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Going to Congress, or actually not the Congress, the Supreme Court decided that military technology is cutting edge, and so there's always a hazard and you cannot sue the manufacturer for a design defect because the military is considered to be the designer of every piece of military hardware. Okay, and it's called — what do they call this — this principle, I can't remember. But anyway, in 1992 the Supreme Court ruled that you can't sue Pratt and Whitney for an engine design defect. You can sue them for a manufacturing defect, but you can't sue them for a design defect, because NAVAIR or the Air Force had huge design teams of all these PhD scientists who worked for the government, and Pratt and Whitney would come in and they would give a presentation — or Boeing if it's an aircraft airframe — they would give a presentation to the government, and the government would approve it. And so therefore the government approved the design, and you can't sue the government. I mean when you signed up, you sign up for hazardous duty, right?

So that's not to say you can't get something. I worked on the V-22 Osprey that crashed at Quantz [Quantico], uh, yeah, Quonset, uh, what, in Quanza — Quantico, Virginia. Ship four. And there were — four — let me get this right — four military on there and a bunch of civilians. If I remember, Boeing and Bell settled, you know, with the military people right away. But the civilians all filed suit anyway. It's a mess, but they finally — it went to trial and stuff, and we can talk about that some other time. Okay, it doesn't have to do with corrosion. So that was one of the things I was going to say. Just make sure you understand that as military officers, your lives are worthless, okay, at least from a business point of view. But you can eject and if you survive the ejection you're fine, okay.

Student: Like you said, planes are expensive, so I'm just surprised that they would even consider like a single engine.

Well, but the engines are about, depending on the aircraft — well, in a military jet the engines are probably more than fifty percent of the cost of the plane, okay. In a commercial jet it's probably twenty, twenty-five percent of the overall acquisition of the aircraft is the — uh, well it depends on the aircraft too, but the engine is going to go for five or ten million on a 747 now, or something like that. And the nacelle, you know, that holds the engine on, that goes for five million. So if you got four engines, there's sixty million dollars — that's not fifty, but — on a 747 which cost about two hundred million, you're talking a third of the cost roughly. So on a military jet the engines actually go for even more.

One of the problems — and this is really getting into a digression from corrosion — is about twenty or thirty years ago, development of military jet engines and development of commercial engines diverged. Anybody know why? Because they were looking for fuel efficiency in the commercial area, because — well, on some of the other lectures you might get some of this — but a fifty degree Fahrenheit increase in the operating test temperature of a commercial jet engine will save just U.S. airlines fifty billion dollars a year in fuel. Okay, that's a lot of money. And that's why they're always talking about fuel economy, and they retire aircraft when they — or they retire the engines — and they're always looking for the latest engines which are more fuel efficient.

Well, the problem is, in order to get a high fuel efficiency, they decided to go to what's called the high bypass jet engine. I mean this has nothing to do with corrosion but it's okay, it's interesting. So you know there's two equations: one-half mv squared, and then there's the momentum equation, mv, right? And so if I have a jet engine — the old jet engines, which are still the military engines, were just a long tube, and whatever came in with a certain amount of mass went out the back with a certain amount of mass, only it had a lot bigger velocity, right? So a lot more energy, and the energy came from burning the fuel in here. But the actual thrust-to-weight ratio and stuff is momentum. And so the problem with momentum is you didn't have a lot of mass going through this little tube. So they came up with a high bypass, and they put these big fans around here, and you see this if you go flying on a commercial jet. You see these engines that are, you know, ten feet in diameter, right? Well, the basic core engine still is this, but you also have — the engine is now driving these fans, which are pulling another mass, a bigger mass, and you get this m little v. So you get lots of high velocity with a little m, but you get lots of mass with a little v by the fans, and now you get tremendous amount of thrust and tremendous efficiency.

The problem is you can't do this with a supersonic engine, and the military engines want to go above the speed of sound, okay. Commercial engines don't want to — well, will go above the speed of sound — so the military engines still are basically the core. They're inherently fuel inefficient, okay, because they want to break the sound barrier. So what's the solution to all of this? Well, it turns out the other solution to all this: you can't run modern engines without a computer. I mean the computer is basically changing parameters in that engine a thousand times a second or faster. The airfoils that control a modern military aircraft, in order to get the maneuverability — I mean when you're going Mach 1 and a half and you want to take a right turn, you can't — no human could respond to all the control surfaces, and so you have computers running those control surfaces. And a lot of the plane designs now are inherently unstable, just like a submarine going through the water. And I — we won't talk about the fact that the deeper you are, the slower you have to go, because if you go unstable and you go deeper than you're supposed to go, it's not a good thing in a submarine.

