Fig. 1 shows a steel water pipe buried near a coastal footpath. The pipe carries water and is joined by a wire to a block of magnesium. The magnesium block ...
Question 1 Report
Fig. 1 shows a steel water pipe buried near a coastal footpath. The pipe carries water and is joined by a wire to a block of magnesium. The magnesium block is replaced during maintenance. A small lead block was found nearby, but the engineer did not attach it to the pipe. Iron in the steel can corrode when water and oxygen reach its surface.
(a) Name the process that damages unprotected iron in the pipe. [1]
(b) State the two substances needed for rusting. [2]
(c) State what happens to the mass of the magnesium block during use. [1]
(d) Explain how the magnesium protects the steel pipe. [3]
(e) Give two reasons why steel is used for a water pipe rather than pure iron. [2]
(f) Explain why the lead block would not protect the pipe in the same way. [2]
Answer Details
(a) Damage to unprotected iron is rusting/corrosion. [1]
(b) Rusting requires water [1] and oxygen. [1]
(c) The magnesium block reacts during use, so its mass decreases. [1]
(d) Magnesium is more reactive than iron. It oxidises in preference to the iron, supplying sacrificial protection and preventing iron from losing electrons and corroding. [3]
(e) Steel is preferred to pure iron because it is stronger and tougher; it is also more durable/less easily bent. Any two. [2]
(f) Lead is less reactive than iron, so it will not oxidise in preference to the iron. It cannot act as a sacrificial metal to protect the pipe. [2]
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