Question

Natural gas transmission pipes are sometimes protected against corrosion by the maintenance of a small potential difference between the pipe and an inert electrode buried in the ground. Describe how the method works.

Short answer

To protect natural gas transmission pipes against corrosion, a small potential difference is maintained between the pipe and an inert electrode buried in the ground. This makes the pipe act as the cathode of an electrochemical cell, preventing the pipe from losing electrons and thus preventing corrosion.

Step-by-step solution

Understanding the Concept of Potential Difference

Potential difference, in physics, is the difference in electric potential between two points, which is defined as the work done by an external agent in carrying a unit positive charge from one point to the other. In this context, the potential difference is maintained between the pipe and an inert electrode.

Role of the Inert Electrode

An inert electrode is one that does not take part in the chemical reaction, but only allows the transfer of charge. In this case, the inert electrode is buried in the ground and is used to maintain the small potential difference with the pipe.

Corrosion Prevention Using Potential Difference

By maintaining a potential difference, the pipe is made to act as the cathode of an electrochemical cell. The corrosion that naturally occurs is an anodic reaction, which is where a metal loses electrons and goes into solution. By making the pipe the cathode, we reverse the natural reaction and prevent the pipe from losing electrons and corroding.

Key concepts

Potential Difference

When we talk about potential difference, imagine it as the driving force behind the movement of electrons in a circuit, similar to how water pressure drives water through pipes. In the context of preventing corrosion in pipelines, a small potential difference refers to a slight charge imbalance that's intentionally established between the metal pipe and another electrode. This imbalance encourages electrons to move in a way that counteracts the corrosive process.

Corrosive reactions require a certain electrical potential to occur. By maintaining a potential difference that doesn't support these reactions, we prevent the pipe metal from corroding. Think of it like setting the voltage in a battery to prevent unwanted chemical reactions that could reduce the battery's life.

Inert Electrode

In our discussion, an inert electrode is a component that serves as a conductor without participating in the actual chemical reactions. That means it doesn't react with the surrounding environment or the material it is meant to protect—kind of like a referee in a sports game who facilitates the play without getting involved in the game itself.

Electrodes made from platinum or graphite are often used as inert because they are sturdy and resilient, resisting deterioration over time. In our pipeline scenario, the inert electrode works in tandem with the pipe and the surrounding soil to create an electrochemical cell. This cell enables the flow of electrical current necessary to maintain the potential difference that helps protect the pipe.

Electrochemical Cell

Imagine an electrochemical cell as a small chemical power station. It's a device that generates electricity through chemical reactions between different substances—usually metals and electrolytes. In the case of pipeline corrosion prevention, the combination of the metal pipe, the soil (acting as the electrolyte), and the inert electrode, forms an electrochemical cell.

Within this cell, the pipe is deliberately made into the cathode, the electrode that gains electrons. By creating conditions unfavorable for corrosion, the chemical reactions that would typically result in the metal pipe corroding are effectively disrupted. Through this managed electrochemical process, we can greatly extend the lifespan of pipelines.

Cathodic Protection

Cathodic protection is a clever method of corrosion prevention that turns the entire pipe into the cathode, or the electron-gaining side, of an electrochemical cell. It utilizes the principle that corrosion occurs at the anode, or the electron-losing side. By making the pipe the cathode, the pipeline doesn't lose the precious electrons required for corrosion to occur.

There are two main types of cathodic protection: galvanic protection, which uses a more reactive metal as the sacrificial anode, and impressed current protection, which uses an external power source to maintain the necessary potential difference. In both cases, the goal is the same: protect the pipeline from corrosion by controlling the electrical environment around it.