Well, the same thing in a jet engine. If all of a sudden one of the control surfaces sends you in another direction you didn't want to go, it's not a good thing. When you're going Mach 1 and a half and you take a ninety degree turn, the g forces were such that the pilot will pass out, okay. And in fact that's one of the problems: the F-22 and the F-35 can do maneuvers that the pilot cannot take above about eight or nine g's. The brain — the blood doesn't get to the brain anymore, okay. You get to the point where the heart is not strong enough to pump it anyway. Because of that, the solution is drones, piloted as pilotless aircraft, which should be one of the solutions for the Navy to think of — ships without people.

Student: I'm sure they are.

Well, you have underwater autonomous vehicles. But, you know, I mean, do you realize what it costs to build a submarine or a surface ship that has to accommodate all these people and their dietary needs and their other needs to take showers and drink water and things like that? If you take the people off, and not only that, they won't die. I mean those people in Tampa that are fighting the war in Afghanistan, they go home, you know, regular time, and you know someone else takes over. You know, these other guys marching through FPS [FOBs?], and that's not fun at all, but in Tampa it's not so bad.

Okay, so maybe we ought to go ahead, start. Anybody have any questions on that? I mean some of this technology is actually pretty neat. But that's another one of my lectures — it's one of the problems nowadays — it's not a lecture, regular lecture, one of my diversions — is the problems with war. I mean, the one war that wiped out more — I don't know if it wiped out more, but probably more of the population of the country than virtually any other war, was the U.S. Civil War. We were killing each other and we did it very effectively. I mean if you got wounded, you were a dead man. I mean medicine couldn't survive you, you know. You just bleed to death, okay.

And then World War One was the war to end all wars, and World War Two. But as we've gone along, casualties have become less and less acceptable. When I was, uh, actually younger than you, was the Vietnam War, and they would publish the body count every day, okay, and every week, on the front page of the paper. And it'd be like a hundred soldiers a day, you know, dying in Vietnam. Well, now we've gotten to the point where — well, by the time the Gulf War, we got to the point where it's unacceptable to have U.S. casualties. And now we get to Afghanistan and it's unacceptable to have the enemy casualties. Which — wait a second, what is war about? Okay, anyway. You know what Clausewitz said, right? War is extension of politics by other means. But anyway, that's — uh, see, I did take ROTC, okay. That's what I learned often, some of these things about the philosophy of war. It's best not to fight wars, it's cheaper too.

Okay, I didn't — we can go on if you want. We on? Okay. Turns out I just happened to get this book — it arrived after class yesterday — on cathodic protection. I'm sure you all are busy purchasing books like this. But it says that — I told you about the cost of corrosion — and it says in the first study was done in 1976, and they found that it cost the U.S. seventy billion dollar loss in 1976, and this was somewhere between three and a half and four and a half percent of the country's gross national product. They redid the study in 1982 and it was a hundred twenty-six billion. So it went from seventy to a hundred twenty-six billion a year. By 1991 it was two hundred seventy-six billion. If you actually calculate that, that's higher than the inflation rate. What it is — lies, damn lies, and statistics — they were including other things. They, for example, in this they assume that every automobile that's junked is junked because of corrosion. That's not necessarily true. A lot of them are junked because of a crash, they're totaled in a crash. That's not corrosion.

However, they've done two studies, one by NIST and one by the National Association of Corrosion Engineers, and they both came up with corrosion costs about 4.2 to 4.9 percent, plus or minus thirty percent statistics, of the gross national product. If it is about five percent of the gross national product, what's that today? Anybody know what the gross national product is today? It's about fifteen trillion. So five percent of that is seven hundred fifty billion. Now do you believe that's the cost of corrosion? Neither do I, okay. But that's what the National Association of Corrosion Engineers will tell you, okay. And I'm sure they have all the data to — but they will do whatever they do to inflate the numbers, because it's in their best interest. But corrosion does cost a lot of money, and if you could stop it — but in fact, the main principle, metals don't want to be metals by nature, and so they will go back to their original state over time. And the way they do it is by corroding. Corrosion, if you look in the dictionary, is the chemical degradation, uh, the slow — it's a — corrosion is a slow chemical process. For a corrosion engineer it's the degradation over time of metals.

