed is a series of buried anodes that are electrically connected and surrounded by certain backfill to reduce their resistance to the earth. The anode bed should be placed remotely from the protected structure
- 12. Impressed current cathodicprotection (ICCP) 24-08-2018 12 The main component of this type of CP is the TR, which forces the current to flow from the anodes to the protected structure (cathode). Three types of anodes are used: Soluble anodes (aluminium and steel), semi-soluble anodes (graphite and high silicon cast iron and non- soluble anodes (platinum, mixed metal oxide, and polymer). The type of anodes used depends on the chemical composition of the electrolyte, to which the substrate is exposed and the area to be protected.
- 13. Impressed Current (ICCP) • Involvesthe use of an external power source – metal to be protected is made cathodic to its surroundings – inert anodes used which are virtually non-consumable – insulated from structure • Early anodes made from scrap steel but most modern ICCP systems use lead silver alloy, titanium or niobium • Has been used in the protection of steel reinforcement in concrete • The use of modern electronics makes the system self regulating • Very costly to run – mainly used in marine applications – oil rigs – large anodes placed on sea bed approximately 100m20/08/2018 NM 13
- 14. Impressed Current Cathodic Protection PowerSupply Shipboard Power Controller Shipboard Power Insulation Pt Anode Reference Electrode Paint Layer Hull e- e - 20/08/2018 NM 14
- 15. Application of CathodicProtection Systems on Buried Pipelines 24-08-2018 15 Pipelines are used for transporting water, petroleum products, natural gas, and other utilities. There’s a huge network of piping systems used in every country all over the world. Pipelines may be onshore or offshore, and are subject to corrosion in both cases. If corrosion isn't mitigated, dangerous and expensive damage can be the result.
- 16. Applications of CathodicProtection • Galvanized Steel Zinc coating • Sacrificial Anodes Ship Hulls Subs (free flooding areas) Los Angeles Class Sub Arleigh-Burke Destroyer 20/08/2018 NM 16
- 17. Application of CathodicProtection Systems on Buried Pipelines 24-08-2018 17 There are several corrosion control techniques used on pipelines; cathodic protection is one of them. It can be applied either to coated pipelines to mitigate the corrosion attack on areas where coating quality may be poor. It is also used on bare pipelines. Both types of CP can be applied to buried pipelines. The application of either of these types depends on several factors, such as the required current, soil resistivity, and the area to be protected.
- 18. Application of CathodicProtection Systems on Buried Pipelines 24-08-2018 18 CP aims to polarize a pipeline to a minimum potential of -850 mv, for carbon steel and for adequate CP. The polarized potential is to be measured through test stations, which are to be installed at the following locations along the route of pipeline: • At frequent intervals (e.g. < 2 km / 1.24 miles) • At crossings with foreign structures • At points of electrical isolation • At some galvanic anode locations • At casings • Near sources of electrical interference • At the location of stray current discharge to earth
- 19. Sacrificial anode systemin seawater Impressed-current cathodic-protection system in seawater 4Al => 4AL+++ + 12 e- 3O2 + 12e- + 6H20 => 12OH- 20/08/2018 NM 19
- 20. Problems Created byCathodic Protection 24-08-2018 20 In large pipeline networks, there are a lot of crossings, parallelism, and approaches, wherein the pipeline has its applied CP system. DC interference may occur between pipelines, accelerating corrosion. In order to overcome this problem, pipelines can be electrically coupled, either directly or through resistance. Cathodic protection is just one method used to prevent corrosion, not just in pipelines, but in ships, offshore oil platforms, and other steel structures. Whether it's the best application for the job, or the only one to be used, is often specific to the structure being protected
Editor's Notes
- #4 &lt;number&gt; Figure: 20-25