Chapter 11: High-Temperature Corrosion

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Welcome back to the Deep Dive.

Today we are turning up the heat.

And I mean that quite literally.

We are taking the safety off, stripping away the water and the humidity we usually talk about and stepping right into the inferno.

We really are.

We're talking about environments so hot that metal doesn't just get warm.

It starts to react.

It starts to change.

And I mean, effectively it tries to destroy itself.

It is a fascinating and honestly a pretty high stakes topic.

We're opening up chapter 11 of Mars G Fontana's classic engineering text, corrosion engineering.

Yep.

The chapter is high temperature corrosion, but I hear it called other things too.

That's right.

In the industry, you'll hear dry corrosion a lot or maybe scaling just to set the context for everyone listening.

Usually when I hear the word corrosion, I have this very specific image in my head.

I'm thinking of my bike chain getting rusty because I left it out in the rain or maybe the wheel well of an old car in a snowy city.

That's wet corrosion, right?

You need water, you need slush, electrolytes.

Exactly.

That is aqueous corrosion.

It's messy.

It's electrochemical in a very obvious like wet battery kind of way.

But the beast we are wrestling with today is it's a completely different animal.

We were talking about environments where liquid water simply cannot exist.

We're looking inside the combustion chamber of a jet engine, the nozzle of a rocket, or say the cracking furnaces of a petrochemical plant.

So we are talking about temperatures north of what?

What's the starting point?

Usually we define high temperature corrosion as starting around 500 degrees Celsius.

But in many applications, we are pushing way past 1000 degrees Celsius.

Wow.

And in these conditions, you don't need water to rust.

The gases themselves, oxygen, sulfur, carbon dioxide, they stop being passive fluids and they start attacking the metal directly.

That is the hook for me.

The idea that air itself, just the air you're breathing, becomes the enemy if you get it hot enough.

It feels like the rules of physics change.

In a way they do, or the pace of the game changes.

The fundamental challenge here is really a battle against thermodynamics.

Okay, unpack that for us.

We have to remember where metal comes from.

We dig up ores from the ground, iron oxide, nickel sulfide.

That is the metal's natural stable state.

It's happy being dirt.

Right.

And then we spend a massive amount of energy smelting it, refining it, beating it into a shiny steel bar.

We put all that energy in.

Thermodynamics dictates that the metal desperately wants to give that energy back.

It wants to return to being an ore.

It wants to go home.

It wants to go home.

At room temperature, the reaction is slow enough that we can mostly ignore it.

But heat is an accelerator.

As the temperature rises,

that urge to revert to an oxide becomes, well, it becomes irresistible.

So our mission today is to unpack exactly how that happens.

We aren't just going to list a bunch of chemical reactions.

I want to how engineers can stop them.

Yeah.

We need to understand the traffic laws of the atomic world.

That is the perfect way to frame it.

We're looking at the battle between thermodynamics, what the metal wants to do, and kinetics, how fast we let it do it.

Let's start with the basics then.

The first line of defense.

The text introduces a concept from way back in 1923, the Pilling Bedworth Ratio.

This seems to be the earliest scientific attempt to predict if a metal is going to survive the heat.

It is.

It's the classic starting point for any student.

Pilling and Bedworth were looking at the oxide layer, the scale that forms on the surface of a metal when it burns, and they realized that the physical volume of that oxide, it matters immensely.

Okay, let's break down the formula.

The Pilling Bedworth Ratio, or PB ratio, is calculated as the volume of the oxide produced divided by the volume of the metal consumed to create it.

Exactly.

You are comparing the size of the corrosion product to the size of the metal replaced.

I found the best way to visualize this is with a clothing ontology.

I love analogies.

Let's hear it.

Okay, so imagine the metal is a person, and the oxide layer is a shirt that the person is trying to put on.

We have three scenarios here based on Table 11 -1 in the text.

Let's look at scenario A first.

This is where the ratio is less than one.

