How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 1150

How does cathodic protection work? — MM, Dominican Republic
The rusting of damp steel is an electrochemical reaction in which iron atoms in the steel are converted into positively charged iron ions (Fe2+) in the water. However, each iron atom that becomes an ion releases two negatively charged electrons and rusting can only continue if there is a suitable destination for these electrons. Normally, the electrons pass through the steel metal and are used together with oxygen molecules to form negatively charged hydroxide ions (OH-) in the water. Overall, the rate at which the steel rusts is limited by how quickly hydroxide ions can be formed to use up the electrons.

Cathodic protection is a scheme in which a piece of reactive metal, typically magnesium, is connected to the steel to form an electrochemical cell. Magnesium ions (Mg2+) form more easily than iron ions and enough electrons are given up by the magnesium atoms as they become positive ions to completely dominate the hydroxide ion formation process. With nowhere for their electrons to go, the iron atoms can't become iron ions and rusting can't proceed. As long as the magnesium metal, often called the "sacrificial anode", remains intact and connected to the steel, the steel won't rust significantly.

As an alternative to this approach, some companies use a power supply to pump negative charges onto the steel to prevent it from rusting. Pipeline companies often do this and that action has led to some interesting complications: metal objects that are brought into contact with such a pipeline can be protected against rusting as well. For example, when people chained their bicycles to protected pipelines, the bicycles became part of the protected materials. This may have been good for the bicycles, but it confused the pipeline companies who found that they needed to pump extra charge onto the pipelines to handle the increased load. It was particularly bad when the bicycles accidentally grounded the pipelines and allowed the negative charges to escape.

         

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