Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased you can join me today for this lesson on corrosion and its prevention.

It's part of the Industrial Chemistry Unit.

I'm here to work with you through this lesson and support you all the way, especially through the tricky bits.

The learning outcome for today's lesson is, I can describe corrosion in detail, including chemical equations, and describe what actions can be taken to prevent corrosion.

And we have some keywords in today's lesson, including rusting, corrosion, sacrificial protection, electroplating, and tarnishing.

Now we're going to look at these keywords in some sentences.

Rusting refers to the corrosion of iron or steel in the presence of water and oxygen.

Corrosion is the gradual deterioration of a substance when it reacts with substances in the environment.

For example, when a metal oxidises in air.

Sacrificial protection refers to the process of using a more reactive metal to corrode preferentially, to protect another metal from corrosion.

Tarnishing is the process by which a thin discoloured layer forms on the surface of a metal due to reactions with the environment.

For example, oxidation.

Electroplating is a process that uses electricity to deposit a thin layer of metal onto the surface of another conductive object.

You may wish to pause the video now and note down these keywords and their definitions so that you can refer to them later on during the lesson.

In today's lesson, there are two learning cycles.

The first learning cycle is on understanding corrosion and the second on preventing corrosion.

So let's get started with our first learning cycle on understanding corrosion.

Corrosion is a gradual deterioration of a substance when it reacts with substances in the environment.

For example, when a metal oxidises in air.

And you can see in this image a corroded metal hinge, which was probably silver looking when it was new and now looks a rust colour.

Corrosion weakens the structure leading to failures and economic losses.

Now we might have to be careful when we think about corrosion, not to confuse it with corrosive chemicals, and the hazard symbol which hopefully you can recognise from previous learning and also some everyday household chemicals.

The hazard symbol is warning about corrosive substances.

These are substances that can cause damage to living tissues and materials through direct contact.

Corrosive chemicals pose an immediate threat upon contact, whereas corrosion refers to the gradual degradation of materials like metals.

They are not the same thing and should not be confused.

Whilst corrosive chemicals can accelerate this process, they do not refer to the same thing.

So that's something that we need to watch out for.

The difference between corrosion and corrosive chemicals.

Quick check for understanding, true or false? Corrosion and the action of corrosive chemicals both refer to the same process.

Well done if you chose false.

Now, let's have a look at the reason why is it A, because corrosion is a process that occurs only in the presence of oxygen.

Or B corrosion refers to the gradual degradation of metals, while corrosive chemicals can cause immediate damage upon contact.

Well done if you chose B.

That is the correct answer, good work.

So there's different types of corrosion.

We're gonna look at them now.

Galvanic corrosion occurs when two different metals are in contact with a corrosive environment.

And we have an example here that you can see in the image.

And what actually happens is one metal, which is more reactive, becomes an anode and that corrodes.

And the other metal, which is less reactive, becomes the cathode and that is protected.

So the example that we can actually see is steel and copper.

If they're in contact in a salty environment, for example, on a ship, the steel will corrode faster.

And this is a process that is actually used to help protect some metals.

Pitting corrosion.

Well, this is a localised corrosion that leads to the creation of small holes or pits in the metal.

And if again, if you have a look at the image here, the photograph, you can see clearly those small holes or pits in the metal.

So what actually happens here? Well first of all, it often occurs in metals that have protective oxide layers, chloride ions from salt can break down the protective layer and cause the pits.

Stainless steel can develop pits when it's exposed to salt water.

So again, this is a potential issue for ships that are made from stainless steel and obviously are in the sea, which contains salt water.

You know from previous learning that metals react with oxygen to form oxides and this is called oxidation.

When metals react with oxygen, they often form metal oxides.

And you can see in the photograph here, a piece of lithium metal.

Now if we were to cut that lithium inside, it would be very shiny, but it is dull on the outside.

Lithium is in group one of the periodic tables and group one metals oxidise very quickly.

And we say the surface has tarnished.

Most metals oxidise slowly.

So that dull colour on the outside is a coat of lithium oxide but we say it has tarnished.

