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Chapter 20: Problem 61

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

Expert verified
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

01

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.
02

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.
03

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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鈥攌ind 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鈥攗sually 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.

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Most popular questions from this chapter

Calculate the quantity indicated for each of the following electrolyses. (a) \(\left[\mathrm{Cu}^{2+}\right]\) remaining in \(425 \mathrm{mL}\) of a solution that was originally \(0.366 \mathrm{M} \mathrm{CuSO}_{4},\) after passage of \(2.68 \mathrm{A}\) for 282 s and the deposition of Cu at the cathode (b) the time required to reduce \(\left[\mathrm{Ag}^{+}\right]\) in \(255 \mathrm{mL}\) of \(\mathrm{AgNO}_{3}(\mathrm{aq})\) from 0.196 to \(0.175 \mathrm{M}\) by electrolyzing the solution between \(\mathrm{Pt}\) electrodes with a current of \(1.84 \mathrm{A}\)

Only a tiny fraction of the diffusible ions move across a cell membrane in establishing a Nernst potential (see Focus On 20: Membrane Potentials), so there is no detectable concentration change. Consider a typical cell with a volume of \(10^{-8} \mathrm{cm}^{3},\) a surface area \((A)\) of \(10^{-6} \mathrm{cm}^{2},\) and a membrane thickness \((l)\) of \(10^{-6} \mathrm{cm}\) Suppose that \(\left[\mathrm{K}^{+}\right]=155 \mathrm{mM}\) inside the cell and \(\left[\mathrm{K}^{+}\right]=4 \mathrm{mM}\) outside the cell and that the observed Nernst potential across the cell wall is \(0.085 \mathrm{V}\). The membrane acts as a charge-storing device called a capacitor, with a capacitance, \(C,\) given by $$C=\frac{\varepsilon_{0} \varepsilon A}{l}$$ where \(\varepsilon_{0}\) is the dielectric constant of a vacuum and the product \(\varepsilon_{0} \varepsilon\) is the dielectric constant of the membrane, having a typical value of \(3 \times 8.854 \times 10^{-12}\) \(\mathrm{C}^{2} \mathrm{N}^{-1} \mathrm{m}^{-2}\) for a biological membrane. The SI unit of capacitance is the firad, \(1 \mathrm{F}=1\) coulomb per volt \(=1 \mathrm{CV}^{-1}=1 \times \mathrm{C}^{2} \mathrm{N}^{-1} \mathrm{m}^{-1}\) (a) Determine the capacitance of the membrane for the typical cell described. (b) What is the net charge required to maintain the observed membrane potential? (c) How many \(\mathrm{K}^{+}\) ions must flow through the cell membrane to produce the membrane potential? (d) How many \(\mathrm{K}^{+}\) ions are in the typical cell? (e) Show that the fraction of the intracellular \(K^{+}\) ions transferred through the cell membrane to produce the membrane potential is so small that it does not change \(\left[\mathrm{K}^{+}\right]\) within the cell.

The gas evolved at the anode when \(\mathrm{K}_{2} \mathrm{SO}_{4}(\mathrm{aq})\) is electrolyzed between Pt electrodes is most likely to be (a) \(\mathrm{O}_{2} ;\) (b) \(\mathrm{H}_{2} ;\) (c) \(\mathrm{SO}_{2} ;\) (d) \(\mathrm{SO}_{3} ;\) (e) a mixture of sulfur oxides.

A test for completeness of electrodeposition of \(\mathrm{Cu}\) from a solution of \(\mathrm{Cu}^{2+}(\mathrm{aq})\) is to add \(\mathrm{NH}_{3}(\mathrm{aq}) .\) A blue color signifies the formation of the complex ion \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\left(K_{\mathrm{f}}=1.1 \times 10^{13}\right) .\) Let \(250.0 \mathrm{mL}\) of \(0.1000 \mathrm{M} \mathrm{CuSO}_{4}(\text { aq })\) be electrolyzed with a \(3.512 \mathrm{A}\) current for 1368 s. At this time, add a sufficient quantity of \(\mathrm{NH}_{3}(\text { aq })\) to complex any remaining \(\mathrm{Cu}^{2+}\) and to maintain a free \(\left[\mathrm{NH}_{3}\right]=0.10 \mathrm{M} .\) If \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) is detectable at concentrations as low as \(1 \times 10^{-5} \mathrm{M}\) should the blue color appear?

In the construction of the Statue of Liberty, a framework of iron ribs was covered with thin sheets of copper less than \(2.5 \mathrm{mm}\) thick. A layer of asbestos separated the copper skin and iron framework. Over time, the asbestos wore away and the iron ribs corroded. Some of the ribs lost more than half their mass in the 100 years before the statue was restored. At the same time, the copper skin lost only about \(4 \%\) of its thickness. Use electrochemical principles to explain these observations.
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