§1. Pourbaix and the thermodynamics of corrosion [00:04]
There was a man in Europe after World War II in Belgium, his name was Marcel Pourbaix. They had no money — in Europe after World War II, they barely got by — and the US Navy started the Office of Naval Research and actually started funding some work. We brought back a lot of the scientists, the von Braun rocket scientists for the Germans. We brought back hundreds from Germany, but not everyone came back, and they had no money to research in the laboratories.
The US Navy started the Office of Naval Research in London and used to fund projects in Europe with some of these top-notch guys who didn't want to come back. Marcel Pourbaix, instead of being an experimentalist, decided to calculate the thermodynamics of corrosion. He called it Atlas of Electrochemical Equilibria in Aqueous Solutions — aqueous means water. Corrosion is an electrochemical process, a chemical process. Most corrosion is also hot corrosion which is not electrochemical. But in an aqueous solution you've got metal ions, and most metals want to go back to their native state. The only metal that is not an oxide or a sulfide in nature is gold. You find metallic gold, you can find native copper, but most metals are in oxide form.
So Pourbaix came up with — and he did this for a number of years — the techniques to calculate diagrams. Now we call them Pourbaix diagrams. Here's a Pourbaix diagram for iron. The thermodynamic data was available in literature, and he ended up coming up with the way to calculate a diagram like this. You're plotting two things here.
You're plotting the pH of the water — seven is neutral pH, zero is really acidic — and this is a log scale. pH goes out to 14, which is considered basic. This is acidic, this is basic, neutral 0 to 7 the way they define it, versus the electrical potential. This goes up to 1.8 volts, this goes down to minus 2 volts. The stability of water is between these two lines. When you were in high school you took water, ran an electric current through it, you made oxygen at one electrode, you made hydrogen at the other electrode. Hydrogen was at the cathode and oxygen was at the anode. So this is anodic up here where things tend to corrode, and down here is cathodic where things tend not to corrode. You can calculate things that aren't even in their stable state, because you're just doing a calculation.
Every metal wants to go back to its natural state. In the beginning of that book he tells you how to apply this to corrosion problems, and to a bunch of other types of problems. We have lots of places where we deal with the reactivity of something in a wet environment. Most of the time we see hot corrosion, but you can do this for every element. Here's zirconium. Remember these two black lines are the stability of water — oxygen up here, hydrogen down here. Above here is stabilized zirconia. There's corrosion by this solution, corrosion by gasification, passivation by oxide or hydroxide layer. Zirconium is completely passive, which means it's got an oxide scale that protects it from corrosion in water between pHs of about five and ten.
Aluminum is not quite as good as zirconium. Hafnium's better. Aluminum, beryllium, titanium. Titanium is almost completely impervious to corrosion in an aqueous environment. That's why we make medical implants out of it — that's an aqueous environment. Now if you start adding other things like chloride ions, sometimes you shrink these regions down by quite a bit. This is why stainless steel is corrosion-resistant: chromium oxide, over a very wide range from very basic like pH of 12 over to pH of four or five, chromium oxide doesn't get attacked. This is theoretical, perfect. You just take an element and say, how is corrosion resisted thermodynamically? Is it energetically favorable? This is basically putting the oxide on the surface in the range where it's protective. Corrosion is up in here, but not down in here. That can change under certain conditions and then you have to understand what those conditions are. That's the thermodynamics approach.
§2. Kinetics, nobility, and the iridium engagement ring [06:41]
So by 1950, we knew the thermodynamics of corrosion. What we don't know, and we can't predict with any regularity, is the kinetics — how fast something corrodes. Zinc actually forms a protective layer. Why does galvanized steel last so much longer than regular steel? The zinc corrodes kinetically at about one-fifth to one-tenth the rate of straight iron. But these diagrams and the thermodynamics, they're not kinetics. Kinetics is time-dependent. Thermodynamics is stability — if it's stable, it's stable for eternity in theory, unless it's metastable. We have an atlas that tells you everything you need to know about thermodynamics. It tells you nothing about the kinetics.
