Video Transcript
In this video, we will learn about
ways to protect metals from corroding. We will investigate how galvanizing
works, what sacrificial protection is, and how effective other surface treatment
methods such as painting a surface are. Let’s start by having a brief look
at what corrosion is. Many metals are not stable in their
pure form. They tend to react and bond with
other elements and compounds to become stable. For example, it is more
energetically favorable for many metals to react with oxygen and water in the
environment then just stay in their pure form. These types of reactions often
result in corrosion.
Corrosion can be defined as an
irreversible destructive process where a metal is converted to a more chemically
stable form by reacting with other elements. This reaction or corrosion process
ultimately damages the metal or material changing its properties, usually making it
weaker. Corrosion happens first at the
surface of a metal and slowly spreads to the inner parts over time as oxygen and
water begin to penetrate. An example of corrosion you may be
familiar with is the rusting of iron in steel. The metal loses its strength and
integrity as the iron in it oxidizes with water and oxygen. This can be problematic because the
rusting of iron or the corrosion of a metal affects its usefulness because its
properties are changed. The specific topic of rusting is
covered in more depth in another video.
Now, let’s have a look at ways to
prevent or slow down corrosion. Alloying, barrier protection, and
sacrificial protection are three ways to hinder corrosion. Alloying involves mixing a metal
with another metal or other elements to develop a new material with unique
properties. The added elements can be carefully
chosen to influence the corrosion resistance of the new material. Barrier protection includes
galvanizing, painting and plastic coating, oiling and greasing, and passivation. Galvanizing involves coating a
metal surface in a layer of zinc. We’ll look at this in more depth in
a moment.
Painting, plastic coating, or
coating a metal with a layer of oil or grease acts as a physical barrier to stop the
metal coming into contact with oxygen and water. The problem with coating a metal in
these ways is that these coatings can be scratched or washed off, and oxygen and
water can get access to the metal again. These methods are not very
effective against corrosion in the long term compared to other methods. Passivation is the process of
treating a metal chemically to decrease its surface reactivity. Metals are cleaned and then placed
in an acidic passivating bath and then rinsed in oxidizing substances. This treatment forms a layer on the
surface of the metal making it more resistant to corrosion. In sacrificial protection,
electrons from a more reactive metal are donated to a less reactive metal and
prevent it from being oxidized.
Now let’s turn our attention in
more depth to the process of galvanizing and sacrificial protection. Galvanizing is the process of
coating a ferrous metal or alloy with zinc. Ferrous refers to iron, so a
ferrous metal or alloy is any metal or alloy containing iron, for example, pure iron
or steel. The iron or steel part is dipped
into liquid, molten zinc. The part is removed from the zinc
bath, allowed to cool, and the zinc solidifies forming a zinc layer on the
surfaces.
The zinc coating protects the
ferrous metal alloy in two ways. Firstly, it acts as a physical
barrier to oxygen and water preventing them from reaching the iron surface. Secondly, even if some of the thin
zinc layer is scratched off and the underlying ferrous metal is exposed to the air,
the ferrous metal still won’t corrode or rust because the zinc layer gives
sacrificial protection to the underlying metal. The zinc will donate electrons to
the underlying metal preventing it from being oxidized. The galvanized zinc layer only
needs to be a very thin layer to carry out barrier protection and sacrificial
protection. Let’s investigate sacrificial
protection in more depth.
Sacrificial protection can be
defined as the use of a more reactive metal to protect a less reactive metal against
corrosion. In the case of galvanized steel,
the more reactive metal is zinc, and the less reactive metal is the iron or the iron
in steel. A reactivity series can help us
determine which metals are more reactive than others. Potassium at the top of the
reactivity series is the most reactive metal. This means it loses electrons and
becomes oxidized the most easily of this list of metals, and gold is the least
reactive on this list. The most energy is required for
gold to lose electrons and become oxidized. Under normal conditions, gold is
inert and is not oxidized.
Now, zinc is above iron on the
reactivity series, which tells us that zinc reacts more easily than iron. In other words, zinc loses
electrons or is oxidized more easily than iron. Little energy is needed for zinc to
lose electrons and be oxidized compared to the energy needed for iron to lose
electrons and be oxidized. If we enlarge this image and if
some of the zinc coating from galvanization was accidentally scratched off or
through wear and tear on the use of this metal part, it won’t be a problem.
The zinc will preferentially donate
electrons to the iron, which we call sacrificial electron donation and will itself
react with oxygen or water to produce zinc oxide, ZnO. The zinc has become oxidized in
place of the iron or instead of the iron. And in this way, the zinc protects
the iron. We say zinc electrons were
sacrificed for the sake of the iron. So the iron doesn’t rust, but
instead a hard coating of zinc oxide covers and protects the iron.
