NDA (II) 2021 GAT Q124: To protect steel and iron from rusting, a thin layer of

Q: What is the answer to NDA (II) 2021 GAT Q124?

B. Correct — B, Zinc. Coating iron or steel with a thin layer of zinc is called galvanisation, and it prevents rusting in two ways: the zinc layer keeps air and moisture off the iron, and even if the coating is scratched, the more reactive zinc corrodes in preference to the iron, giving sacrificial protection. Galvanised sheets, buckets and wires are everyday examples, and zinc is the metal routinely applied for this purpose.

NDA (II) 2021 General Ability Test · Question 124 of 149

Question text verified against the official NDA question paper.

To protect steel and iron from rusting, a thin layer of which one of the following metals is applied?

(a)Magnesium
(b)Zinc
(c)Aluminium
(d)Lead

Answer

Why

Correct — B, Zinc. Coating iron or steel with a thin layer of zinc is called galvanisation, and it prevents rusting in two ways: the zinc layer keeps air and moisture off the iron, and even if the coating is scratched, the more reactive zinc corrodes in preference to the iron, giving sacrificial protection. Galvanised sheets, buckets and wires are everyday examples, and zinc is the metal routinely applied for this purpose.

Why the others are wrong

(a)Magnesium — Magnesium serves as a sacrificial anode for large structures such as ships and pipelines, but the thin protective coating on iron sheets and wires is zinc, applied by galvanising.
(c)Aluminium — Aluminium resists corrosion itself by forming an oxide layer, but in ordinary galvanising the iron is protected by a zinc coating, not an aluminium one.
(d)Lead — Lead is toxic and is not used to coat iron against rust; it appears in solder and older piping, not in galvanising.

Concept

Rusting is the corrosion of iron in the presence of both air (oxygen) and moisture (water), forming hydrated iron oxide. Coating the iron with zinc, called galvanisation, is a common way to prevent it.

The standard, cheap protection combines a barrier with sacrificial action, and that is a zinc coat, the process named galvanising. Because zinc is more reactive than iron, it corrodes first and shields the iron even where the coating is broken.

Key facts

Coating iron or steel with zinc is called galvanisation.
Zinc acts as a barrier and, being more reactive, corrodes sacrificially to protect the iron.
Rusting requires both air (oxygen) and moisture (water).
Galvanised iron is widely used for roofing sheets, buckets and wires.

Close-up of a galvanised, zinc-coated steel sheet showing the characteristic crystalline spangle pattern. — The thin zinc coating protects the iron beneath from rusting, in the process called galvanising.Image: Splarka · Wikimedia Commons (Public domain)

Study next

Corrosion and rusting of ironGalvanisation and other rust-prevention methodsThe reactivity series of metalsSacrificial or cathodic protectionAlloys of iron such as stainless steel

Common traps

Choosing magnesium, which protects big structures as a sacrificial anode, when the coating asked for is zinc.
Assuming any metal coating works, whereas the answer is specifically zinc (galvanising).

Asked which metal coats iron to prevent rust; recall zinc and the term galvanisation.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
The process of coating iron with zinc to prevent rusting is called
- (a)electroplating
- (b)galvanisation
- (c)annealing
- (d)tempering
Answer(b) galvanisation — a zinc coating on iron or steel.
practice — not a real PYQ
Rusting of iron requires the presence of
- (a)oxygen only
- (b)water only
- (c)both oxygen and water
- (d)nitrogen
Answer(c) both oxygen and water.