Now then you get into questions like Three Mile Island — the sister reactor at Three Mile Island, where they wanted to restart that nuclear reactor. They started up the steam generator, and there's a type of acid that you can only produce — you can't produce regularly in the laboratory, but under the high pressures and the startup temperatures you can generate, and you have some sulfur contamination, you get polythionic acid. It's a polymerized sulfur compound. And this nickel-based superalloy heat exchanger on reactor 2 — not the one that had all the problems, but they were trying to restart it so it could generate electricity a couple years after Three Mile Island — they started it up, and within a half an hour they had stress corrosion cracks, and it cost them thirty million dollars to repair, okay.

Well the fundamental question there, for the insurance carriers, was: if all the damage was done in a half an hour, was that a slow process, was that corrosion? Okay. The first press releases to come out of Three Mile Island, from GPU Nuclear which was the utility, the first day and a half, they called it stress corrosion cracking, SCC. By the time the attorneys had gotten hold of it, they called it stress-assisted cracking, because the attorneys went and looked at their insurance contract which excluded corrosion, because corrosion is just a slow gradual wearing away. And they knew that if they called it stress corrosion cracking, the insurance company was going to deny the thirty million claim. If they could change the name and call it stress-assisted cracking — well, what cracking is not stress-assisted? Okay. But I mean, you start getting into — this is when we get into the story about lawyers, okay. You can go back to the Bible and see what the Bible says about lawyers. But in any case, that's what they do.

The second point from yesterday: metals may possess immunity, or we called it nobility, or they can have practical nobility, which means they've got a coating of some sort of protective oxide or something else. And to give you an example of something that has a protective oxide — anybody know what this is? [Tom holds up a length of pipe.] Some of you must be mechanical engineers. You should call this a pipe nipple, okay. It's threaded on both ends. Bought it at Home Depot, cost me like a buck, okay. Three-quarter inch by six, black nipple. Why is it a black nipple? Because it's got a coating of Fe₃O₄, magnetite, on the surface. When you heat up steel to above about five or six hundred degrees Fahrenheit — and this thing gets to temperatures of eight or nine hundred degrees when they're forming it in the steel mill — it will form, not Fe₂O₃ which you know as rust, it will form Fe₃O₄ which is called magnetite, okay. It's slightly magnetic. Ever play that little game with the magnets where you put the mustaches and the beards on these little figures? You know, that's magnetite. That's another use for magnetite.

But in fact, the black iron pipe has its own magnetic, or not magnetic — it has its own inherent coating, okay. If you go down to the basement of MIT and you look up, you'll see the fire sprinkler system, and it's black iron pipe. You can tell it's a fire sprinkler system because the couplings are either orange or red, right? You've seen these Victaulic-type couplings that have gaskets. You probably have something similar types of things on board ship, okay. And it's black iron pipe. They don't do anything other than just take the regular mill finish, although this one someone put varnish on, which doesn't hurt it. But anyway, and that keeps it from corroding. How come rust is not protective, Fe₂O₃? Because it can be easily — flakes off, right? It flakes off. Rust is flaky.

So it turns out, if you can keep a protective coating on something, you can actually get pretty good protection. This comes out of one of these marine books that I showed you yesterday, and it shows you what happens to the corrosion rate in millimeters per year versus explosion [exposure] time, exposure time in days. So a year is right about in here. And if you have attached rust, the corrosion rate in millimeters per year is something on the order of a tenth of a millimeter per year. What's that in inches? You're Canadian, you know the metric system, okay. A millimeter is forty thousandths of an inch, so a tenth of a millimeter is four thousandths of an inch. I did that calculation in my head, okay. And that is the typical corrosion rate for steel in the atmosphere, or in lots of other humid or moist environments. This happens to be sea water, but — and it was probably painted, okay — but where the paint was worn away and stuff, it did rust, and it was rusting at about four thousandths of an inch per year. That is a good general rule of thumb for the corrosion rate of steel in water, okay.

