Video Transcript
Which of the following is not a
method for lowering the rate of rusting? (A) Alloying, (B) greasing, (C)
welding, (D) galvanizing, or (E) electroplating.
First, let’s define what we mean by
rusting and how it happens. Rusting is a reaction between water
or oxygen in metal parts that contain a high proportion of iron. Rusting is a form of corrosion. Corrosion is an irreversible,
destructive process where a metal reacts with other substances to form more stable
compounds. To protect a metal from rusting,
something must be done to slow down or stop the other substances from coming into
contact with the iron. Let’s look at our answer choices to
see if they are likely to lower the rate of rusting.
Answer choice (A), alloying, is the
process of mixing a metal with other elements to produce a new material with unique
properties. The other elements are often
metals. Some alloys of iron are more
resistant to rusting than pure iron. For example, stainless steel is an
alloy containing iron and chromium and other elements. The chromium helps the alloy form a
protective passivation layer that blocks oxygen and water from getting to the
iron. Passivation is the process of
chemically treating a metal part to form a protective barrier on its surface. This question asks us which method
would not be used to slow down rusting. Since alloying could be used to
slow the rusting of iron, we can eliminate answer choice (A).
Answer choice (B), greasing, is
when we add a layer of grease to a metal part. The grease provides a physical
barrier against oxygen and water, slowing the rate of rusting. So, we can eliminate answer choice
(B).
Answer choice (C), welding, is a
process for connecting metal parts. It usually involves melting metals
in a small area so that the metal parts can be connected to each other. Since the composition of a weld is
usually the same as the original metal parts, welding is unlikely to affect the rate
of rusting. Answer choice (C) is the most
likely answer, but let’s rule out answer choices (D) and (E) just to be sure.
Answer choice (D), galvanization,
is the process of coating an iron-containing metal part with zinc. This is usually done by dipping the
iron part into molten zinc. This coating stops rusting, even if
it is scratched. A scratched coating will act as a
sacrificial anode, corroding in place of the iron. Therefore, galvanization can
prevent rusting.
Electroplating can produce the same
effects as galvanization. Electrolysis is used when
electroplating to deposit one metal onto another. This, like galvanization, can
prevent rusting.
Therefore, the answer choice that
is not a method for lowering the rate of rusting is answer choice (C), welding.
