Lesson Explainer: Corrosion Chemistry • Third Year of Secondary School

In this explainer, we will learn how to describe methods for the prevention of rusting of iron and identify metals less susceptible to oxide formation.

Corrosion is a natural process that has caused costly damage to many things that people have made.

Corrosion can be benign and even beautiful, like the green color of the Statue of Liberty (which was originally copper colored).

Corrosion can cause minor damage, like the rust on a knife.

Or, corrosion can be fatal. In 1967, the Silver Bridge over the Ohio River (Ohio, USA) collapsed. A mixture of stress and corrosion weakened part of the bridge enough that it snapped under the load of the traffic. 46 people died.

Incredibly, around of iron production each year is necessary due to the amounts of iron lost through corrosion.

Before we examine the impacts of corrosion in more detail, we need to understand what it is.

Corrosion is a process that many metals go through over time, where they transform from a metal to a metal compound. We tend to hear the word corrosion when the process is slow, occurs naturally, and occurs to a solid object made of metal. A metal part will get weaker as it corrodes; the corroded areas will not stick together as strongly as the metal, and they can fall away completely. For this reason, corrosion is usually seen as a bad thing.

Most pure metals are reactive enough to form more stable compounds with common substances. The common substances that are most often involved in corrosion or rusting are oxygen and water.

Corrosion starts at the surface of a metal part and can get deeper with time.

Iron is the most popular metal in machinery and commercial products. In the presence of oxygen and water, a solid piece of pure iron will rust quite quickly. Steels are alloys of iron with other elements, most commonly carbon. Iron in steel will generally rust more slowly, and special types of steel are called “stainless” if they rust extremely slowly or not at all. Common stainless steels are alloys of iron and chromium, often with some carbon and nickel.

When some metals and metal alloys react with oxygen and water, the compounds that form break apart and peel away from the surface, exposing more metal. Layers continue to react and peel away until the whole piece of metal has corroded away.

However, some metals and alloys form a passivation layer; this is a layer of metal compound that does not peel away but sticks well to the surface of the metal and does not crack. This passivation layer stops or slows oxygen and water getting to the rest of the metal. As long as the passivation layer is not scratched, cracked, or broken off, the metal beneath will corrode much more slowly than without it.

Passivation occurs naturally with metals like aluminum and chromium as they are both reactive metals that bond readily with oxygen. However, we can also use special chemical treatments on other metals that create different types of passivation layers.

Sensitive electrical components are sometimes chemically passivated; this slows corrosion without adding to the component’s thickness.

As we have seen with stainless steel, the process of alloying is sometimes sufficient to turn a reactive, corroding metal into something that stays strong and shiny. Therefore, alloying is another way of slowing or stopping corrosion.

As alloys can be a mixture of metals, at an atomic level, differences may occur in terms of the rate and order of corrosion. This heterogeneity of alloys means that in any given alloy, for example, an alloy of aluminum and iron, the aluminum would corrode before the iron rusts. This difference is due to aluminum’s greater reactivity and inability to form a protective oxide layer once alloyed.

There is a simple alternative to using chemicals to make a physical barrier: we can coat metal parts in various ways. Some of these methods produce barriers that are purely physical, while others also provide electrochemical protection.

Perhaps the simplest way of forming a barrier against corrosion is to coat a metal part in oil or grease. This forms a temporary barrier that is cheap to reapply. Coating moving mechanisms with oil often produces better results than painting them; for example, oil is the best option for protecting bicycle gears. Paint on a moving part like a gear will break up and fall off quickly. However, oil and grease will stick to the surface for much longer.

For nonmechanical parts, paint is often a great choice. A coat of paint is a more long-term physical barrier against corrosion, and it can also improve how things look; car bodies are painted to take advantage of both.

The downside of these barriers is that if they get scratched, the metal underneath will corrode.

While paint, oil, and grease (or varnish or primer) may be effective under many circumstances, there are some scenarios where a more durable coating is needed.

Galvanization is the process of coating a metal part with zinc. The part will be made of iron or an iron alloy, and it will usually be dipped into molten zinc to form the protective layer.

Galvanic protection serves two purposes: the layer of zinc will be a physical barrier against corrosion and if the zinc coating is incomplete, the zinc will form a galvanic cell with the metal part. Zinc is more reactive than iron, so rather than the iron reacting, the zinc reacts instead.

Normally, the reaction of iron with oxygen and water would result in the oxidation of iron:

For a part with a zinc coating, it is more stable overall if the electrons come from the zinc rather than the iron:

Rather than the iron rusting, the zinc coating corrodes instead. As electrons ultimately come from the zinc, this part of the process is known as sacrificial electron donation; as electrons are leaving the zinc and entering the metal part, the zinc is the anode and the metal part is the cathode. This is why the zinc coating is also known as the sacrificial anode.

Galvanization will often be used in scenarios where metal parts containing iron are exposed to the weather, for example, steel fencing.

This is a form of sacrificial protection, where one metal is sacrificed in order to stop another metal from corroding.

What is really useful is that we do not need a whole coating of zinc for it to work. If a block of zinc is in electrical contact with the metal part, the whole of the metal part will be protected. The zinc block will gradually corrode and will eventually need to be replaced. Magnesium can be used for the same purpose and is used to protect iron pipes in wet soil and ship hulls in salty seawater.

We can use the reactivity series to remind us of which metals will work when protecting iron from corrosion.

Zinc, aluminum, and magnesium are more reactive than iron (they will be oxidized in place of iron). Calcium, lithium, sodium, and potassium are more reactive than iron, but they are also so reactive that they will react vigorously with water—not a good attribute for this application.

Copper, silver, and gold are less reactive than iron, so they will not be suitable for sacrificial protection of ferrous metal parts.

Zinc stands out from the other options: it is economical, easy to apply, abundant, and it corrodes at a rate that provides good protection.

Paint is often not enough to protect metal parts in seawater, and the salt in seawater increases the rate of corrosion. Zinc blocks are often attached to the hulls of ships and the frames of oil-drilling platforms. The zinc blocks are routinely replaced, ensuring that corrosion of the structure is minimal. Underground steel pipes are also in an ideal environment for corrosion, and an electrical cable in contact with a zinc block on the surface is sufficient to protect them.

Example 2: Identifying Which Metal Cannot Be Used as a Sacrificial Metal for Tin

Which of the following can not be used as a sacrificial metal to prevent corrosion of tin?

1. Lead
2. Magnesium
3. Iron
4. Zinc
5. Aluminum

Answer

A sacrificial metal is a metal that is a more reactive metal that corrodes in place of a less reactive one. The sacrificial metal provides sacrificial protection to the less reactive metal by oxidizing in its place.

Tin corrodes when it reacts with oxygen in the air:

A sacrificial metal only works if it is more reactive than the metal it is protecting. For our answer, we can look at the following reactivity series.

Magnesium, aluminum, zinc, and iron are more reactive than tin, so they could theoretically be used as sacrificial metals for tin. Lead is the only one that is less reactive than tin. Lead would not work as a sacrificial metal for tin because it has a lower tendency to oxidize than tin does.

Therefore, of the metals given, the only one that cannot be used as a sacrificial metal for tin is lead, answer A.