The numbers I use are typically 0.002 to 0.010 inches per year — two to ten thousandths of an inch per year. So sometimes people come to me and say, oh, we had this piece of black iron pipe which is about an eighth of an inch thick and it corroded through in one year. And I said, no it didn't, okay. It should last ten years even in a very severe ten-thousandths-an-inch [environment], and at four thousandths of an inch per year it should last about twenty-five years. And those of you who have been aboard ship and you have carbon steel piping, how long does it last? About twenty-five or thirty years before you really need to sort of get rid of the whole thing. And that used to be the limiting lifetime on ships, okay. But then someone at NAVSEA said, oh, we need ships that will last fifty years, right? Because we can't afford to replace them so often.

So — I'm not sure if the person understood what happens to carbon steel, but eventually they learned that the carbon steel is worn out after twenty-five or thirty years. And so they replaced it with — you've been at the shipyards — what types of metals are they replacing the carbon steel piping with?

Student: They might be on some of the modern ships that are mostly aluminum ships.

Aluminum actually is what we build most sewage treatment plants out of, okay. So aluminum actually has very good long-term corrosion resistance — and not so in the good water — but on nuclear subs, it's, they first went to cupro-nickel, Monel. Now they're into titanium.

Student: Depends on what the piping system is, I know.

You don't think — what?

Student: Well, there are, you know, at least, you know, since in my experience in submarines, that you know, if the system is built one way it's pretty much going to stay that way. I know like some of the seawater systems have gone titanium, and some of the kind of special unit, more specialized builds.

Well, they are using a lot more titanium than they were forty years ago, believe me. I mean forty years ago they just assumed you scrapped the ship after thirty years, and they would go with the lowest cost, which was carbon steel. Now — and actually it's true even on subs and critical systems like the reactor systems and stuff, they would use cupro-nickel or Monel in critical systems — but there's a lot more titanium in piping now than there used to be, okay. There was no titanium twenty years ago in piping on ship, in general. But the problem is — and you all know the problem is — titanium is, as fabricated, you don't have to use something as thick if you don't need the pressure, but the cost is still a factor of ten, twenty, or thirty times carbon steel.

Student: We saw in one of your slides yesterday that sewage systems are very susceptible to corrosion. Yep. So how come we don't use like plastic piping on ship or something for that?

Well, there's a couple of reasons about plastic piping. Can I hold that just a second until I finish? Let me just finish this. Is, if you look at the top of this, is up in the shear strake region — you know, the top part of the ship where it's bowing from bending — and the rust is flaking off, okay. And it's the detachment of the partially protected rust. So, let me — okay, so why haven't we switched to plastic? We have been switching to plastic. It turns out plastics are sort of a recent development, okay. The history of polyethylene, which is one of our most common plastics and one of our cheapest plastics — polyethylene really didn't exist in World War Two. I mean they knew about it but it wasn't a commercial development.

In the 1950s, I can't remember which big plastics company — what is now a big plastic company — tried to build a plant because they could. They decided they're going to be able to produce hundreds of millions of pounds of plastic polyethylene every year. And they started producing it, but they didn't really have a market. And their original quality of manufacturing of this was sort of variable. And so they tried to take it into somebody and say, oh why don't you use this for plastic pipe for whatever the application was. Because it's easy to extrude, you form everything at, you know, temperatures you hardly burn yourself in the plant because everything's low temperature. Well it turns out, all the production trials they were running, they couldn't sell their junk because it had no consistency. They hadn't come down the learning curve of manufacturing to get consistent quality.

And then a magic thing happened. Someone thought of an application for all these fifty million pounds of scrap polyethylene that was sitting there. And it was called the hula hoop, okay. It had no real requirements other than it be a round tube, and it could be made in different colors and stuff. And in the 1950s the hula hoop became the big craze. Anybody ever used a hula hoop? Okay. Well all of a sudden they were able to sell their fifty million pounds of polyethylene, and start producing more polyethylene, and over time they learned to make it a better quality, okay.

The problem — and I went looking this morning for a piece of polyethylene, actually a special type of polyethylene — problem with polyethylene is at about a hundred forty or a hundred fifty degrees Fahrenheit it will creep, it will bulge, it will get an aneurysm, and it will explode on you, or you know, it'll burst and you'll have a flood. So people tried better things — so people tried better things like polybutylene, okay. You know, longer chain hydrocarbon. And they put that in lots of houses. Anybody ever have a house that had polybutylene fittings? Yeah, what happened?

Student: [PEX?]

