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The equilibrium potential of zinc is -763 mV vs. SHE (-1000 mv vs SCE) and therefore it is anodic to iron (equilibrium potential -440 mV vs. SHE). However, the polarity of the zinc-iron couple could reverse, with the iron becoming anodic to the zinc when the temperature is raised above 60oC in an aerated electrolyte. This limits the use of zinc anodes to ambient temperature applications. Zinc anodes are generally specified to strict chemical compositions because of harmful impurities which impair the performance of the anode. The most detrimental impurity is iron. If present in excess of 0.0014%, iron will precipitate out as discrete intermetallic particles which will set up local galvanic cells on the zinc anode.
This has the effect of reducing anode efficiency. Moreover, the corrosion products of this self corrosion, namely the electrically non-conductive zinc hydroxide, tend to induce passivity making the anode redundant. This problem is effectively overcome by the addition of aluminum and silicon which combine with the iron to form less harmful intermetallics. Cadmium is also added to promote uniform corrosion and the formation of non-adherent corrosion products. As a result anode efficiencies greater than 90% can be achieved. Zinc anodes, because of their driving voltage, are suitable for the protection of coated pipelines in high conductivity electrolytes, such as in seawater, and at ambient temperatures. Zinc anodes are also used in applications where spark hazards need to be avoided as in storage tanks containing flammable hazards.
