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Illustration - Corrosion of Metal - Indicative of current movement between Anodic and Cathodic Areas through the Electrolyte. The more conductive the Electrolyte [e.g. more dissolved salts, chlorides or acids], the higher rate of current movement & more accelerated the rate of corrosion.


The rate of corrosion is a function of the 'Corroding Current'. This current is dependent on the voltage that is causing the current to flow; therefore, increasing the driving voltage accelerates the metal propensity to corrode or increases the rate of corrosion. Connecting one metal to another metal that has a greater manufacturing energy content increases the driving voltage of the corrosion cell.


Any 'Galvanic Cell' corrosion has five factors that determine the rate at which the elements will equalize their charge. These are;

The relationship of the various metals and the voltage difference is listed on the anodic scale, some elements of which are listed in table 1 below.
















Table 1: Galvanic Series (See also the note below)

Note: In a given environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electron gas, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a Galvanic series and can be a very useful design guideline when choosing materials for construction.


To identify the driving voltage of a 'Galvanic Circuit', the relative positions in the electromotive series can provide the simple answer; i.e.


By treating the resistance of the electrolyte as negligible, the current in the circuit can be calculated from the relative exposed surface areas of the dissimilar metals and provide a cell 'Rot Rate' or rate of corrosion. Typically a DC current of one amp flowing for one year will corrode [rot] away 9 Kg of steel, 10Kg of copper, 11.8 Kg of zinc, 22 Kg of lead or 3.5 Kg of aluminium. The eroded material is returned to the universally present electrolyte.


Secondary features of corrosion cell action is that elevated temperature will result in an increased rate of corrosion, as will the impurities in any water / electrolyte modify the cathodic / anodic relationship. For example in the presence of water borne sulphates [hard water] at elevated temperatures [500C+] copper becomes anodic with respect to steel, in many cases causing copper pipes in heat exchangers or water heating systems to corrode & leak due to internal corrosion("Metals Handbook" refers, see Bibliography below).



Bibliography;

1. AS 2832: 1998 Cathodic Protection of Metals

2. IEEE Standard 142 - 1991 - Connection to Earth

3. Cathodic Protection - L.G Rankin, "The Indonesian Pipeliner October 1999"

4. Metals Handbook - Volume 13, Corrosion. 9th Edition ASM International, Metals Park OHIO, (1987)

CATHODIC PROTECTION  (CONTINUE)