Chapter 7: Mineral Acids

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Welcome back to the Deep Dive.

Today we're doing something a little different.

We're leaving the clean, comfortable, and frankly somewhat abstract world of classroom theory, and we're walking right onto the factory floor.

It's time to get our hands dirty,

or perhaps more accurately, keep our hands very, very clean because the materials we're dealing with today,

well, they're unforgiving.

They really are.

We are diving into chapter seven of Mars G.

Fontana's Corrosion Engineering, and if you're an engineering student, a plant manager, or just someone fascinated by how the industrial world stays held together without, you know, melting into a puddle, this one is for you.

Oh, yeah.

You're talking about mineral acids.

The heavy hitters.

Yeah.

The absolute workhorses of the chemical industry.

We have a full docket today.

We're covering what I like to call the big three.

That's sulfuric, nitric, and hydrochloric acid.

The inorganic giants.

But we're also going to tackle the glass eater hydrofluoric acid, and then wrap up with phosphoric acid.

And the mission here, it's really about survival,

material survival.

When you're designing a plant, a storage tank, a piping system for these chemicals, you're playing a really high stakes game.

You need to select materials that won't dissolve or crack or, in the worst case scenario, fail catastrophically and hurt someone.

The economic stakes are massive too, right?

I was reading in the notes that sulfuric acid consumption is actually used as a yardstick for a nation's economic health.

That kind of blew my mind.

It's true.

It really is.

It's produced in such massive quantities, more than any other chemical in the world, that if a country is making a lot of sulfuric acid, it means they're making steel and fertilizer, batteries, they're refining oil.

It's the literal blood of the industrial economy.

So if production dips.

If sulfuric acid production drops, the economy is almost certainly slowing down.

It's a very reliable indicator.

So our approach today isn't just to memorize a bunch of corrosion tables.

I mean, anyone can look up a why a metal might survive in a tank of boiling acid one day and then fail the next, just because someone opened a valve too fast or let a little air in.

That's the key distinction.

That is the art of it.

It's rarely just about acid A versus metal B.

It's about concentration and temperature, aeration, velocity.

All these other variables.

All of them.

A material that works beautifully at 70 % concentration might fail disastrously at 50 % or at 99%.

It's a nuanced shifting landscape and you have to understand the underlying mechanism to navigate it.

Okay.

So let's start with the king, sulfuric acid,

HOA.

A big one.

Before we get into metals, we should probably briefly mention how this stuff is even made because the notes mentioned the contact process and that actually dictates the corrosion problems you face in the plant itself.

Right.

So broadly speaking, you burn sulfur to get sulfur dioxide, S -O -A -R -O.

Then you convert that to sulfur trioxide, S -O -A -R -O, using a catalyst.

That's the contact part.

But here's the tricky part.

You have to absorb that gas to make the liquid acid and you can't just dump S -O -A -R -O's into water.

Why not?

What happens?

It's way too exothermic.

It creates this massive choking fog of acid mist that you simply can't condense or capture.

It's a total mess and it's dangerous.

So the trick is you absorb the gas into existing sulfuric acid.

You use acid to make more acid.

Which means the equipment you use to manufacture the product has to be able to survive the product at its most aggressive stages.

And this leads us to probably the most counterintuitive fact in this whole chapter.

We're talking about ordinary carbon steel.

The odd couple relationship.

It really is.

I mean, if I told a lay person, I have a tank of super concentrated aggressive sulfuric acid and I'm going to store it in a cheap ordinary steel tank, they would think I was negligent.

They'd think you were crazy.

But that's the standard, isn't it?

It is, but with a massive asterisk.

A huge one.

Steel is widely used, but only at concentrations above 70 percent.

Okay, let's unpack that because this is where engineers get into trouble.

We have a visual and source material, figure seven to two.

It's the ISA corrosion chart for steel.

Can you walk us through what this graph actually looks like mentally?

I want the listener to be able to see this grid.

Sure.