Tarnishing is a surface phenomena where a metal develops a film due to a chemical reaction with substances in the air.

Often metals like silver, copper, and brass form a thin layer of oxide or sulphide on the surface.

And we say that is tarnished, it's just dulled.

It doesn't look as attractive, but it doesn't cause structural damage.

So tarnishing often results in decolorization.

So if you've got that silver picture frame that was once very shiny, after maybe several years of just standing on the shelf, it becomes a very dull, unattractive colour that is the result of tarnishing.

So another quick check for understanding.

True or false? Tarnishing leads to structural damage of metals.

True or false? Well done if you picked false.

Now let's have a look at the reason why? Is at A, tarnishing doesn't cause damage but weakens the internal structure of the metal.

Or B, tarnishing can easily be removed without affecting the metal underneath.

Well done if you chose B.

We can easily remove the tarnishing.

And if we go back to that example of the silver picture frame, if we get some silver polish, we can just rub it and then clean it over with a cloth and the tarnish will be removed, and the frame will look a shiny, silver colour again.

Some metals form thin, stable oxide layers preventing further corrosion.

For example, some metals can make the metal appear very dull.

And aluminium is a good example of this.

When aluminium reacts with oxygen, it forms aluminium oxide, Al2O3.

And that layer protects it.

So it's a very thin layer over the aluminium metal.

It adheres or sticks strongly to the metal and it prevents further oxidation and corrosion.

It's effectively transparent compared to some metal oxide layers.

So you can see the aluminium foil in the image.

It is covered or protected in the layer of aluminium oxide.

So it doesn't appear to be quite as reactive as the position in the reactivity series of metals would suggest.

Metals that form protective layers include aluminium, that goes to aluminium oxide and it's one of the reasons why it's used for food containers.

Chromium, which goes to chromium oxide, which is essentially used in stainless steel.

And titanium, which goes to titanium oxide, is used in the aerospace industry.

And here is an image of some stainless steel which is used for cutlery, a knife and a fork.

Iron goes to iron oxide, which is rust.

And this is slightly different because it's porous and flakes off leading to more corrosion.

So another quick check for understanding.

Which metals are known to oxidise quickly causing surface tarnish? Is it A, iron and steel.

B, group one metals like sodium and potassium.

C, stainless steel.

Or D, aluminium and titanium.

Well done if you chose B, group one metals like sodium and potassium cause surface tarnishing.

Rusting is a specific type of corrosion that only affects iron and steel.

And you can see from this rusty chain something that probably looks quite familiar to you, that brown, flaky, rusting look that I'm sure you've all seen when things have been left outside in the winter.

So for rusting to occur, we need special conditions.

We must have iron, we must have oxygen or air, 'cause of course air is 20% oxygen and we must have water.

Without these three things present together, rusting will not occur.

And the overall reaction, the chemical reaction that takes place is iron + oxygen and water, and that forms something what we call hydrated iron three oxide.

So that brown colour we can see on the chain is hydrated iron three oxide, otherwise known as rust.

So true or false? Another quick check for understanding.

Corrosion only occurs when metals are exposed to water.

True or false? That is false, so well done if you chose that.

Now, let's have a look at the reason why? Is it A, corrosion can occur due to exposure to various environmental factors including oxygen and acidic conditions.

Or B, corrosion occurs because metals react with moisture to form rust.

Well done if you picked A, that is the correct answer.

Corrosion can occur due to exposure to various environmental factors including oxygen and acidic conditions, so that is as well as the water.

All right, I have another question for you here, which the following conditions are necessary for rusting to occur? Oxygen and water.

B, any metal, oxygen and water.

C, any metal and salt water solution.

D, iron, oxygen, and water.

So this is rusting.

Well done if you chose D.

Rusting is that special case and you must be iron, oxygen and water.

So the rusting process speeds up when ion is in the presence of the electrolytes, for example, salty water.

And here we have got a diagram showing the electrolyte and our iron.

So an electrolyte is an aqueous solution containing ions capable of conducting electricity as they are free to move.

The ions produced by the electrolytes carry and transfer electrons and they facilitate electrochemical reactions in rusting.

