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Methods and Materials in Cathodic Protection

Methods and Materials in Cathodic Protection The following is an abstract from a paper read before the Manchester Metallurgical Society in January, by Dr. W. F. Higgins, A.R.I.C., of Magnesium Elektron Ltd. has the largest difference of potential pipeline should give a value of about ATHODIC protection is com­ from other metals, it has a favourable 0.85 v. negative to a copper/copper plementary to the physical pro­ tection of metals, since it is used in electrochemical equivalent and is sulphate cell. Determining the extent conditions where painting and similar free from polarisation, which is of to which protection is required entails a wide survey, both as regards the methods are not practicable, or not course disadvantageous in the anode pipeline itself and the ground in which wholly satisfactory; in the case of since it reduces output of current. it is buried, and all results will have metals which are buried in damp The most favoured alloy is one con­ taining 6% aluminium, 3% zinc and a bearing on the final analysis. A ground or immersed in aqueous fluids. The underlying principle is that, if approximately 0.2% manganese, plus current of anything between 1 to 20 metal surfaces in need of protection a small percentage of impurities such ma./sq.ft. has been found necessary to prevent corrosion. Where the coun­ can be made completely cathodic, no as iron and nickel. This gives an alloy try is uniform an applied current corrosion will occur. This is effected with a fairly uniform corrosion pattern. system is desirable, but a galvanic by the introduction of an artificial Impurities on the surfaces of the anode might result in the corrosion of the anode is advantageous in variable anode which sets up a flow of current, the circuit of which is completed by adjoining metal, thus reducing current ground, since it can be placed exactly the aqueous medium acting as the output, and a self-corrosion resistance where the pipeline is most subject to corrosion. electrolyte. is essential in the alloy, therefore the magnesium alloy must be free from the impurities causing self corrosion. Effects of alkalinity Oi l tankers Several important factors influence A specific instance of the application cathodic protection. Firstly, the dis­ Protection of a steel pipeline of cathodic protection is its use in the charge of hydrogen at the cathodes interior of cargo compartments in an It is obvious that pipelines and similar causes the neighbouring ground to oil tanker. These carry oil and sea installations are affected by very varied become increasingly alkaline. This is water alternately, and at times are conditions, since they cover a con­ predominantly beneficial in result, as empty but damp with sea water. Protec­ siderable distance. It has been found iron and steel are less prone to cor­ tion can therefore only be given at that, where the ground resistivity is rosion in an alkaline condition, pro­ intervals. Magnesium anodes are more than 3,000 ohm/cm., magnesium tection is afforded by the precipitation used for a period of very high current anodes are usually unable to pass of magnesium and calcium salts, and intensity, detaching the corrosion scale sufficient current into the ground to also harmful anaerobic bacteria become and leaving in its place a calcareous render the pipeline negative enough to dormant in degrees of higher alkalinity. scale which protects the compartment prevent all corrosion. In this case, On the debit side, a high alkaline until it can again receive cathodic pro­ an applied current system capable of content may affect any previous tection. By this means there is a net producing direct current of a suitable physical protective coating, and may saving of 80% in repair costs. voltage is preferable. A fully protected give rise to the phenomenon known as cathodic corrosion. Secondly, the hydrogen ion discharged at the cathode is a strong reducing agent, clearing the metal of rust and scales and leaving the surface clean and bright. Anode size Cathodic protection is only effective where there is a certain volume of the electrolyte. Metals' which are merely exposed to a moist atmosphere allow only a restricted path for the current, but where there is complete immersion in a large volume of water the current has unlimited freedom of movement, and a small anode is able to supply the current for a wide cathodic area. Once the hydrogen ions have started to dis­ charge at the cathode they set up a resistance to further discharge, in other words polarisation takes place. This characteristic is useful in deter­ mining the amount of protection that is required. The most widely used metal in cathodic protection is magnesium, in the form of a high-purity alloy. It CORROSION TECHNOLOGY, April 1955 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Anti-Corrosion Methods and Materials Emerald Publishing

