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.
Corrosion is an irreversible, destructive process where a metal reacts with other substances to form more stable compounds.
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.
Reaction: Metal with Oxygen
Reaction: Metal with 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.
Passivation is the process of chemically treating a metal part to form a protective barrier on its surface.
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.
Alloying is the process of mixing a metal with other elements to produce new material with unique properties; the other elements are often other metals.
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.
Example 1: Identifying the Main Reason for Using Oil over Plastic to Protect a Bicycle Chain
What is the main reason for preferring oil over plastic as a protective coating for a steel bicycle chain?
- Oil is less likely to react with the steel chain.
- Oil can spread to maintain a uniform coating during movement of the chain.
- Oil is less heavy than plastic so is less likely to impede the chain movement.
- The oil layer reduces oxidation by acting as a sacrificial coating.
- Oil is less expensive than a plastic coating.
A steel bicycle chain works because the links can move. It also works because it fits neatly over the teeth of the gears.
If a steel bicycle chain rusts, it will change shape, parts will wear away more quickly, and links may stick together so badly that the chain stops working entirely. Clearly, we want to stop that from happening.
We could use stainless steel, but that is expensive. It is easier to just coat the metal with something.
The options we have been given are oil and plastic. Oil, as a sticky liquid, will coat the chain very well, but it is going to wear off or evaporate eventually, so it needs to be maintained.
Paint, as a solid, cannot be easily wiped off, but a gear is constantly moving and changing shape when in use. Paint will quickly crack and fall off, and it is very hard for paint to stay in the very small gaps between parts.
The downsides of paint are not as important as the upsides of oil.
An oiled mechanical part will automatically spread oil over the areas that are in contact, keeping the most mechanically stressed parts protected from corrosion.
Therefore, the answer is that oil can spread to maintain a uniform coating during movement of the chain, answer B.
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.
Galvanization is the process of coating a ferrous metal part with zinc.
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.
Definition: Sacrificial Protection
Sacrificial protection is when a more reactive metal protects a less reactive metal from corrosion.
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?
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.
Example 3: Identifying Which Method Is Not Used to Lower Rusting Rates
Which of the following is not a method for lowering the rate of rusting?
Rusting occurs to metal parts that contain a high proportion of iron; rusting is a form of corrosion. To protect a metal part from corrosion, something must be done to slow down or stop oxygen and water from getting to the iron.
Welding is a technique and process for connecting metal parts. There are many types, but they all essentially involve melting metal in a small area so that it sticks one metal part to another. The composition of a weld is usually the same as the rest of the metal parts, so welding is unlikely to affect the rate of rusting of a metal part.
Greasing means adding a layer of grease to a metal part. This helps mechanical parts move over each other, but it also provides a physical barrier against oxygen and water, slowing the rate of rusting.
Alloying is simply the process of mixing a metal element with other elements (usually metals) at the atomic scale. The alloy produced will often have very different properties to the pure elements of which it is made. Some alloys of iron are significantly more resistant to rusting than pure iron. For example, stainless steels are alloys containing iron and chromium; the chromium helps the alloy to form a protective passivation layer that blocks oxygen and water from getting to the iron below the surface. Therefore, alloying is a method that could be used to reduce the rate of rusting of iron-containing parts.
Galvanization involves coating a metal part in zinc, usually by dipping the part in 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 metal part.
Electroplating can produce the same effects as galvanization. Metals that are less reactive than iron can stop iron rusting if the electroplated coat is complete. Metals that are more reactive than iron can stop iron rusting even if the electroplated coat is incomplete and areas of iron parts are left exposed.
Therefore, the only method that is not used to lower the rate of rusting of iron-containing parts is welding, answer A.
- Many metal elements are reactive in their pure form.
- Objects made from reactive metals may corrode over time.
- Corrosion is an irreversible, destructive process where metals react with other substances to form more stable compounds.
- There are a few common ways of preventing or slowing corrosion:
- Alloying is the process of mixing a metal with other elements to produce new material with unique properties (these other elements are generally also metals).
- The heterogeneity of different alloys affects the rate of corrosion depending on the constituent metals.
- Coating can involve galvanization, painting, oiling, or greasing:
- Galvanization is the process of coating a ferrous metal or alloy part with zinc.
- Sacrificial protection involves a more reactive metal protecting a less reactive metal against corrosion.
- Passivation involves chemically treating a metal part to form a protective barrier on its surface.
- Methods of preventing or slowing corrosion work in one or more of the following
- reducing the reactivity of the metal surface (alloying and passivation),
- blocking other substances from getting to the metal surface with a physical barrier (coating),
- using another substance that will oxidize in place of the metal (sacrificial protection).
- Galvanization provides both a physical barrier and sacrificial protection to the metal part if it is scratched.