Question

(a) Magnesium metal is used as a sacrificial anode to protect underground pipes from corrosion. Why is the magnesium referred to as a "sacrificial anode"? (b) Looking in Appendix \(\mathrm{E}\); suggest what metal the underground pipes could be made from in order for magnesium to be successful as a sacrificial anode.

Short answer

(a) Magnesium is referred to as a "sacrificial anode" because it is more reactive than the metal it is protecting, so it corrodes instead of the protected metal. It sacrifices itself (corrodes) to protect the less reactive metal, thus preventing corrosion of the underground pipes. (b) To select a suitable metal for the underground pipes, we need to choose a metal that is less reactive than magnesium. Some possible options could be metals like iron, copper, or zinc, which are typically considered less reactive than magnesium according to the reactivity series.

Step-by-step solution

(a) Understanding the function of magnesium as a sacrificial anode

A sacrificial anode is a piece of metal that is more reactive than the metal it is protecting, so it corrodes instead of the protected metal. In this case, magnesium is the sacrificial anode protecting underground pipes. It is called "sacrificial" because it sacrifices itself (corrodes) to protect the less reactive metal. This corroding process helps lessen the chances of the underground pipes corroding.

(b) Suggesting a suitable metal for underground pipes

To find a suitable metal for the underground pipes, we need to look at Appendix E (which is not provided here) and select a metal that is less reactive than magnesium. As we know that metals lower in the reactivity series are less reactive, we will want to choose a metal that is placed lower than magnesium in the reactivity series. Some possible options for the underground pipes could be metals like iron, copper, or zinc, which are typically considered less reactive than magnesium.

Key concepts

Corrosion Protection

Corrosion protection is a critical consideration for maintaining the longevity and functional integrity of metal structures. Specifically, when discussing underground pipes, which are constantly exposed to moist soil and other corrosive elements, it becomes crucial to implement strategies that prevent or slow down the process of corrosion. One such strategy is the use of sacrificial anodes.

A sacrificial anode is a piece of metal that is deliberately placed to corrode preferentially in place of the protected metal. In essence, it 'sacrifices' itself to protect the more valuable or functional part of the structure. For underground pipes, a metal such as magnesium is often used as the sacrificial anode because it has a higher reactivity compared to the material of the pipes. This higher reactivity leads to a potential difference between the anode and the pipe metal, causing the anode to corrode rather than the pipes. Over time, the anode will degrade, necessitating its replacement, but the pipes themselves remain relatively untouched by corrosion.

Understanding corrosion protection is not only vital for infrastructure and industrial applications but also plays an important role in various fields, including maritime, automotive, and even domestic environments.

Metal Reactivity Series

The metal reactivity series is a list that ranks metals by their reactivity. Reactivity, in this case, refers to how easily a metal can undergo chemical reactions resulting in its oxidation, which is a common form of corrosion. At the top of the series, metals like potassium and magnesium are highly reactive, meaning they are more likely to lose electrons and corrode quickly. Conversely, metals at the bottom, such as gold and platinum, are considered noble or less reactive, which means they resist corrosion and are often used in fine jewelry.

When selecting materials for structures like underground pipes, an understanding of this series is essential. Using the reactivity series, engineers can predict how different metals will interact with their environments and with each other. In the context of using a sacrificial anode for corrosion protection, one would choose a metal higher in the reactivity series than the metal of the structure it is intended to protect. Magnesium, being high up in the series, is often used as a sacrificial anode for metals like iron or steel that are lower in the series and thus form the body of the underground pipes.

The strategic placement of metals based on their position in the reactivity series is an application of electrochemistry that takes advantage of the natural tendency of metals to oxidize at different rates under similar conditions.

Underground Pipes Material

The material choice for underground pipes is a decision that balances durability, cost, and resistance to environmental factors such as corrosion. Common materials include plastics like PVC and metals like copper, steel, or ductile iron.

Metals are often preferred for their strength and durability but are prone to corrosion, especially when buried in soil that can be moist and contain varying degrees of corrosive substances. To counteract this, the concept of sacrificial anodes is used, where a more reactive metal, like magnesium, is connected to the underground pipe network. The robust nature of materials like copper or steel, when paired with the sacrificial anode system, ensures that the underground pipes can withstand the test of time and the harsh subsurface environment.

It's crucial that the underground pipes are made of materials placed lower than magnesium in the metal reactivity series to allow for effective protection through the sacrificial anode process. This layered approach to material selection, informed by chemical properties and reactivity, ensures a well-planned defense against corrosion for essential infrastructure like water and gas supply lines.