So imagine a graph on the bottom, the x -axis.

You have the concentration of sulfuric acid going from zero percent on the left all the way up to over 100 percent on the right, which we call oleum or fuming sulfuric acid.

Right, fuming acid.

And on the vertical side, the y -axis, you have temperature going from freezing up to boiling and beyond.

Okay, so we're looking for the safe zones, which usually means a corrosion rate under, what, 20 mils per year?

20, yeah, that's a common cutoff for acceptable general corrosion.

All right.

So let's trace the line.

If I start at the left zero percent acid, just water and move right, what happens?

At low concentrations, say 10, 20, 30 percent, the corrosion rate is sky high.

The curve shoots straight up.

So steel just dissolves.

Rapidly is completely useless.

You would have a leak in days, maybe even hours.

But as I keep moving right on that chart towards higher concentrations.

As you cross 60 percent and hit 70 percent, the curve drops off a cliff.

I mean, it plummets.

The corrosion rate becomes very, very low.

Above 70 percent, specifically at room temperature and even a bit higher,

the steel is quite safe.

The stronger the acid gets, the safer the steel becomes.

That feels completely backwards.

Usually stronger chemicals mean more damage.

It does feel backwards, but there's a really neat chemical reason for it.

In strong acid, the steel reacts initially to form ferrous sulfate.

Now, because the acid is concentrated, meaning there's not much water around, that ferrous sulfate is insoluble.

It can't dissolve.

So it precipitates right onto the surface of the steel.

It forms a scab, like a protective layer.

Exactly.

It's a protective salt film.

It creates a physical barrier that stops the acid from eating deeper into the metal.

But, and here's the why.

That film is soluble in water.

So if the acid is dilute, if there's a lot of water present, the film just dissolves as fast as it forms.

So the metal is naked again.

The metal is naked and it gets eaten away.

You need the acid to be strong enough to keep that scab, that raincoat intact.

Got it.

But looking at this chart, figure 7 -2, there's a trap waiting at the very end, isn't there?

The dip.

This is where it gets really dangerous.

So you're cruising along at 90, 95, 98 % concentration, and steel is performing great.

You feel confident.

Right.

But when you hit about 101 % again, that's acid containing free.

So the corrosion rate spikes up again, sharply.

Then it drops, then it spikes.

It's extremely erratic.

The text mentions the 20 -amp line just stops it at 150.

Because they don't even want to guess what happens after that.

It's too unpredictable.

The flow conditions, the exact temperature, it all makes the behavior volatile.

So you have to be very, very careful right at that 100 to 101 % boundary.

You can't just assume stronger is better indefinitely.

There's another practical danger with this whole protective film mechanism that I want to highlight.

Moisture.

The text talks about grooving.

Yes.

This is a classic, classic failure mode.

Imagine you have a steel storage tank half full of 98 % sulfuric acid.

Okay, so the steel is happy.

The steel loves 98%.

It's safe.

But you leave a vent open to the air and sulfuric acid is hygroscopic.

It greedily absorbs moisture from the air.

So the layer of acid right at the top, the surface, it starts grabbing water from the humid air coming in the vent.

Correct.

And as that top millimeter of acid grabs water, its concentration drops.

It becomes dilute.

It drops below that magic 70 % number.

And the film dissolves.

Suddenly the protective film dissolves and the acid eats a perfect groove right into the tank wall exactly at the liquid line.

You could literally cut a tank in half that way.

You can.

It acts like a liquid saw.

That is why you use desiccant breathers on these tanks to make sure the air entering the tank is bone dry.

That is just wild.

All from letting humid air in.

Now let's move from steel to its cousin cast iron.

Specifically, gray cast iron.

Right.

Historically, cast iron was used a lot.

It behaves similarly to steel.

It's good in concentrated acid, but it has a fatal flaw.

Which is?

It's called graphitization.

That sounds like some kind of disease.

It is.

It's a structural disease for the metal.

See, gray cast iron contains graphite flakes, little microscopic chips of carbon floating in the iron matrix.

