EEI MASS Practice Test 12

Corrosion and Cathodic Protection

¹Buried piping, tank bottoms and the water boxes of condensers all face a slow but steady enemy: corrosion. Corrosion is not simply metal wearing away. It is an electrochemical process, which means that it works much like a small battery. Understanding that comparison explains both why metal corrodes and how plants protect it.

²Four things must be present for this kind of corrosion to occur. There must be an anode, the area where metal gives up electrons and dissolves; a cathode, the area that receives those electrons; an electrolyte, such as water or damp soil, that carries current between them; and a metal path that connects the anode and the cathode. Remove any one of the four and the process stops. The metal at the anode is the part that is lost, while the cathode is left largely unharmed.

³Anodes and cathodes can form on a single pipe because of small differences in the metal or its surroundings. They form even more readily when two different metals are joined in an electrolyte. In such a pair, the more active metal becomes the anode and corrodes faster than it would alone, while the less active metal is protected. This effect, called galvanic corrosion, is why a steel fitting attached directly to copper piping in wet conditions can fail early.

⁴Cathodic protection puts this same effect to work. Its goal is to make the entire structure being protected act as a cathode, so that the corrosion happens somewhere else. In one method, blocks of a more active metal such as zinc or magnesium are connected to the structure and buried or submerged nearby. These sacrificial anodes corrode in place of the structure and must be replaced when they are used up. In the second method, called impressed current, a rectifier supplies direct current that flows through long-lasting anodes and into the structure. This method can protect large structures, such as long pipelines, that sacrificial anodes alone could not.

⁵Cathodic protection is usually combined with a protective coating. The coating covers most of the surface, so the protection system needs to supply current only at small breaks and scratches. Workers check the system by measuring the electrical potential between the structure and the surrounding soil or water at test stations. A reading that drifts out of its normal range may mean that an anode is used up, a wire is broken or a rectifier has failed, and it should be reported.