No, PEX is cross-linked polyethylene. I'll talk about that in a second. That was what I was looking for, is a piece of PEX. Well, PEX actually — the PEX tubing is fine. Some people are blaming the fittings. There's a billion dollars worth of lawsuits out in Las Vegas and the West Coast on PEX fittings, brass fittings, but that's another story. For which is a corrosion problem called de-alloying. Maybe we'll talk about it when we get there, to dealing-type of attack. But so anyway, they put in polybutylene, and the fittings are susceptible to stress corrosion cracking. So I talked about metals, you know, energetically unfavorable, they want to go back to nature. And then it turns out, if I went back thirty, forty years ago, people said, oh well, plastics don't corrode. And they said ceramics don't corrode. Well, that's not true. They corrode by different mechanism. Metals corrode by going back to their oxide or their sulfide. Plastics can degrade, and this is not necessarily by a chemical process.

So maybe the corrosion engineers were right, as they started getting into the degradation of these other materials. You can have separation, you can have cracking. The polybutylene fittings started cracking. People had mold growing in their walls because they'd start leaking. I had a daughter who about 1998 bought a condo down in Raleigh, North Carolina, and it had polybutylene, and she knew about it, and she got in just under the wire of the class action suit, and they paid her twenty-five hundred dollars to have all the fittings replaced, okay. So I mean it's not the end of the world, but they don't use polybutylene because it stress corrosion cracks in certain types of water.

One of the best stories about plastics corroding was DuPont came out with something called Delrin, which was poly — you can call it polyformaldehyde, it's called acetal, but basically it's polyformaldehyde. Formaldehyde's one of the simplest hydrocarbons, okay. What is it — C, CH₂O or something, I think that's formaldehyde, you can Google it. But I think it's CH₂O, and if you polymerize this, you can make a ten thousand psi plastic which is very strong, easy to injection mold. And DuPont came out with a paper in like 1958 that tested this polyformaldehyde in two hundred different solvents, and it was resistant to all these chemicals, and this was the plumbing material of the future.

In 1962, some Germans did a study and showed that over the long haul, at the types of stresses you had, you would get a form of stress corrosion cracking in polyformaldehyde with small amounts of chlorine, okay. So about fifteen or twenty years later, Hoechst — we know — paid a nine hundred million dollar class action lawsuit to take Delrin off, or to pay for all the Delrin failures. What they didn't call it Delrin, they called it their trade name was different. DuPont paid an even bigger settlement, well over a billion, but it's confidential how much they paid in their settlement for Delrin, okay.

Now it turns out, Delrin — the problem with Delrin is a little bit of chlorine. Now how much chlorine? Ten years ago if you went to the DuPont website and it listed the materials that it was resistant to — oh, by the way, the 1958 paper, the one solvent that they didn't test this material in was water. They actually never tested it in water. So you look at all these organics and stuff, they just never — this was the plumbing material of the future, and they figured the water wouldn't corrode it. Ten years ago you went to the DuPont website, and you looked at Delrin and said water, and it said corrosion resistant. And they had an asterisk and said with less than half part per million.

Now how much — you know, you want to make coffee out of the Cambridge water this morning, how much chlorine is in the Cambridge water? Anybody have an idea? Well, it's basically rain water, but it comes out of Fresh Pond up here since the 1640s, and it has five or ten percent chlorine. So if you have water that's cleaner — ten times cleaner than what you get out of the streams and brooks, but really good water in the Northeast will be five or ten ppm chlorine — and DuPont says it's corrosion resistant to something that is ten times better than some of the better potable water. The EPA says you can have up to a hundred parts per million chlorine and it's still potable water. So if you're in San Antonio or Las Vegas or something, you could have seventy, eighty, ninety parts per million chlorine. Delrin is not so good, okay.

Well, what do they make out of Delrin? They had the little water closet valves. You know, you flush the toilet and you know it fills up with the float and everything and then it shuts off. Well, they were making those things out of Delrin. And after about two or three years, the chlorine in the water would cause the Delrin just to turn into a brittle — just full of little cracks. And they had what they called the exploding toilets, okay. Because when this thing would let go, the sixty psi water pressure would hit the little ceramic top to your water closet, and it would go — you know, you'd hear it blow off. It wasn't really an explosion. It wasn't a release of gas pressure. It was water pressure knocked the top off, and it was ceramic, it fell to the ground. You hear the crash of the breaking top, and then you have a flood in your home, and people would be in their home, they'd hear this loud noise which they called an explosion. And so you had what they called exploding toilets back in the 1980s from Delrin, okay.

