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GALVANIC CORROSION TABLE |
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| When two different metals are immersed in a corrosive solution, each will develop a corrosion potential. If the corrosion potential of the two metals is significantly different, and they are in direct contact and immersed in an electrolyte, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. The corrosion rate of the anode will be increased and the cathode decreased. The increased corrosion of the anode is called "galvanic corrosion". | |||||||||||||||||
Requirements for Galvanic Corrosion |
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| In order for galvanic corrosion to occur, three elements are required. | |||||||||||||||||
| 1) Dissimilar
metals 2) Metal-to-metal contact 3) Metals in the same conduction solution (usually called an electrolyte) |
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| If any of these elements is missing, galvanic corrosion cannot occur. If, for example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) there cannot be galvanic corrosion. | |||||||||||||||||
| Listed below is the latest galvanic table from MIL-STD-889 where the materials have been numbered for discussion of characteristics. However, for any combination of dissimilar metals, the metal with the lower number will act as an anode and will corrode preferentially. The table is the galvanic series of metals in sea water from Army Missile Command Report RS-TR-67-11, "Practical Galvanic Series." | |||||||||||||||||
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The greater the separation between the two metals listed, the greater the corrosion potential. For example, if zinc (think galvanized steel) which is an active material and near the top of the list and Silver a noble metal and near the bottom of the list were in direct contact and in the presence of an electrolyte (water), galvanic corrosion will probably occur. |
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In addition to the three elements sighted above, the relative area of each of the exposed metal surfaces is also a consideration. If the area of the cathode (noble metal) is very large, and the anode (active metal) is very small, the current produced is likely to be very high and the anode will corrode quickly. For example, if there is a window frame made of stainless steel and it is attached with carbon steel screws, the screws will probably corrode. If the area of the cathode (noble metal) is very small, and the anode (active metal) is very large, the current produced will be very low and the anode will corrode very slowly, if at all. If the window frame is made of carbon steel and it is attached with stainless steel screws there will be very little, if any, corrosion. |
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