Cathodic protection readings must be at least how negative?

• A. -0.75
• B. -0.95
• C. -1.0
• D. -0.85

Answer: (D)

In cathodic protection, you assess effectiveness by how negative the metal’s potential is relative to a reference electrode. For steel in soil or water, a common protection criterion is that the potential must be at least -0.85 volts with respect to a Cu/CuSO4 reference electrode. Reaching -0.85 V or more negative means the protective current is sufficient to suppress the corrosion reactions. If the reading is less negative than that, protection is inadequate. Readings more negative than -0.85, such as -0.95 or -1.0, still meet the criterion, but -0.85 V is the minimum threshold to guarantee protection under typical conditions.