So plastics do corrode, many times in very subtle ways. Some plastics will stress corrosion crack in very clean water, nearly pure water, okay. Water can attack the plastics. Same thing with ceramics. They often want to use ceramics at very high temperatures, like in some engine part or something. Well, sulfur compounds just destroy ceramics in general, okay. So these other things do corrode. They — environmentally maybe they environmentally degrade rather than corrode. But why don't they use it now — the PEX? PEX is cross-linked polyethylene, and there are two types of PEX. There's PEX-A and PEX-B, in that clever way to call it. So PEX-A — PEX-B is electron beam polymerized, okay.

Remember Charles Goodyear and sulfur and putting it in latex and making rubber, okay? You know, that's how he learned how to make rubber, you know, in the 1830s or whatever, by putting some sulfur in with the latex, and the sulfur would create crosslinks between the long polymer chains. I'm not getting a lot of nods here of remembering Charles, okay. Well, it turns out, uh, Charles Goodyear, I guess, got a patent on it, but that's because he didn't know that the ancient Mayans or whatever actually used to use the — it's a vine that had natural sulfur compounds — and they would mix that, and so their ball, remember they used to play this — what they call it — was the game they played that was over miles and miles with this rubber ball, and things down there. Anyway, Mike Tarkanian, who runs the lab over here, for his bachelor's thesis was the one who rediscovered the fact that the Mayans from thousands of years ago used the morning glory vine, a type of morning glory that had natural sulfur compounds. They mix it with the rubber and they get crosslinking.

Well you got these polymer chains, which in polyethylene — polyethylene is basically just C with a bunch of H's. So you got a backbone of carbon, and it could be ten thousand carbons long, with hydrogen, and then at the end you have to have another hydrogen. But in fact — so we've got hydrogen — it's just a chain of car— it's actually C₂ₙ — eight, no, Cₙ — H — 2n plus 2, okay. So for every carbon you got two hydrogens, and you have to have a hydrogen on the ends, both ends, so it's 2n plus 2. That's the formula for polyethylene. Has the exact same formula as paraffin wax. And so one way to tell if something is polyethylene is you just take a little piece of it, you burn it, and see if it smells like a candle, okay. This is actually one of the scientific tests for polyethylene. You need to understand these sophisticated tests we use in the world. Anyway.

PEX-A, they basically get crosslinking by hitting it with electron beam, which breaks some of these bonds, and you can actually get links, and now you have a carbon coming off this way and this way in this way. Well, you can't have that, anyway. You can only have two carbons at a time, but you can have crosslinks with carbon, and you get the same type of thing as taking latex and vulcanizing it, making rubber, okay. So they can do it with an electron beam, or they can do it chemically where they add compounds, and the chemical compounds help break down some of the polymer chains. Here they're just using a lot of energy to break the polymer chain and hope that it reforms, and it does. This is made by a company called Uponor, this is made by almost everybody else. And now you have enough strength in all directions, and these things don't just burst on you.

So the best anyone knows on the PEX, which has been in service for about twenty-five years now, and it shows no evidence of deterioration — the PEX will probably last about a hundred years, okay, or more, okay. But the best estimate right now — but the people are complaining about the brass fittings, but we'll talk about that when we get to the alloying. It's convenient. Well, in fact that's why people like it, because, A, you don't have to use much copper. I mentioned yesterday some people are tearing the copper out of their homes to sell the copper and putting PEX back in, okay. So they're taking the investment value in their home, the copper pipes, and replacing it with PEX.

The problem is, some attorneys in Las Vegas got about a quarter billion dollar settlement for corrosion of some of the brass fittings, and so now there's a feeding frenzy by attorneys claiming that this causes leaks, okay, except no one's ever seen one of the leaks. But they corrode — they — anyway, we'll talk about de-alloying later. They do get corrosion in Las Vegas. Water is not the best water in the world, okay. You can drink it, but it's not good for your pipes. But that's another story, okay.

So the scale of the point here is the scales can be protective. We sometimes put on artificial scales which are called paint, okay, or galvanizing, which will try to protect things from corrosion, for the protective — for the general corrosion protection. Now the next thing about corrosion, here, third point, is you need free oxygen or sulfur to get corrosion, okay. That's the way it's found in nature. And if you're going to have a metal corrode you need some free oxygen. Now what do we do to prevent that? Or actually, let me put it this way: if I have black iron pipe, I can make my boiler system, my heating system of my home out of black —