This article traces the history and development of the —0.850 V E (copper sulfate electrode) potential criterion over the last 60 years with respect to thermodynamic considerations and empirical testing in both the laboratory and field. The significance of reference electrode position and IR-drop voltages for the protective potential measurement and the influences of soil resistivity, sulfate-reducing bacteria, and temperature on the effectiveness of the minimum potential value are reviewed. In addition, the hazards of overprotecting a steel structure with respect to hydrogen embrittlement and coating disbondment are considered.

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