That's what makes it cast iron.

Right.

The sulfuric acid can actually penetrate the metal along these graphite flakes.

So it seeps into these little internal cracks.

Essentially, yes.

And corrosion products, the rust and iron salts, build up inside those tiny spaces deep inside the metal wall.

As they build up, they expand.

Iron oxide takes up more volume than the iron it replaced.

So it creates a wedging action.

A tremendous wedging action.

It generates immense pressure from the inside out.

So the metal is literally ripping itself apart from the inside.

Yes.

The iron can split open or crack.

And you have to remember, cast iron is brittle.

It doesn't stretch or yield like steel.

If a cast iron pipe bursts under pressure because of this graphitization, it shatters.

Like glass.

Exactly.

It can send shrapnel flying.

Which is a massive safety hazard.

Huge.

That's why the text notes that steel is generally preferred for piping and valving now.

If steel fails, it usually leaks or yields first.

It gives you a warning.

Cast iron just, it just goes boom.

Okay, so if steel and cast iron own the concentrated end of the spectrum, who owns the dilute end?

Because we said steel fails miserably below 70%.

Enter the compliment.

Chemical lead.

The text shows figure 7 -3, which is cool because it looks like an X.

Where Skil's corrosion rate goes up, Leeds goes down.

It's the perfect opposite.

In dilute sulfuric acid from 0 % all the way up to about 70 % lead is incredibly resistant.

Why?

Same mechanism.

Similar, but inverted.

It forms a lead sulfate film that is insoluble in the dilute acid.

It's rock solid.

But then, as you get into the concentrated stuff.

The lead sulfate becomes soluble in strong acid.

So at high concentrations, lead dissolves.

It is the exact mirror image of steel.

So if you're running a plant, you might literally have lead -lined tanks for the weak acid and steel tanks for the strong stuff.

That was very common practice for a long time, yes.

But lead has mechanical issues too, right?

I mean, it's soft.

Very soft.

You can't use it if there's high velocity or turbulence because the liquid will just scour that protective film right off.

It's strictly for low velocity or static storage.

And it creeps.

It creeps.

If you heat it up, it sags under its own weight like candle wax in the sun.

So you can't just build a tank out of it.

No, you have to strap it to steel supports.

You build a steel cage and then you line it with lead sheet.

Okay, so we have steel for the strong stuff, lead for the weak stuff.

What if we need something that handles everything or handles it when it's boiling hot?

That chart showed steel fails if you heat the acid up too much.

Then we enter the realm of the super materials.

The text highlights two main ones.

High silicon cast iron, known by the trade name durian, and high alloy stainless like DERM at 20.

Let's talk about durian first.

High silicon cast iron, 14 .5 % silicon.

This stuff looks invincible on the charts.

It nearly is.

If you look at figure 7 -5, the corrosion rate is practically zero all the way up to the boiling point for almost all concentrations.

It creates a silica -rich film that almost nothing can get through.

It's the ultimate shield.

What's the catch?

There is always a catch in engineering.

Oh, there's a big catch.

The catch is mechanical.

Durian is essentially glass.

It's incredibly hard and incredibly brittle.

If you hit a durian pipe with a wrench, it might shatter.

If you heat it up too fast, thermal shock, it cracks.

So you can't weld it.

No, it has to be cast.

And it's so hard you can't machine it with standard tools.

You have to grind it.

So it's expensive to shape and fragile to handle.

But for pure corrosion resistance, it's a beast.

OK, and the other one, Duramet 20.

That's Carpenter 20, right?

The 20 alloy.

Yes, that's the one.

It's a bridge material.

It's a specialized stainless steel containing nickel, chromium, molybdenum, and copper.

It fills the gaps where the others can't go.

So if you have a batch process that swings from dilute to concentrated, you can't use lead because it fails at high concentration.

And you can't use steel because it fails at low concentration.

Exactly.

You use Duramet 20.

It handles the entire transition.

