Machinery's Handbook, 31st Edition
Corrosion 551 Selecting metals with similar electrochemical potentials usually minimizes galvanic cor - rosion. One method of comparing potentials involves referencing a galvanic series. While it should be representative of anticipated environmental conditions, this tool is not used to pre - dict corrosion rates, but rather provides a qualitative evaluation of coupled metal behavior. To develop a series, a reference half-cell and samples of the target metals are immersed together in an electrolyte solution chosen and circulated to match the expected environ - mental conditions. Over time, potentials of the target metals are measured relative to the reference half-cell. There are several standard reference half-cell compositions that will yield different values; the appropriate reference is compatible with the electrolyte. A useful standard is ASTM G82-98 (2014), “Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance.” Most published galvanic series data applies to specific flowing seawater environment conditions. While saltwater is highly conductive, freshwater has low conductivity, and dynamic electrolyte conditions will greatly affect potential measurements. Therefore, for critical applications, it is good practice to develop application-specific series data, rather than using published galvanic series information. For examples of seawater applications, refer to Table 2, which is based on Army Missile Command Report RS-TR-67-11, “Practical Galvanic Series.” Materials closer together along the arrow in the series have less corrosion-inducing potential difference between them in that environment. However, use this data with caution in predicting whether cor - rosion will be a risk. This series indicates which material will be the anode in a couple, though polarity reversals can occur in which a metal normally anodic to another will be - come cathodic to that same metal. Examples include high-temperature reversals of zinc/ iron, aluminum/iron, and aluminum/zinc. Table 2. Sample Galvanic Series, General Seawater Environment
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