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Question Video: Identifying the Prevention of Rusting by a Current from a Sacrificial Metal Chemistry • 10th Grade

A steel oil pipeline is protected via an electrical connection to a buried block of sacrificial metal. Which diagram correctly illustrates the composition of the sacrificial metal and the direction of the electrical current?

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Video Transcript

A steel oil pipeline is protected via an electrical connection to a buried block of sacrificial metal. Which diagram correctly illustrates the composition of the sacrificial metal and the direction of the electrical current?

We are told that an oil pipeline is made from steel. Steel is a ferrous metal, in other words an iron-containing alloy. Steel is strong. But in the presence of oxygen and water or water vapor from the air, steel turns to red rust. Rust is weak and flaky. So without protection, a steel pipe would lose its strength and integrity over time. We are asked to identify which diagram correctly illustrates the composition of the sacrificial metal, either zinc, copper, tin, again copper, or lead, and to identify the direction of the electrical current.

Let’s clear some space to answer this question. We’ve already seen that a steel pipe as it is exposed to oxygen and water or water vapor would begin to rust. Continual exposure will lead to a loss of integrity and the oil that has been carried by the pipeline may indeed leak out. Rusting is a specific type of corrosion. Corrosion is an irreversible, destructive process where a metal reacts with other substances to form more stable compounds. This degradation of steel pipes can be slowed down and prevented by using a sacrificial metal. We need to decide which metal is the sacrificial metal in this example, zinc, copper, tin, or lead.

Sacrificial protection is when a more reactive metal protects a less reactive metal from corrosion. A portion of the reactivity series of metals is shown with the more reactive metals at the top. We know iron is in steel. And any metal above it in the series is more reactive than iron or will be oxidized more easily. And any metal below iron in the series is less reactive than iron and will be oxidized less easily than iron; in other words, iron is oxidized more readily.

If we looked at our definition, we can see that sacrificial protection uses a more reactive metal to protect a less reactive metal from corrosion. So if we want to protect iron from corrosion, we need to use a metal that is more reactive than iron as the sacrificial metal. From our possible sacrificial metals zinc, copper, tin, and lead, we can see that zinc is higher up on the reactivity series than iron or more reactive than iron, while tin, lead, and copper are less reactive than iron. So, tin, lead, and copper are not suitable sacrificial metals to protect iron from corrosion. So, let’s erase lead, tin, and copper as possible answers.

We now know that the sacrificial metal used to protect a steel pipe would be zinc, but what is the direction of the electrical current flow? Don’t be confused; it must be a closed circuit for current to flow. But which is the source of electrons, the zinc or the steel? We know that zinc is more reactive than iron. This means that zinc is oxidized more easily. So, the loss of electrons from zinc instead of from iron will be preferred; this is more energetically favorable. So, we know that zinc will donate electrons into the wire, and these will flow to the iron in the steel.

Iron in the steel is prevented from being oxidized and is kept in the reduced state; it does not corrode. We say that zinc sacrifices its valence electrons to iron and in this way protects the iron in the steel from rusting. Now that we know the direction of the electrical current, we can identify the correct diagram. Which diagram correctly illustrates the composition of the sacrificial metal and the direction of the electrical current? The answer is (A).

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