CATHODIC PROTECTION THEORY AND PRACTICE V ASHWORTH PDF

Abstract As Volta already observed in early s, a current that is exchanged between a metal surface and the electrolyte to which it is exposed modifies the behaviour of the metal, increasing the corrosion rate or reducing it. Some years later, Davy put in practice such concept demonstrating the possible implications, which, after about a dormant century, started to have effective industrial use: with Davy, cathodic protection was born. Both techniques are set up by establishing a cathodic or anodic current that is exchanged with the electrolyte, which means supplying or taking electrons. The following chapter deals with these techniques from a theoretical and practical point of view: mechanism, protection criteria, protection potential and current density, applications, monitoring. Electrical current [exchanged between metal and electrolyte ] modifies exceptionally the oxidation state.

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Steels used in platform and pipework construction are not pure materials. The amount of carbon determines the type of steel — for example from mild steels up to hard tool steels. The carbon takes the form of iron carbide Fe3C , and, because it is not spread quite evenly through the steel, there are adjacent surface areas of pure iron and iron carbide throughout the steel structure.

These individual areas are minute in size and are very close together a matter of thousandths of an inch. Iron carbide has a lower potential than pure iron when in the presence of conducting electrolyte such as seawater or damp soil.

Each cell, which can be regarded as a primary battery, is short-circuited by the structure metal work 3. This current returns through the main structure metal conducting path, as shown in Figure below: 4. As the iron metal is removed, atom by atom, from the anode and deposited at the cathode, a corrosion pit is formed as shown above.

Hundreds of thousands of such pits are formed, which appears as general corrosion of the structure member or pipe. On the other hand marine growth may tend to inhibit the process with time, but no reliance can be placed on it. Since corrosion is due to the outflow of iron atoms carried by the sea or soil current, it can be reduced or even prevented if such currents could be stopped or reversed, so causing all the submerged or buried metal parts to receive current instead of giving it out — that is, by making all areas cathodic.

This can be done by placing other, independent electrodes in the sea or soil near the structure and causing them to force current into the structure. Either method may be used on an offshore installation and on either type of platform, steel-jacket or concrete;in the latter the well risers and their guide-tube structures still have to be protected.

Either method may be used on an offshore installation and on either type of platform, steel-jacket or concrete; in the latter the well risers and their guide-tube structures still have to be protected. The following material was researched and was submitted as a guideline for calculating the requirements of sacrificial anodes for your application: a Calculate the AREA to be protected.

The sacrificial anode system design requires the current density to achieve this potential. Note that it is important to consider the environment that the metal is exposed to since the "current demand" may vary with different combinations. Ashworth and C. Booker, eds.

This will give you the weight in pounds.

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Cathodic Protection Design

You are on page 1of 34 Search inside document Cathodic protection prevents corrosion by converting all of the anodic active sites on the metal surface to cathodic passive sites by supplying electrical current or free electrons from an alternate source Cathodic protection shall be design with due to environment condition, neighbouring structures and other activities The general design procedure for both sacrificial anode and impressed current systems is similar Cathodic protection design Modifications to the structure to incorporate requirements are best made at the early design and pre-construction phase of the structure For underground structures it may be necessary to visit the proposed site, or for pipelines the proposed route, to obtain additional information on low-resistivity areas, availability of electric power, and the existence of stray dc current or other possible interaction. It is common practice for a survey to be made before design This survey is often combined with a study to establish economic justification for the recommended anti-corrosion proposal while the principal data necessary for design chemical and physical are also collected If the structure already exists, measurement of existing structure-to- soil potentials is essential to give valuable information as to which areas are anodic and which are cathodic If the new structure, the design of a cathodic protection system should include the calculation of: Current demand Resistance to earth of the anodes Quantity and location of anodes Electrical supply requirements 2. Drawings and material specifications Engineering drawings: To establish the size and shape of the structure to be protected in order to design an effective cathodic protection system To check all other metallic structures in the vicinity presence may affect the operation of the system being designed Material specifications: To establish material and surface conditions, particularly the presence and quality of protective coatings. Site survey To establish the actual environmental condition Media characterizations.

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Cathodic protection : theory and practice

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Cathodic and Anodic Protection

Steels used in platform and pipework construction are not pure materials. The amount of carbon determines the type of steel — for example from mild steels up to hard tool steels. The carbon takes the form of iron carbide Fe3C , and, because it is not spread quite evenly through the steel, there are adjacent surface areas of pure iron and iron carbide throughout the steel structure. These individual areas are minute in size and are very close together a matter of thousandths of an inch. Iron carbide has a lower potential than pure iron when in the presence of conducting electrolyte such as seawater or damp soil. Each cell, which can be regarded as a primary battery, is short-circuited by the structure metal work 3.

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CATHODIC PROTECTION THEORY AND PRACTICE V ASHWORTH PDF

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