It allows for operational flexibility that the other materials just can't match.

Got it.

So that is our sulfuric acid landscape.

Steel for strong, lead for weak, Dura -Iron if you're brave and careful with the wrench, and Duramet 20 if you need that flexibility.

Precisely.

You've got the map.

All right, let's pivot to the second member of the big three.

Nitric acid.

HNO.

The oxidizer.

This requires a total shift in our mental model, doesn't it?

It does.

Sulfuric acid, especially when it's dilute, acts largely through hydrogen evolution.

It's a classic acid attack.

But nitric acid is strongly oxidizing.

That changes the rules of the game completely.

How so?

What does that mean for our material choices?

In the corrosion world, oxidizing environments are usually great for materials that rely on passivity.

Passivity.

That's that invisible oxide film that forms on the surface, right?

A little force field?

And chromium is the magic element here.

In the presence of an oxidizer like nitric acid, chromium instantly forms a tight, stable, passive chromium oxide layer.

For nitric acid, stainless steel, specifically the 18 -8 varieties like type 304, is the absolute champion.

The text references figure 715 for the 18 -8 steels.

It describes a massive safe area.

It's a huge area.

You look at that chart and it's almost all white.

Low corrosion rates, less than five mils per year, covering almost all concentrations all the way up to the boiling point.

If you are handling nitric acid, your default choice is almost always going to be stainless steel.

It's the standard.

But looking at the chart, there is a danger zone above the boiling point.

Right.

If you're in a pressurized system where you can heat the acid above its atmospheric boiling point, then yeah, the corrosion rate spikes.

The chemical activity becomes too intense and that passivity can start to break down.

For most standard storage and transport.

For that, stainless is king.

Now, here's a curveball the text threw at me.

Aluminum.

I usually think of aluminum as pretty weak against acids.

If I put aluminum foil in the hydrochloric acid, it's gone in seconds.

But the text says it works in strong nitric acid.

This is the aluminum surprise.

It's another one of those counterintuitive facts.

In concentrations above 80%, so we're talking fuming, nitric acid aluminum becomes very resistant.

Why?

Because the acid is so strongly oxidizing that it overpowers the aluminum's tendency to dissolve and instead instantly oxidizes the surface.

It creates a thick, hard, protective aluminum oxide barrier almost immediately.

So again, strong acid equals strong protection.

But you have to be extremely careful.

If you dilute that acid, say you drop it below 80%,

that oxidizing power drops, the protection fails, and the aluminum dissolves rapidly.

So don't do it.

Do not use aluminum for dilute nitric acid.

It is a very specific tool for a very specific concentration range.

Okay, noted.

And what about titanium?

Titanium is generally excellent in nitric acid because, like stainless, it relies on a very tough oxide film for protection.

But there's a very specific and frankly terrifying danger mentioned in the text.

A red -fuming nitric acid.

Yes.

This is nitric acid with dissolved nitrogen bioxide gas.

If you have red -fuming nitric acid and the water content drops below one and a half percent, titanium can become pyrophoric.

Pyrophoric meaning?

Meaning it can spontaneously explode or burn.

Whoa.

Okay, that is not just a leak, that's a crater.

Yes.

The reaction becomes violently exothermic.

It's actually a classic safety lesson in the aerospace and rocket propellant industries where they use these materials.

I can see why.

So while titanium is great for standard nitric service, you have to be religiously careful about the water content in these fuming acids.

You need that little bit of water to maintain the stability of the oxide film.

Without it, the metal just consumes itself in fire.

That is a safety tip worth remembering.

Okay,

so nitric is the oxidizer and stainless steel loves it.

Now let's move to the difficult one,

hydrochloric acid.

HCl.

The headache of the chemical industry, truly.

Why is it so much harder to handle than the others?

Two main reasons.

First, chemically, it's a reducing acid.

That means it's the opposite of nitric.

It strips away oxygen.

It wants to evolve hydrogen.

Second, and this is the big one, it contains chloride ions.