So mathematically, the volume of the oxide is smaller than the volume of the metal.

Right.

So in my analogy, this is the too tight shirt.

You have a large person trying to put on shirt that is like three sizes too small.

That is a perfect visualization.

If the shirt, the oxide is too small, it has to stretch to cover the metal surface.

But we have to remember oxides are ceramics.

They're brittle.

They don't stretch.

So what happens?

It rips.

It cracks.

It becomes porous.

It essentially leaves the metal naked because the oxide takes up less space.

It can't form a continuous barrier.

So the oxygen just flows right through.

Oxygen molecules from the air just flow right through those cracks and eat the metal underneath.

There's no protection.

And looking at the table, the metals that suffer from this too tight shirt syndrome are things like lithium, sodium, potassium, and magnesium.

Magnesium is the classic example.

It has a ratio of about 0 .81.

It's significantly under one.

This is why, if you ignite a strip of magnesium, it burns so fiercely and brightly.

Right.

In high school chemistry class?

Exactly.

The oxide skin cracks open immediately as it forms, constantly exposing fresh metal to the fire.

It cannot protect itself.

Okay.

So that's scenario A, a disaster.

Now let's look at the opposite extreme,

scenario B.

The ratio is way bigger than one.

Let's say two or three.

This is the oversized coat.

The oxide takes up way more space than the metal it replaced.

You might think, hey, a big, sick coat.

That sounds protective.

You might think that, but imagine that coat is growing on you and it's getting tighter and tighter because there's just too much fabric for the space available.

This creates massive compressive stock at the surface.

The oxide is pushing against itself.

It is.

And eventually something has to give.

It pops off.

It buckles, creates blisters.

And eventually it undergoes a process we call spalling.

It literally flakes off.

Spalling?

That sounds expensive.

Oh, it is catastrophic.

Once the scale flakes off, you have fresh naked metal exposed again.

The oxygen attacks and new thick layer forms.

It gets too stressed.

It flakes off.

And the cycle just repeats until there's no metal left.

And the villain here, who's a good example of this?

Tungsten.

Tungsten has a ratio of 3 .4.

It produces so much voluminous oxide that it essentially pushes itself off the metal.

It's useless for oxidation resistance in air.

So we don't want a ratio of 0 .8 and we don't want a ratio of 3 .4.

We want the Goldilocks zone.

Scenario C.

Scenario C.

The perfect fit.

You want a ratio that is close to one or perhaps slightly above it.

You want just enough extra volume to ensure the shirt is tight, a compression fit.

So it seals perfectly against the wind, but not so tight that it rips or buckles.

And who are the winners here?

What materials fit this profile?

Aluminum is the gold standard.

It has a ratio of 1 .28.

That is a beautiful number.

It forms a continuous, tight, non -porous skin of alumina.

Aluminum is aluminum oxide, right?

All 2033S.

That's the one.

Chromium is also a heavy hitter around 1 .99 or 2 .0.

That's pushing the limit on the high side, but it still forms a very coherent protective scale.

Iron is around 1 .77.

So when we see engineers mixing aluminum and chromium into their high -tech alloys, they are basically trying to force the metal to wear this perfect fit shirt.

Exactly.

We aren't relying on the base metal.

We are relying on the alloy to generate that specific Pilling Bedworth -approved skin.

Now, I have to ask.

If I'm an engineering student and I have this table, can I just look at the ratio and know everything?

Is it that simple?

I wish it were.

And the text is very clear on this.

The Pilling Bedworth ratio is a rule of thumb.

It is a qualitative screening tool.

It tells you if a metal has a chance of being protective.

But it's not the whole story.

No, because it fails to predict a lot of things because it is too simple.

It treats the oxide scale like a solid, static wall.

It assumes that if the wall is there, nothing gets through.

But that's not what's happening.

No.

In reality, at high temperatures, that wall is alive with movement.

It ignores the complexity of how atoms actually travel through the solid.