So another quick check for understanding.

Why do electrolytes speed up the rusting process? A, they provide a protective coating on the iron surface.

B, they facilitate electrochemical reactions by providing ions that can carry and transfer electrons.

C, they increase the temperature of the water.

D, they absorb moisture from the air.

A, B, C, or D.

Well done if you chose B.

They facilitate electrochemical reactions by providing ions that can carry and transfer electrons.

And this brings us to our first task.

We have three questions that we'd like you to answer.

The first one, describe what is meant by the terms corrosion and rusting, giving details on how they are similar and different.

Question two, explain how a metal oxide layer can protect the underlying metal from further corrosion.

Provide an example of a metal that does this.

And question three, explain what oxidation means in the context of rusting.

Identify which element is oxidised during the rusting process and describe how you can tell.

And we have the equation given here, iron + oxygen + water gives hydrated iron three oxide, which you might want to refer to during your answer.

So pause the video now and have a go at these questions.

And then when you are ready, we'll have a look at the answers together.

Let's have a look at the answer to question one.

Describe what is meant by the terms corrosion and rusting, giving details on how they're similar and different.

So to start this answer, we need to sort of state clearly what is corrosion and what is rusting.

So your answers should be something like this.

"Corrosion is the general process of metal degradation, whereas rusting is specifically the corrosion of iron or steel with water and oxygen." The similarities, well both involve the deterioration of metals and both can weaken metal structures.

Differences, corrosion affects various metals, whereas rusting only affects iron and steel.

Rusting specifically needs oxygen and water.

So very well done if you've got that answer correct.

Moving on to question two, explain how a metal oxide layer can protect the underlying metal from further corrosion.

And give an example.

So first of all, the protective mechanism, where the metal oxide layer forms a physical barrier and this prevents oxygen and water from reaching the underlying metal.

An example is aluminium.

Aluminium forms aluminium oxide.

This oxide layer is thin, adheres strongly, and prevents further oxidation.

The thin layer of aluminium oxide is also nearly transparent, so it doesn't significantly alter the appearance of the metal.

So very, very well done if you've got that answer correct.

Let's have a look at the answer to question three.

Explain what oxidation means in the context of rusting.

So the context of rusting oxidation is that's the iron reacts with oxygen to form a new compound.

The element that is oxidised during the rusting process is iron.

Iron is oxidised during the rusting process and we can tell because iron gains oxygen to form oxide, which is rust.

And we can see that from the word equation and the symbol equation that you have been provided with.

So very well done if you've got that correct.

Excellent work.

That brings us to the end of our first learning cycle, and now we move on to our second learning cycle on preventing corrosion.

So desiccants are substances that absorb moisture from the air and help to keep sealed environments dry.

And there's typically two kinds of desiccants that you may have come across.

First of all, silica gel.

These are small, transparent beads, and you may have noticed these little silica gel packets, if you have bought any electronic devices such as a laptop.

They often are used in packaging for electronics and also food products, and they absorb the moisture from the air.

Another type that's used a lot is calcium chloride, and this looks more like white powder, or granules of powder, and they're used in more industrial applications such as dehumidifiers, where we want to have the whole atmosphere dried out.

Desiccants can reduce metal oxidation.

Rust prevention can be achieved by eliminating oxygen or water, the air.

Now we can actually carry out a practical activity experiment to investigate rusting.

So in this example, we have an iron nail placed in four test tubes, A, B, C, and D.

So in the first test tube we are placing boiled water and oil.

In the second test tube, we are placing some salt water solution.

Then the third one is gonna be left open to the air.

And the fourth 1D is a sealed container with some calcium chloride in.

So having left these for a period of time, let's have a look at the results.

So that first one A, the boiled water and oil layer.

Well, what you'll see is there is no rust because the boiling water removes oxygen and the oil stops new oxygen from entering.

The salt solution water, B.

If we look at that nail, it's quite brown.

It does look rusty.

The salt water solution acts as an electrolyte, so it speeds up that rusting reaction.

C, open to the air.

Well, we've got some rust on that.