I think chloride ions are famous for being trouble makers in the corrosion world.

They are the worst.

They're small, they're aggressive, they're penetrators.

They act like little needles that poke holes in protective films.

They cause a type of corrosion called pitting.

So our champion from the last section, stainless steel.

What happens to it here?

It fails miserably.

The chloride ions drill tiny holes right through that beautiful chromium oxide passivity layer we just praised.

Type 304, even type 316, they are practically useless in hydrochloric acid.

They'll pit, they'll crack, and they'll fail.

So if chromium and stainless are out, who do we call?

Who's on the team?

We need the molybdenum squad.

That sounds like a superhero team.

In a way, they are.

To fight HCl, we generally rely on high nickel alloys that are fortified with a lot of molybdenum.

The classic examples are hastole B and its cousin chloramate 2.

These are the nickel molybdenum alloys.

Right.

They have excellent resistance to reducing acids like HCl.

They work even at boiling temperatures.

Molybdenum is the key element that imparts resistance to reducing environments, and especially to chlorides.

But, and there is always a but, there's a trap here.

The text calls this the aeration trap.

This seems crucial for any engineer to understand, because it's not about the material itself, it's about the process.

This is where careers are ruined.

Seriously, picture this.

You have a tank of hydrochloric acid.

You've done your homework.

You built it out of hastole B expensive high quality nickel molybdenum alloy.

It's working perfectly.

Okay.

Then, one day, a pump seal leaps and starts sucking tiny air bubbles into the line.

Or maybe some ferric ions fio from a rusty pipe upstream get into the mix.

Exactly.

Oxygen and ferric ions act as oxidizers.

Wait, I thought oxidizers were good.

We just said for nitric acid, oxidizers help form protective films.

That's true for stainless steel.

But for nickel molybdenum alloys and hydrochloric acid, oxidizers are kryptonite.

They completely change the game and accelerate the corrosion reaction.

So the very thing that saves stainless steel destroys hastole B.

Precisely.

It shifts the electrochemical potential of the system into a zone where the alloy becomes unstable.

It changes the cathodic reaction from hydrogen evolution, which is slow to oxygen reduction or ferric ion reduction, which is very, very fast.

How much faster?

The corrosion rate of hastole B can skyrocket by 10 or 100 times just by adding a few air bubbles.

The text describes failures where pumps or valves made of this stuff just dissolved in days because of aeration.

That's insidious.

So if you can't guarantee that your acid is 100 % air -free, which is really hard in the real world, what do you do?

You have to switch alloys.

You have to move to the nickel molybdenum chromium alloys.

Like hastole C.

Exactly, hastole C or chloramate 3.

Because the chromium helps with the oxidation.

Exactly.

The chromium adds that bit of resistance to oxidizing conditions.

So hastole C is the safer all -around bed if you have mixed conditions or the potential for contaminants.

It's not quite as good as the B type in pure boiling, reducing acid, but it won't fail catastrophically if some air gets in.

It gives you a safety buffer.

That is a subtle but absolutely critical distinction.

Pure reduction.

Use B.

Might have air.

You better use C.

Yes.

And since most industrial processes are messy and unpredictable, C is often the default choice.

What if we have zero tolerance for corrosion?

Like we're making pharmaceuticals or something where we absolutely cannot have any metal ions leaching into the product.

Then you bring out the nuclear option.

Tantalum or zirconium?

Tantalum.

I've heard of that.

Expensive.

Very, very expensive.

But it is virtually immune to hydrochloric acid.

It behaves almost like gold or platinum.

It's used for things like thermometer wells or very thin heat exchanger tubes where you simply cannot afford any corrosion whatsoever.

Wow.

Okay, so HCl is tough.

No stainless.

Use nickel moly.

Yeah.

And watch out for air.

Yeah.

Now let's talk about the acid that sounds like it came from a horror movie.

Hydrochloric acid.

A chef.

The glass eater.

