Galvanic Corrosion - GALVANIC CORROSION by Stephen C Dexter...

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Galvanic corrosion, often misnamed “electrolysis,” is one common form of corrosion in marine environments. It occurs when two (or more) dissimilar metals are brought into electri- cal contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater. When contact with a dissimilar metal is made, how- ever, the self-corrosion rates will change: corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop. We can use the seawater Galvanic Series, shown in Table 1, to predict which metal will become the anode and how rapidly it will corrode. The seawater Galvanic Series is a list of metals and alloys ranked in order of their tendency to corrode in marine environ- ments. If any two metals from the list are coupled together, the one closer to the anodic (or active ) end of the series, the upper end in this case, will be the anode and thus will corrode faster, while the one toward the cathodic (or noble ) end will corrode slower. For example, suppose we have an aluminum alloy with a voltage range of -0.7 to -0.9 V (an average of -0.8 V) as shown on the series, coupled to a 300 series stainless steel with an average voltage of -0.07 V. The Galvanic Series pre- dicts that aluminum will be the anode, and the voltage differ- ence between the two alloys will be about 0.73 V (obtained by subtracting the two average voltages). It is this voltage dif- ference that drives the current flow to accelerate corrosion of the anodic metal. The two major factors affecting the severity of galvanic corrosion are (1) the voltage difference between the two metals on the Galvanic Series, and (2) the size of the ex- posed area of cathodic metal relative to that of the anodic metal. Corrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area. The approximate voltage difference for any two metals can be taken directly from Table 1. It is worth noting that marine slime films composed primarily of microscopic bacteria and diatoms can change the potentials of many of the alloys near the noble end of the Galvanic Series as indicated. The poten-
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