Not quite as much as B.

There is air and moisture.

Remember, moisture contains water and that causes the normal rusting effect.

And in D, it's sealed with calcium chloride.

So this means we can see here, there is no rust on the air.

The reason is the calcium chloride dries out the air, it removes the moisture from the air.

So still oxygen present but no moisture.

So this confirms what we learned in learning cycle one, that for corrosion or rusting to occur, there must be iron in the presence of water and oxygen.

So true or false? Desiccants reduce metal oxidation by absorbing moisture from the air.

Well done if you picked true.

Let's have a look at the reason why? Well, is it A, desiccants keep the environment dry, preventing water from reacting with the metal.

Or B, desiccants coat the metal forming a protective barrier against oxidation.

Well done if you chose A, the way desiccants work is they remove the water from the atmosphere.

They're like an air dryer.

That's one of the ways to think of it.

So very well done if you got that correct.

So a simple way of preventing corrosion is to cover up the metal and that basically will provide a physical barrier.

And there's several different ways in which we can do this.

So we can grease something, we can cover something with a layer of oil or grease.

And if you think about riding a bicycle, the bicycle chain needs to be nicely oiled and that stops it from going rusty over time, stops it from corroding.

We also paint things again, railings outside are often painted.

The paint provides a physical barrier and protects the metal underneath from corrosion or we can put a plastic coating over something.

So for example, this metal test tube holder has been painted to reduce corrosion and it, yeah, it looks a bit better as well.

The coatings can be damaged easily.

So do require regular maintenance.

So for example, if a painting, or plastic coating gets a massive scratch into it, that place where the scratch is will get oxygen, and water there, and that part of the metal will start to corrode.

Sacrificial protection is a method where a more reactive metal is used to protect a less reactive metal.

And this is because the more reactive metal corrodes, or we can say it is sacrificed, instead of the protective metal.

And this is something that is often used on ships.

And you can see an image here of the underneath of a ship where you have got a rudder.

So what actually happens is zinc blocks are placed onto the ship's hull and the zinc blocks act as an anode, they will corrode instead of the steel hull.

So basically, it will protect the actual hull of the ship, the steel.

The zinc will corrode away, but eventually, it will need to be replaced.

And if the zinc is not replaced, then the steel will start to corrode and we don't want that to happen because that will damage the structure of the ship.

Galvanising is a special type of sacrificial protection.

And what happens here is you coat iron or steel with a layer of zinc.

And it's usually done by hot dipping.

So you can see a galvanised steel handrail here.

And if you kind of just look at it, what actually happens is a zinc starts to corrodes away, but the steel coat or the iron handrail is protected.

So structurally it is still very strong.

Okay, so let's check our understanding.

True or false? Sacrificial protection involves using a less reactive metal to protect a more reactive metal.

True or false? Well done if you picked false.

Now let's have a look at the reason why.

Sacrificial protection involves coating the metal with oil to protect corrosion.

So is it A? Or is it B? Sacrificial protection involves a more reactive metal corroding instead.

Well, well done if you chose B.

Sacrificial protection always involves a more reactive metal corroding instead of a less reactive metal.

Well done if you got that correct.

Electroplating is the process of coating a metal object with a thin layer of another metal using an electric current.

And its use to enhance appearances and improve corrosion resistance.

So there are some basic principles to electroplating.

First of all, the metal to be plated is the cathode.

And you can see this in the diagram, it's the grey metal that's labelled Me.

The metal salt solution containing ions of the plating metal is the electrolyte.

So in the diagram we are going to cover this metal in copper, so our electrolyte must have Cu2 plus ions in it.

The electric current causes the metal to deposit on the object.

So when this cell is switched on, the Cu2 ions are attracted towards the cathode and are deposited on the metal.

So before electroplating can occur, the object must be cleaned and it's often submerged in the metal salt solution as part of the cleaning process.

Why do we do electroplating? Well, there are quite a lot of benefits.

First of all, the end product looks shiny and has a decorative finish.

And in the image that you can see here, we have some silver-plated copper lions at V&A in London that look a lot nicer.