The one thing everyone remembers from high school chemistry class, you cannot put HF in a glass peaker.

No, don't do it.

It attacks silica.

Silicon dioxide, which means glass, stoneware, porcelain, and even our old friend high -silicon iron duran, which was our hero in sulfuric acid, are all completely useless here.

They'll just dissolve.

Rapidly.

It reacts to form silicon tetrafluoride, which is a gas.

So the material literally just evaporates away.

Even concrete.

Concrete is full of silica.

It eats concrete.

If you spill HF on a concrete floor, it is going to be a very, very bad day.

So what on earth holds it?

Surprisingly, magnesium.

Magnesium.

The white metal that flares up in chemistry experiments.

That stuff.

The very same.

In most acids, magnesium dissolves instantly.

It's actually used as a sacrificial anode because it corrodes so easily.

But in hydrochloric acid, it forms an insoluble magnesium fluoride film.

It creates a hard protective shell.

That is bizarre.

So another one of these weird protective film stories.

It is.

Magnesium is actually used for shipping containers for HF.

But for plant equipment, like piping and valves,

we probably aren't using magnesium.

It's not strong enough.

No.

For that, the industry workhorse for HF is monol.

Monol.

It's a nickel -copper alloy.

Roughly 70 % nickel, 30 % copper.

It resists all concentrations up to the boiling point.

It's tough, ductile, and reliable.

It's the go -to material.

But does it have an Achilles heel?

They all seem to.

It does.

Just like the molybdenum alloys in HCO, monol is sensitive to aeration.

It's a copper -based alloy.

And copper really does not like oxidizing conditions in acid.

So if you have air in your HF, the corrosion rate on monol goes up significantly.

What about simple steel?

Can we use plain old carbon steel for HF?

Similar story to sulfuric, actually.

Steel works in strong HF above 60 % or 70%.

But there's a specific manufacturing detail the text highlights that engineers often miss.

The steel must be killed.

Killed steel.

What does that mean?

It's an old steel -making term.

It just means fully deoxidized steel.

But crucially for HF service, you have to watch the silicon content.

Ah, because HF eats silica.

Exactly.

So if your carbon steel has high silicon impurities or little inclusions of silicates, the acid will seek those out and attack those specific spots.

So you get these little pockets of corrosion deep inside the steel.

Exactly.

It can lead to honeycombing or blistering under the surface.

You need clean, low -silicon, killed steel for it to be reliable.

Got it.

So that's HS.

No glass.

Yes, magnesium.

Monol is the standard.

And watch out for air and silicon.

Now let's quickly touch on the last of the specific acids.

Phosphoric acid.

ASP.

This one is generally considered less aggressive than the others.

The nice acid.

Relatively speaking, yes.

For phosphoric acid, the standard material is usually type 316 stainless steel.

It handles most concentrations and temperatures just fine.

For the hotter, tougher spots, you might upgrade to Durmet 20.

But there was warning in the text about commercial phosphoric acid versus pure acid.

Yes.

This is a supply chain issue that can really bite you.

Phosphoric acid is made from phosphate rock.

And that rock, as it comes out of the ground, often contains fluoride.

Ah, so you unintentionally get hydrofluoric acid in your phosphoric acid.

Exactly.

As we just learned, fluorides attack glass and silica.

So while pure food -grade phosphoric acid might be perfectly fine in glass -lined equipment, the raw commercial stuff, they call it wet process acid, might eat it alive because of that fluoride contamination.

You absolutely have to know your impurities.

That brings us to the topic of mixed acids.

This is where chemistry gets really interesting.

Sometimes mixing two nasty acids makes them nicer.

Sometimes.

It's all about the electrochemistry.

Take the sulfuric -nitric mixture.

Two of the strongest acids combined.

You'd think that's a nightmare.

You would think it would destroy everything.

But remember our lesson on nitric acid.

It's an oxidizer.

It passivates steel.

So if you mix nitric with sulfuric, the nitric component can actually help maintain a passive film on ordinary steel, allowing you to use steel in conditions where pure sulfuric might attack it.

