Machinery's Handbook, 31st Edition
552 Corrosion Another galvanic compatibility guide is an anodic index. Table 3 is an example based on US military specification MIL-F-14072, “Finishes for Ground Based Electronic Equipment.” This table shows electrochemical potentials normalized relative to a gold reference. Subtract one index value from another to determine the electrochemical po - tential difference between the metals. The standard recommends differential limits for three types of non-immersed application environments (controlled, normal, and harsh). Again, use caution when referring to any anodic index, because, as with a galvanic series, actual conditions can significantly change the corrosion behavior of a galvanic couple. Table 3. Anodic Index (Gold Reference)
Note that the “Electrochemical Series,” also known as the “Electromotive Force (EMF) Series,” which ranks metals with respect to inherent reactivity, from active to noble, de - pends on theoretical, static conditions. Therefore, galvanic corrosion of metals listed is likely to vary in real-world environments. Protective Area Ratios for Galvanic Couples: The wetted cathode/anode area ratio for parts in contact has a significant effect on the rate of galvanic corrosion: if the wetted cath - ode is large and the wetted anode is small (high ratio), corrosion is intensified; if the cath - ode is small and the anode is large (low ratio), corrosion slows. Protective area ratios can offset large potential differences between coupled metals. This effect makes it possible to use stainless steel screws to anchor large aluminum parts in conductive environments. By contrast, use of zinc or galvanized fasteners to secure large stainless steel parts will result in intense fastener corrosion in the presence of moisture. When choosing which part to coat in a galvanic couple, coat the more noble (cathodic) part; if only the more active metal (anodic) is coated, intense corrosion can occur at sites of small coating defects due to the resulting high area ratio.
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