Sometimes jewellery is electroplated.

So to make a cheaper metal look more expensive and attractive such as gold, we electroplate a cheap metal with gold, for example.

But it also protects the base metal from corrosion, which is another really important reason for using electroplating.

The downside of electroplating is it's expensive because it requires specialist equipment and users electricity in the process.

So let's have a quick check for understanding.

In the process of electroplating, what role does the metal object to be plated play? Is it the anode, the cathode, electrolyte, or the solvent? Well done if you chose B, it is the cathode.

Regular checks and maintenance ensure long-term protection.

So you can see here we've got some divers in the image checking the ship's hull, which is really important to make sure that no corrosion is taking place.

So what the divers will do to start off with, they will have carry out some inspections to see what condition the hull is actually in.

After that, there may be some cleaning and some touch-ups.

This prolongs the lifespan of metals, reduces the cost of replacements, repairs and increases safety and reliability.

So it's actually important that those regular checks and maintenance are in place.

Now, some alloys are corrosion resistant.

So stainless steel is an example of an alloy and you can see an image of some stainless steel cans, so made of iron, nickel, and chromium as well as other elements.

The metals like chromium form a metal oxide protective layer making stainless steel, very corrosive resistant.

Just quick check for understanding, true or false.

Alloys are always more corrosion resistant than pure metals.

False.

I wonder why.

Let's have a look at the answer and let's have a look at the reasons.

Is it A, an all alloys are less resistant to corrosion than pure metals.

Or B, only certain alloys like stainless steel are more corrosion-resistant due to their specific composition.

So well done If you chose B, that is the correct answer.

So that brings us to Task B.

We have a table here.

What we'd like you to do is to fill in the table with the information about different methods used to prevent corrosion.

For each method, describe how it works, provide an example and list its advantages and disadvantages.

So pause the video, have a go at the question, and then we'll look at the answers together.

Here we have our completed table and I'm just gonna talk through each of the different methods.

So desiccants, well, these absorb moisture from the air, reducing humidity around the metal to prevent rusting.

An example is calcium chloride or silica gel in sealed containers.

The advantage is they're simple and effective in enclosed spaces.

The disadvantages, they're limited to enclosed or sealed environments.

Physical barrier.

Well, this physically separates the metal from environmental factors.

Examples include painting, plastic coating and greasing.

The advantages.

They're inexpensive, easy to apply, and provides immediate protection.

The disadvantages, they can be damaged easily and require regular maintenance.

Alloying.

This is when we have mixing metals with other elements to improve properties.

An example is stainless steel, which of course, is iron, nickel, and chromium.

Advantages.

We get high resistance to rust and its long lasting.

The disadvantage is it can be more expensive than the pure metals.

Sacrificial protection.

This is where the more reactive metal corrodes in place of the protective metal.

An example is zinc anodize the steel on ship hulls.

And advantage is it provides a long-term protection.

The disadvantages requires periodic replacement of the sacrificial metal.

Galvanising.

Coating iron or steel with a layer of zinc to protect it against rust.

The example is hot-dip galvanised steel handrail.

Advantage is it provides durable protection even if scratched.

The disadvantage is the zinc layer can eventually wear away and need recoating.

And finally, electroplating.

This is when we coat metal objects with a thin layer of another metal using electric current.

An example is silver-plated jewellery.

The advantages, it enhances appearances and is corrosion resistance.

The disadvantage is it can be expensive and requires specialist equipment.

So very, very well done if you manage to complete that table and get everything correct because there is quite a lot of different methods that we have covered during this lesson.

And now we come to our summary of the key learning points.

So we'll just go through them together.

Corrosion is a general process of degradation of metals through various courses.

Rusting is a specific type of corrosion affecting iron in the presence of air and moisture.

A metal oxide layer can protect the underlying metal if the oxide forms a physical barrier.

Some coatings are reactive and contain a more reactive metal to provide sacrificial protection.

For example, zinc to galvanise.

Electroplating can be used to improve the appearance and / or the resistance of corrosion of metal objects.

I hope that you have enjoyed this lesson today and look forward to learning with you again very soon.