Methods and Materials in Cathodic Protection

Anti-Corrosion Methods and Materials , Volume 2 (4): 1 – Apr 1, 1955

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0003-5599
DOI
10.1108/eb019045
Publisher site
See Article on Publisher Site

Abstract

The following is an abstract from a paper read before the Manchester Metallurgical Society in January, by Dr. W. F. Higgins, A.R.I.C., of Magnesium Elektron Ltd. has the largest difference of potential pipeline should give a value of about ATHODIC protection is com­ from other metals, it has a favourable 0.85 v. negative to a copper/copper plementary to the physical pro­ tection of metals, since it is used in electrochemical equivalent and is sulphate cell. Determining the extent conditions where painting and similar free from polarisation, which is of to which protection is required entails a wide survey, both as regards the methods are not practicable, or not course disadvantageous in the anode pipeline itself and the ground in which wholly satisfactory; in the case of since it reduces output of current. it is buried, and all results will have metals which are buried in damp The most favoured alloy is one con­ taining 6% aluminium, 3% zinc and a bearing on the final analysis. A ground or immersed in aqueous fluids. The underlying principle is that, if approximately 0.2% manganese, plus current of anything between 1 to 20 metal surfaces in need of protection a small percentage of impurities such ma./sq.ft. has been found necessary to prevent corrosion. Where the coun­ can be made completely cathodic, no as iron and nickel. This gives an alloy try is uniform an applied current corrosion will occur. This is effected with a fairly uniform corrosion pattern. system is desirable, but a galvanic by the introduction of an artificial Impurities on the surfaces of the anode might result in the corrosion of the anode is advantageous in variable anode which sets up a flow of current, the circuit of which is completed by adjoining metal, thus reducing current ground, since it can be placed exactly the aqueous medium acting as the output, and a self-corrosion resistance where the pipeline is most subject to corrosion. electrolyte. is essential in the alloy, therefore the magnesium alloy must be free from the impurities causing self corrosion. Effects of alkalinity Oi l tankers Several important factors influence A specific instance of the application cathodic protection. Firstly, the dis­ Protection of a steel pipeline of cathodic protection is its use in the charge of hydrogen at the cathodes interior of cargo compartments in an It is obvious that pipelines and similar causes the neighbouring ground to oil tanker. These carry oil and sea installations are affected by very varied become increasingly alkaline. This is water alternately, and at times are conditions, since they cover a con­ predominantly beneficial in result, as empty but damp with sea water. Protec­ siderable distance. It has been found iron and steel are less prone to cor­ tion can therefore only be given at that, where the ground resistivity is rosion in an alkaline condition, pro­ intervals. Magnesium anodes are more than 3,000 ohm/cm., magnesium tection is afforded by the precipitation used for a period of very high current anodes are usually unable to pass of magnesium and calcium salts, and intensity, detaching the corrosion scale sufficient current into the ground to also harmful anaerobic bacteria become and leaving in its place a calcareous render the pipeline negative enough to dormant in degrees of higher alkalinity. scale which protects the compartment prevent all corrosion. In this case, On the debit side, a high alkaline until it can again receive cathodic pro­ an applied current system capable of content may affect any previous tection. By this means there is a net producing direct current of a suitable physical protective coating, and may saving of 80% in repair costs. voltage is preferable. A fully protected give rise to the phenomenon known as cathodic corrosion. Secondly, the hydrogen ion discharged at the cathode is a strong reducing agent, clearing the metal of rust and scales and leaving the surface clean and bright. Anode size Cathodic protection is only effective where there is a certain volume of the electrolyte. Metals' which are merely exposed to a moist atmosphere allow only a restricted path for the current, but where there is complete immersion in a large volume of water the current has unlimited freedom of movement, and a small anode is able to supply the current for a wide cathodic area. Once the hydrogen ions have started to dis­ charge at the cathode they set up a resistance to further discharge, in other words polarisation takes place. This characteristic is useful in deter­ mining the amount of protection that is required. The most widely used metal in cathodic protection is magnesium, in the form of a high-purity alloy. It CORROSION TECHNOLOGY, April 1955

Journal

Anti-Corrosion Methods and MaterialsEmerald Publishing

Published: Apr 1, 1955

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