And that leads us directly into the next section, which I think is the most mind -bending part of the chapter, the electrochemical nature of oxidation.

This is often the aha moment for students.

We tend to write oxidation as a simple chemical equation.

Metal plus oxygen equals metal oxide.

M plus O goes to MO.

Boom.

Done.

Right.

It looks like simple addition.

But physically, that is not what is happening.

The text explains that oxidation is actually an electrochemical circuit.

It's a battery.

Wait, hold on.

You said this was dry corrosion.

How do you have a battery without liquid?

Don't you need an electrolyte soup for ions to swim in?

That is the intuition we have to break.

In high temperature oxidation, the solid oxide scale itself acts as the electrolyte.

The solid rust is the electrolyte.

It is.

Let's try to visualize figure 11 to 1 from the text.

Right.

I want everyone listening to close their eyes for a second, unless you're driving, of course.

Okay.

I'm imagining a cross section of a metal bar.

Good.

On the far left, you have the pure metal.

On the far right, you have the oxygen gas.

And in the middle, separating them, is the oxide scale, that rust layer.

Got it.

Metal, scale, gas.

Now, look at the interface where the metal meets the scale.

We call this the anode.

Here, the metal atom gives up electrons.

It becomes a positively charged ionication.

The reaction is M goes to M2++2 electrons.

So new metal ions are born on the inside.

Correct.

Now look at the other side.

The interface where the scale meets the gas.

We call this the cathode.

The oxygen molecules land there.

They pick up electrons and become negatively charged oxygen ions.

Okay.

So we've got positive metal ions on the left and negative oxygen ions on the right.

They're attracted to each other like magnets, but they have this solid wall of oxide separating them.

Exactly.

And this completes the circuit.

For the reaction to continue, for the metal to keep rusting, two things must flow through that solid rock scale.

What are they?

First, the electrons.

They have to travel from the metal through the scale to get to the oxygen.

So the scale must have electronic conduction.

Okay.

Electricity has to flow.

And the second thing.

The ions themselves.

Matter must move.

Either the metal ions have to migrate out through the scale to the surface, or the oxygen ions have to migrate in through the scale to the metal.

We call this ionic conduction.

This is the part that feels impossible.

How does a big chunky metal atom move through a solid crystal lattice?

I look at a piece of steel and it looks solid.

Nothing is moving.

It looks static to our eyes.

But this brings us to the most critical concept in the chapter.

Diffusion and defect structures.

The reaction rate, how fast your engine destroys itself, is controlled by how fast these ions can push their way through the solid.

It's diffusion controlled.

So we need to understand the traffic laws of the solid highway.

The text breaks us down into defect structures.

Right.

If you had a perfect crystal lattice,

every atom exactly where it should be, perfectly aligned diffusion would be essentially impossible.

Like a gridlock city.

It would be like a gridlock city where every square inch of pavement has a car on it.

Nothing moves.

But real crystals aren't perfect?

No.

They have defects.

And these defects are the open lanes that allow corrosion to happen.

The text categorizes oxides into two main types based on these defects.

N -type and P -type.

I want to spend some time here because this helps us understand how to stop it later.

Let's start with N -type oxides.

N -type usually stands for negative type, meaning conduction is largely by excess electrons.

But structurally, the text links this to metal excess or anion deficiency.

A classic example is zirconia or zinc oxide.

The mental model here is what we call interstitial sites.

Interstitial.

That's a fancy word for in between.

Yes.

Imagine a movie theater.

Every seat is taken.

That represents the crystal lattice.

But in an N -type oxide, you have small metal ions squeezing into the aisles, the spaces between the seats.

They don't belong there.

No, they're uncomfortable.

But because they're in the aisles, they can run up and down the rows much faster than the people sitting in the seats.

In an N -type oxide, the metal ions migrate by moving through these interstitial spaces.

Okay, so N -type is running in the aisles.

What about P -type?

P -type oxides like nickel oxide, nidol, or cobalt oxide are metal deficient.

They rely on collocation vacancies.

Vacancies.

So empty seats.

Right.

Think of those sliding tile puzzles.

The plastic grid with one square missing so you can slide the others around.

I was terrible at those.

I could never get the picture to line up.

But you know the mechanic.

To move a tile, you have to slide it into the empty spot.

Effectively, the tile moves one way and the hole moves the other way.

That makes sense.

In a P -type oxide, the nickel ions move exactly like that.

A nickel ion jumps into an empty hole of vacancy.

Then another ion jumps into the spot it just left.

The metal migrates outward by using these empty spots as stepping stones.

So if there are no vacancies, no empty spots, the metal can't move.

The sliding tile puzzle is glued shut.

Precisely.

And this leads to the most practical engineering application in the chapter.

Doping and the Wagner -Hoff rules.

This is where it gets really interesting.

This is where we stop just observing the corrosion and start actively manipulating it.

We can dope the metal.

Doping just means adding a small amount of an impurity, another element, to change the number of defects in the crystal.

We are essentially playing traffic controller.

Let's walk through the example in the text because the logic is a bit tricky.

We are dealing with nickel oxide.

We established it's a P -type semiconductor.

It moves ions via vacancies, the sliding tile puzzle.

Correct.

So if we want to slow down the corrosion, what is our goal?

We want fewer vacancies.

We want to fill the empty seats so the ions can't move.

Exactly.

So let's see what happens if we add lithium to the nickel.

Lithium is a plus one ion.

Nickel is a plus two ion.

We are replacing a plus two charge with a plus one charge.

We have a deficit of positive charge.

Now the crystal lattice has a strict rule.

It must remain electrically neutral.

It cannot have a net charge.

If you put in a bunch of weak lithium ions, you have lowered the total positive charge in the system.

So how does the crystal fix that?

How does it balance the books?

It fixes it by reducing the number of empty spots.

Remember, a vacancy is a missing plus two charge.

By having fewer vacancies, the crystal keeps its density of positive charge higher to balance the negative oxygen.

That is some 4D chess right there.

So simply by adding lithium, the crystal structure naturally closes up its vacancies to stay balanced.

And fewer vacancies means?

Fewer vacancies means the sliding tile puzzle gets stuck.

The ions can't move.

And if the ions can't move, the oxidation rate slows down.

So doping nickel with lithium creates a more corrosion resistant material.

That is incredible.

But the text warns us about the opposite scenario.

What if we add chromium, CR3 -cremium plus 3 -serie to nickel?

Chromium has a higher valence, plus three.

You are replacing a plus two nickel with a plus three chromium.

You are adding extra positive charge into the system.

So the crystal has to dump positive charge to balance it out.

And the easiest way to dump positive charge is to kick out some nickel ions.

Yeah.

It creates more vacancies.

This opens up more empty seats.

Bingo.

You add chromium to nickel oxide and you create a highway of vacancies.

The sliding tile puzzle becomes mostly empty space.

The ions can rush through.

The oxidation rate increases.

Wait, stop.

Isn't chromium the stuff we put in stainless steel to stop rust?

You're telling me adding chromium makes it rust faster.

I love this question because it is the classic trap for engineering students.

The Wagner -Hoff rules apply to the defect structure of the parent oxide.

If you have a dilute alloy, say 0 .5 % chromium and nickel, the scale is still basically nickel oxide just contaminated with chrome.

And yes, in that specific range, the oxidation rate goes up.

It gets worse.

But we use chromium in jet engines?

We do, but we use a lot of it.

If you add 15 % or 20 % chromium,

you aren't just doping the nickel oxide anymore.

You overwhelm the system.

You switch over to forming a completely new oxide scale, chromium oxide or chromium.

And chromium is naturally a much better barrier than nickel oxide ever could be.

So the rule works for small amounts, the doping range, but if you add enough, you change the game entirely.

Exactly.

But understanding these doping rules is critical because it explains why random impurities in your metal can have massive unexpected effects on how fast your equipment degrades.

Okay, let's zoom out from the atoms for a minute.

As engineers, we need to predict the future.

We need to know how long a part will last.

This brings us to kinetics, the mathematics of growth.

Right.

We usually look at a graph like figure 11 to 6.

On the vertical axis, you have weight gain, basically how much oxygen has been absorbed, and on the horizontal axis, you have time.

The text outlines three main laws or shapes that these curves take.

Let's run through them.

Law number one, the linear law.

This looks like a straight line going up and to the right.

The equation is W equals KL times T.

Linear means constant speed.

Which is bad news.

It means the reaction isn't slowing down.

As the oxide layer gets thicker, it's not providing any extra protection.

This usually happens when the scale is porous or cracked, like the magnesium example we talked about.

The too tight shirt scenario.

Exactly.

If your material follows a linear law, the metal will eventually be consumed entirely at a constant rate.

There's no stopping it.

Law number two, the parabolic law.

This is what we want.

The curve starts steep, but then it bends over and flattens out over time.

The equation is W squared equals KP times T.

So it slows down as it gets thicker.

Because it is diffusion controlled.

Remember our traffic model.

The thicker the scale, the further the ions have to travel to find each other.

The diffusion path gets longer.

So the process naturally throttles itself.

It's a self -limiting process.

Yes.

Iron, cobalt, copper.

Most of our engineering alloys aim for this behavior.

We accept a specific amount of corrosion initially to build that shield,

and then it stabilizes.

And the third one, the logarithmic law.

This one flattens out incredibly fast.

It's usually seen in very thin layers at lower temperatures, like aluminum in air at room temperature.

It forms a thin skin and then essentially stops dead.

So practically speaking, engineers are usually hoping for and dealing with parabolic behavior in high -temp equipment.

Yes, we rely on that parabolic flattening.

But sometimes things go wrong, very wrong.

This leads us to the horror stories of section six, catastrophic oxidation.

The name alone sounds expensive.

It is.

Imagine you have a metal following a nice slow parabolic curve.

Everything looks safe.

Then suddenly the temperature bumps up just a little bit and the rate shoots straight up, even faster than linear.

It goes vertical.

What happened?

Did the scale fall off?

Usually a liquid phase formed.

The rust melted.

The oxide melted.

We tend to think of metal oxides as these super heat -resistant ceramics.

But not all of them.

The tech specifically calls out molybdenum trioxide and vanadium pentoxide.

These have surprisingly low melting points.

So if you use molybdenum in your alloy, if that molybdenum oxidizes and the temperature is high enough, that oxide can turn into a liquid.

It drips off the surface.

Or worse, it acts like a flux.

It runs down the grain boundaries and dissolves the solid metal.

So you're constantly exposing fresh metal to the air.

And here's the kicker.

Oxidation is exothermic.

It releases heat.

So the reaction gets hot, which melts more oxide, which exposes more metal, which reacts and releases more heat.

It's a thermal runaway train.

The text mentions that molybdenum can literally evaporate as smoke.

Yes.

If you have high moly steels and stagnant air, they can disintegrate rapidly.

The metal just vanishes into smoke and a puddle of slag.

There is another sneaky form of destruction mentioned.

Internal oxidation.

This sounds like a medical condition.

It acts like one.

It's rusting from the inside out.

How does oxygen get inside without rusting the outside?

It happens in dilute alloys.

Suppose you have a silver alloy with a tiny bit of indium or a copper alloy with a bit of aluminum.

Oxygen can actually dissolve into the base metal, the silver or copper, and diffuse inward.

So the oxygen bypasses the border patrol and infiltrates the country.

Exactly.

It's essentially a Trojan horse.

The oxygen diffuses in faster than the aluminum atoms can diffuse out to form a scale.

So the oxygen finds the reactive aluminum atoms deep inside the metal matrix and oxidizes them right there.

So you get rust forming inside the solid metal bar.

You end up with these tiny oxide particles precipitating at the grain boundaries deep inside the material.

Figure 11 -2 shows this.

It doesn't look like a layer on top.

It looks like spots or speckles deep inside.

And those spots act as stress risers.

They make the alloy incredibly brittle.

You might look at the part and think, hey, the surface looks clean, but the structure inside is compromised.

It increases notch brittleness.

The metal loses its ductility.

If you hit it, it shatters like glass instead of bending.

That is terrifying.

Just invisible structural failure.

Speaking of invisible enemies, we have to talk about hydrogen attack.

Hydrogen is the smallest atom in the universe.

This makes it a unique threat.

At high temperatures and high pressures like in the giant hydrocracking units at an oil refinery, hydrogen gas can diffuse right through solid steel like a ghost through a wall.

And it's not just passing through.

It's looking for something.

It's looking for carbon.

Steel gets its strength from carbon, specifically iron carbide.

When that infiltrating hydrogen meets the carbon inside the steel, they react chemically.

What do they make?

Methane.

The reaction is C plus 4H goes to CH4.

Methane.

And that's a gas.

Methane gas.

And here is the problem.

Hydrogen is tiny.

It can drift in and out of the lattice.

Methane is a large molecule.

It is trapped.

It cannot diffuse out.

So you are creating gas bubbles inside the solid steel.

Yes.

The hydrogen keeps flowing in, feeding the reaction, and the methane keeps building up.

The pressure in these microscopic voids can build up to enormous levels, thousands of atmospheres.

What happens to the steel?

It rips apart.

It blisters.

The text calls it fissuring.

The metal literally cracks from the inside out due to internal gas pressure.

And since the carbon is gone, turned into methane, the steel also loses its strength.

It's called decarburization.

Exactly.

So you have weak, cracked steel holding back high -pressure flammable gas.

It is a recipe for a massive explosion.

This is a huge issue in the oil and gas industry.

To prevent it, the text introduces the famous Nelson Curves, figure 1112.

If you work in a refinery, you know this chart.

It plots temperature on the vertical axis and hydrogen partial pressure on the horizontal axis.

It draws a series of curves for different types of steel.

It looks like a safety mat.

It is.

If you're operating conditions, your temp and pressure fall below the line for your steel, you are safe.

The reaction is too slow to matter.

If you are above the line, boom, you will get hydrogen attack.

And how do we move the line?

How do we make the steel safer so we can run hotter?

We change the chemistry.

We add chromium and molybdenum.

You'll hear about chromaely steels.

Why do they help?

They form very stable carbides.

They hold onto the carbon much tighter than iron does.

They essentially lock up the carbon so tightly that the hydrogen can't steal it to make methane.

It's like putting the carbon in is safe.

Exactly.

Okay.

We've covered oxygen, hydrogen,

but there is one more villain in the high temp story.

Perhaps the worst of them all.

Sulfur.

Ah, sulfur.

If oxygen is the enemy, sulfur is the nemesis.

Generally speaking, sulfidation is much faster and more destructive than oxidation.

Why?

What makes it so much worse?

Two main reasons.

First, sulfide scales tend to have more defects, more vacancies, so ions diffuse through them much faster.

But the real killer is the melting point.

The liquid phase again?

The nickel -sulfur eutectic.

Explain eutectic for us.

A eutectic is a specific mixture of two substances that melts at a temperature lower than either of them would alone.

Pure nickel melts at a very high 1 ,455 degrees C.

Pure nickel sulfide is also high.

But if you mix them at a specific ratio, that mixture melts at only 645 degrees C.

645 is low.

A jet engine runs way hotter than that.

Exactly.

If sulfur gets into a nickel -based engine, maybe from bad fuel, you can form this liquid puddle that eats through grain boundaries at terrifying speeds.

This leads to the phenomenon of hot corrosion.

This is a specific term in the text, right?

It's not just corrosion that is hot.

Correct.

Hot corrosion specifically refers to an accelerated attack involving molten salts, usually sodium sulfate.

Where does the salt come from in a jet engine?

Imagine a Navy jet or a ship operating near the ocean.

It ingests sodium chloride from the sea air.

It burns fuel that contains sulfur.

In the combustion chamber, the sodium and sulfur react to form sodium sulfate.

And this stuff coats the turbine blades.

It condenses on the blades and melts.

Now remember that protective oxide scale we worked so hard to build?

The alumina or chromia skin.

The scenario C, perfect fit skin.

This molten salt acts as a solvent.

It essentially dissolves the protective skin.

We call it fluxing.

It can dissolve it either acidically or basically depending on the chemistry.

It effectively strips the armor off the knight in the middle of the battle.

It strips the metal naked, allowing the sulfur and oxygen to attack the base metal continuously.

The text notes that this is a major life -limiting factor for marine gas turbines.

So we have all these threats.

Oxygen, hydrogen, sulfur, molten salts.

It feels hopeless.

How do we actually build anything that flies?

What materials can withstand this?

We rely on three base metals, iron, nickel, and cobalt.

But as we discussed, their own oxides like FeO or NiO, they're just not good enough.

So we treat the base metal as a canvas.

And we paint with alloys.

We alloy them to form specific super protective scales.

We have two main heresies in the high temp world.

Chromium, which forms chromia, and aluminum, which forms alumina.

Let's talk about the trade -offs.

Why choose one over the other?

Chromia is the workhorse.

It's what makes stainless steel stainless.

It's excellent up to about 900 or 1 ,000 degrees C.

But at extremely high temperatures or high gas velocities, chromia has a weakness.

It can oxidize further into something called CRO3D3.

And what is CRO3D3 alls?

It's volatile.

It turns into a gas and evaporates.

So if you run a chromia former too hot, your protective scale literally blows away in the wind.

So for the really hot stuff, the jet engines, we need aluminum.

Yes.

Alumina 2033 there is the gold standard for superalloys.

It grows very slowly.

Remember the parabolic law.

And it sticks tight.

It's very, very stable.

Hence the superalloys.

Nickel -based superalloys.

They are designed to maintain mechanical strengths at high heat to resist creep, while simultaneously supplying enough aluminum to the surface to grow that protective aluminum scale.

But there is a trade -off, right?

If aluminum is so good, why not make the whole engine out of aluminum?

Well, aluminum metal melts at 660 degrees C.

We need the nickel for strength.

But even adding aluminum to nickel is tricky.

If you add high levels of CR or Al, it helps corrosion.

But it can make the alloy brittle or hard to work with.

It's a balancing act.

It is the fundamental struggle of materials engineering.

You are constantly trading mechanical ductility for chemical stability.

It's fascinating.

We are essentially tricking the metal into protecting itself.

We are.

We are using the metal's own tendency to react to create the barrier that stops the reaction.

So to wrap this up, what does this all mean for the listener?

It means that high -temperature corrosion is a fundamental constraint on human technology.

We want to run engines hotter to be more efficient, to save fuel, reduce emissions.

Thermodynamics says, no, I will melt you.

And kinetics says?

Kinetics says, maybe if you slow me down.

And we use these oxide scales to negotiate that deal.

I have a final thought for our listeners.

We usually think of rust as decay, a sign of neglect.

But in a jet engine, that rust, that oxide scale, is the only thing keeping the plane in the air.

That is a beautiful way to put it.

We literally fly on wings of rust, carefully controlled, engineered rust.

And just remember, keep your scales tight, your vacancies low, and watch out for that sulfur.

Words to live by.

Check the show notes for the Pilling Bedworth table and the Nelson curves.

Thanks for diving deep with us.

Ideally, stay parabolic.

Goodbye, everyone.