That's that enemy of my enemy is my friend logic.

It is.

But then you have the other mixture, nitric plus hydrochloric.

Aquaregia.

The king's water.

This is the death zone.

The nitric oxidizes and the hydrochloric supplies those aggressive chloride ions.

It overcomes almost everything.

It dissolves gold.

It dissolves platinum.

Do not put that in a steel tank.

Definitely not.

That is reserved for glass or, if you must, tantalum.

Okay.

We have toured the acids.

Now I want to bring it all together.

The text provides a fantastic master chart, figure 7 -9.

It combines all the materials for sulfuric acid onto one map.

This is a great tool for visualization.

I always think of it as a territory map.

Okay, so let's build that map.

Looking at the bottom left, corner dilute acid,

cool temperatures.

Who owns that territory?

That's lead.

Lead owns the dilute cool zone, no question.

And the bottom right, corner concentrated acid, cool temperatures.

That's steel's kingdom.

Steel owns the concentrated cool zone.

Now what about the middle?

There's this narrow band crossing the chart right between the lead and steel territories.

That is the dormant 20 zone.

It connects the two.

It's the bridge that lets you operate in those tricky middle concentrations where lead and steel both fail.

And finally, the high ground.

High temperatures, strong acid.

Up at the top of the chart.

That is the domain of high silicon iron, dry iron.

When the heat is on, you need that glass -like resistance.

It really clarifies things when you see it spatially like that.

It's not random.

It's a grid with defined regions.

Exactly.

And briefly, before we wrap up, the text mentions non -metals.

Rubber, plastics.

Right, they have their place.

Rubber linings, PVC, Teflon.

They are great for corrosion resistance.

I mean, they simply don't rust.

But they have mechanical limits.

They're not as strong.

Not at all.

They melt.

They creep.

They get brittle in the cold.

Generally, you're limited to temperatures below, say,

150 or 200 degrees Fahrenheit, except for Teflon, which can go a bit higher.

So good for storage.

Maybe not for a high -pressure reactor.

Correct.

You usually line a steel tank with them to get the strength of the steel and the chemical resistance of the plastic.

This has been a serious tour.

Let's synthesize this for the listener.

If you're walking away from this deep dive,

what are the acid personalities you need to remember?

Okay, so sulfuric is the tricky one.

It's all about concentration.

Dilute eats steel.

Concentrated saves it.

And you have to watch out for that weird dip around 101%.

Nitric.

The oxidizer.

Stainless steel is your best friend.

Aluminum works in the really strong stuff, but nowhere else.

Hydrochloric.

The reducer.

No stainless, ever.

You need nickel and molybdenum.

And you must, must, must keep the air out.

And hydrofluoric.

The glass eater.

No silica.

Use monel.

And again, watch for air.

It is amazing how much personality these simple chemicals have.

You really do.

And the final thought I'd want to leave with is about the art of it all.

The art.

Yeah.

We can make a container out of tantalum that lasts forever.

It would never corrode in almost any of these acids, but it would cost millions of dollars and bankrupt the company.

Right.

Not practical.

Or we can make a tank out of skill for pennies.

But if we get the concentration or the temperature wrong, it bursts and hurts people.

The art of corrosion engineering is navigating these charts, figure 7 -2, figure 7 -9, to find that sweet spot, that perfect balance where safety meets economics.

It's a balancing act.

It's the entire job.

So next time you're driving down the highway and see a tanker truck with a diamond placard on it saying sulfuric acid, take a closer look.

Chances are that tank is just simple carbon steel.

Nothing fancy.

And the only thing keeping that acid from eating right through the metal and spilling onto the highway is a microscopic invisible film of ferrous sulfate and the simple fact that the acid is strong enough to keep it there.

It's a fragile equilibrium.

Very fragile equilibrium.

Engineering is wild.

It certainly is.

Keep those oxide films intact, and we'll see you in the next deep dive.