2) Galvanic or Two-Metal corrosion

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2 2) Galvanic or Two-Metal corrosion Galvanic corrosion occurs when two metals or alloys are electrically coupled while exposed to an electrolyte. The more reactive metal in the particular environment will experience corrosion, the more inert metal, will be protected from corrosion. For example, steel screws corrode when in contact with brass in a marine environment; or if copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. 2

3 Mechanism of Galvanic corrosion For the formation of a galvanic cell, the following components are required : (1) A cathode. (2) An anode. (3) An electrolyte. (4) A metallic path for the electron current. In case of copper and steel pipe joint, Iron is more negative potential in the emf series ( V ).It is the anode,moisture acts an electrolyte and metal surface provides a metallic path for electron current to travel. A piece of copper is the cathode.galvanic cell will formed and galvanic corrosion proceeds. 3

4 Factors affecting galvanic corrosion The following factors significantly affect the magnitude of galvanic corrosion:- ( A ) Position of metals in the Galvanic series : For galvanic corrosion. The magnitude of galvanic corrosion primarily depends on how much potential difference exists between two metals. For a particular environment, the metals selected should be close to each other in galvanic series to minimize galvanic corrosion. ( Al should not be joined to Fe ). 4

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6 6 ( B ) The Nature of Environment : The environment that surrounds the metal must be consider. Water containing copper ions like seawater is likely to form galvanic cell on a steel surface of the tank. The marine environments, galvanic corrosion may be accelerated due to increased conductivity of the electrolyte. Galvanic corrosion of buried material is reduced because of the increased resistivity of soil. Tantalum is very corrosion resistant metal. It is anodic to platinum and carbon, but the cell is active only at high temperatures. Zn is anode with respect to iron at room temp. with presence of water, but if the water temperature became higher ( C o ). The polarity will reverse and iron will be anode ( corroded ). ( C ) Area, Distance and Geometric Effects : Effect of Area : The anodic to Cathodic area is extremely important as the magnitude of galvanic corrosion is seriously affected by it.an unfavorable area ratio consists of a large cathode and a small anode.corrosion of anodic area may be 100 or 100 times greater than if the anodic and Cathodic areas were equal in size. Effect of Distance : It is a known principle that the solution conductivity varies inversely with the length of the conduction path. The greatest galvanic damage is likely to encountered near the junction of two metals and severity would be decreased with increased length.

7 Effect of Geometry : Geometry of components and their design also influence galvanic corrosion. As current does not flow around the corners, the geometry of the circuit affects the degree of galvanic corrosion.any obstacle to polarization would accelerate galvanic corrosion. The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte. For a given cathode area, a smaller anode will corrode more rapidly than a larger one. 7

8 8 Measures taken to reduce galvanic corrosion are: 1 - If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series. 2 - Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area as large as possible. 3 - Electrically insulate dissimilar metals from each other. 4 - Electrically connect a third, anodic metal to the other two; this is a form of Cathodic protection.