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CORROSION MECHANISM
THE CORROSION PROBLEM
1. Steels used in platform and pipework construction are not pure materials. Although iron(Fe) forms the major part, steel contains a certain quantity of carbon (C) up to about 1%. 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.
2. Iron carbide has a lower potential than pure iron when in the presence of conducting electrolyte (such as seawater or damp soil). That is to say, a mass of iron/iron-carbide “cells” are formed in electrolyte, with the pure iron areas positive and the iron-carbide areas negative. Each cell, which can be regarded as a primary battery, is short-circuited by the structure metal work
3. At the iron surface some of the electrons surrounding the iron atoms break away, leaving the atoms of iron positively charged (Fe++). These are called iron“ions”, and they are, in fact, positively charged atoms of the metal. They travel through the sea or soil down the potential gradient from the positive iron areas to the negative iron-carbide areas, forming an electric current through the sea or soil as they do so. This current returns through the main structure metal conducting path, as shown in Figure below:
4. The positive areas from which current flows into the sea or soil are called “anodes” (or anodic areas) and the negative areas into which the current flows are “cathodes” (or cathodic areas). 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. This process can be slowed by painting or by other forms of encapsulation, but it appears increasingly as the covering wears off. On the other hand marine growth may tend to inhibit the process with time, but noreliance can be placed on it.
THE CURE
5. 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.
6. There are two quite distinct methods of achieving this, known as the “sacrificial anode” and the “impressed-current” systems. 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.
There are two quite distinct methods of achieving this, known as the “sacrificial anode” and the “impressed-current” systems. 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.
Zery Engineering Company Ltd offers design, consultancy and installation services for Cathodic Protection of Pipelines and Vessels in oil and gas exploration and production.
The following material was researched and was submitted as a guideline for calculating the requirements of sacrificial anodes for your application: