Sign Confusion

It is to be remembered that anodes are negative in naturally occurring electrochemical processes and they are positive in forced electrolyte processes when an external current is introduced. It is to be noted that oxidation always occurs at the anode regardless of whether the process is natural or forced.

Corrosion is a natural process in which the electrons on a corroding surface flow from the anode to cathode and a similar exchange takes place between cathode and anode in the electrolyte that completes the circuit. Figure below shows the corrosion process in a buried pipe. In a process forced by an external current, the signs are reversed; anode is positive and cathode is negative.

Thus when a cathodic reaction is impressed upon a metallic surface to eliminate the naturally occurring anodes (which cause corrosion) the metal surface becomes completely cathodic with a negative sign and corrosion stops due to elimination of anodic areas. However, despite the sign change, reduction still occurs on the cathode and oxidation on the anode. The best way to avoid confusion is to remember anode as the site of oxidation and cathode as the site for reduction.