What is interesting and beneficial
about sacrificial protection is that a full coating of zinc is not always necessary
to protect the underlying steel. Very large steel structures such as
oil rigs and ship hulls would be extremely costly and inconvenient to try to
galvanize with zinc. So instead, small blocks or bars of
zinc are attached to the steel. The action of sacrificial
protection from the zinc still works, even though the zinc does not cover the entire
surface of the steel.
Over a long period of time, all the
zinc in the blocks is oxidized to zinc oxide. All that needs to be done is the
zinc oxide blocks are removed and replaced with new zinc blocks. And so the sacrificial protection
of the iron continues. Now, potentially any metal that is
more reactive than iron could work as a sacrificial protector. But zinc is both cost-effective and
easily available, and so it is the metal of choice. Let’s have a look at some suitable
methods for preventing rusting in different applications.
We’ve seen that alloying, barrier
protection of different kinds, and sacrificial protection are the main ways of
protecting a metal from corroding. Here are some specific real-life
examples of where these protection methods are used. Stainless steel is an alloy,
usually of iron, carbon, chromium, and nickel. The presence of chromium and nickel
help prevent the iron from rusting. Cutlery and surgical equipment are
purposefully made from alloys to prevent corrosion. Steel palisade fencing and steel
ladders are often galvanized to prolong their lifespan. We saw that oil rigs and ship hulls
are often protected from corrosion using blocks of zinc, and this is called
sacrificial protection. Underground steel pipes come into
contact with moisture in the ground. They are prevented from rusting by
sacrificial protection, either by attachment to zinc blocks or another metal that is
more reactive than iron.
The barrier method of protection is
most suitable for protecting steel bicycle chains from corroding. Painting won’t work because the
paint will flake off due to friction. And so oiling and greasing is the
preferred method of choice here. But painting can be used where
there is no friction, for example, on base steel fencing or over galvanized steel
fencing. Soil erosion on the banks of rivers
is sometimes prevented using steel gabion baskets filled with heavy rocks. Because there is much water and
oxygen present, the steel would rust very quickly. To slow down the corrosion process,
these river gabion baskets are often coated with plastic.
What about passivation? Many steel parts in various
applications that we’ve already discussed are also passivated. Treatment of electronic components,
in other words, passivation, protects their performance even though over time they
are exposed to oxygen and water from the air. Now it’s time for some
practice.
Which of the following metals could
be used as a sacrificial coating on iron to prevent rust formation? (A) Sn, (B) Pt, (C) Cu, (D) pb, or
(E) Al.
The answer options are tin,
platinum, copper, lead, and aluminum. The question talks about a
sacrificial coating on iron to prevent rust formation. When iron comes into contact with
oxygen and water, it is converted or oxidized into iron oxide, which is rust. Rust is red-brown and flaky with
little strength, so iron often needs to be coated to prevent it from rusting so that
it can maintain its strength. We can use a metal reactivity
series to choose which metal is suitable for a coating on iron.
The metals on this reactivity
series are listed in order of increasing reactivity going from the bottom to the
top. Potassium at the top is the most
reactive metal. It loses electrons and is oxidized
the most easily of all these metals. Platinum at the bottom of the list
is the least reactive. It loses electrons or is oxidized
the least easily. Platinum and gold are inert and
unreactive under normal conditions. Under normal conditions, they are
not oxidized. When a thin metal coating is placed
on iron, oxygen and water are prevented from coming into contact with the surface of
the iron and thus prevents it from rusting.
The metal coating should be made
from a metal above iron in the reactivity series. Potassium, sodium, lithium,
calcium, magnesium, aluminum, and zinc are all more reactive than iron. This means they will be oxidized
more easily than iron, and it means that they can donate electrons to the iron
preventing the iron from being oxidized itself. So even if some of the thin metal
coating is accidentally scratched off the surface and even if the metal iron comes
into contact with oxygen and water, it will not rust. Instead, the coating is
oxidized. We say the coating is
sacrificial. It sacrifices or gives its
electrons to the iron, and in this way, the iron is protected although the coating
itself is oxidized.
Of the possible answer options,
only aluminum is higher than iron in the reactivity series. Tin, platinum, copper, and lead are
all less reactive than iron and so cannot be sacrificial protectors of iron. So the metal which could be used as
a sacrificial coating on iron to prevent rust formation is Al, aluminum.
Let’s have a look at the key points
from this video. We learned that pure metals often
react with other elements to become more stable, and this can result in
corrosion. We defined corrosion as an
irreversible destructive process where a metal is converted to a more chemically
stable form by reacting with other elements. We looked specifically at corrosion
by the reaction of a metal with oxygen and water. We saw that corrosion prevention
can be achieved by alloying a metal with other elements, by painting or
plastic-coating the surface of a metal, by oiling or greasing a surface of a metal,
by passivation, which is the chemical treatment of the surface of a metal usually
with acids or oxidizing agents. And we focused our attention on
galvanizing, which is the coating of a metal with a thin layer of zinc, and
sacrificial protection, which is the protection of a metal using a more reactive
metal.