My rule of thumb is carbon steel corrodes at about 4,000 microns per year in the atmosphere in New England. You put a piece of steel out there, come back a year later, you'll have lost four thousand microns of thickness in rust. So if someone comes to me and says, we have this quarter-inch steel pipe and it corroded in one year, I know that for it to corrode through would take maybe two to two and a half years even with chloride ions speeding things up. The quarter-inch steel pipe — if you start thinking about your seawater piping on some of your older ships where these are carbon steel, you can project why you have 30 years life with this type of thing. That's the corrosion rate of iron without any protection.
So we know how fast something corrodes in a given environment, and there are lots of databases that will tell you about those things. Pourbaix has a chart where he talks about thermodynamic nobility and practical nobility. Thermodynamically, what is the most noble metal? The most noble is gold, and the least is magnesium. I told you, we don't use magnesium for structural things because of corrosion. In fact it's not just bad, it's worse. Take virtually anything you want on the periodic table — magnesium is worse from the thermodynamic standpoint. Number two is iridium. Number three is platinum.
That's why when I made my wife's engagement ring I electron-beam melted an iridium alloy, because I happen to know it's immune from corrosion. Anyway, I'm resorting to a story, but there's more to that story.
I was working on the electron beam melter where the last time someone got killed in the laboratory was Christmas Eve of 1962. They used to have sparks here, and they knew it was this one wire. He reached out — it had been raining that night, Christmas Eve, the soles of his feet were wet — he reached down to pull the wire away so it would stop sparking, and electrocuted himself. Six thousand volts. In '73 I was using the same piece of equipment. I'd be sitting there for a couple of days, and I'd hear a zap inside the vacuum system, melting platinum-iridium. Of course there's a Faraday cage around it. This went on for about three weeks while I was working on it, before the technician finally realized we had to fix this. I didn't tell my wife the story for about 32 years.
I heard the zap and we finally found inside — this was an Army surplus power supply they were using from the Korean War — they had these screw-on Cannon connectors. Inside there were two leads in an insulated area, and there was arc tracking between them. It was zapping inside the Cannon plug. So I simply said, we'll just fix that.
Anyway, you've got a whole bunch of things. The best practical nobility in terms of passivation — meaning you form a protective oxide on the surface — is chromium, then niobium and tantalum. They form such stable oxides. Tantalum capacitors are the best capacitors for long-term stability. We use them in computers, storage devices. Tantalum capacitors form a tantalum oxide film, extremely stable even at very high voltages.
If we go back to the Pourbaix diagram, you can decompose virtually any metal with two or three volts. I was working with Johnson & Johnson back in the '80s, and they use electric cautery to cauterize a wound. They go in there with a couple of electrodes and 100 volts, pass the current through your skin, and it basically dries up the blood and the proteins. In cardiac surgery, you've got a leaky vein or something, blood all over. What's to stop the bleeding? They go in there with these two little probes, put a hundred volts there, and essentially clot the blood right there locally. But one's positive, one's negative, and the anode — which is positive — was always corroding. 100 volts will corrode any metal. They were using stainless steel and nickel alloys, and all of them were corroding. What are we going to use? I said nothing. Because if it's a metal, two volts will corrode any metal. Not even platinum — you name it — because of the thermodynamic immunity, you can't get the stability of the oxide or anything else.
I said, well yeah, you can use gold, but even gold will dissolve in chloride solution. Aqua regia is just an oxidizing solution — hydrochloric acid, nitric acid mixed together — and it will dissolve gold. So if you take the chlorine out of the person you could use gold, but then they'd be dead, because you're made out of salt water. So there was no solution. I said make the tip cheap, and make it disposable — they're going to corrode no matter what.
So you can see things change places. Hafnium, beryllium, very reactive metals, aluminum — they give corrosion resistance from a very stable surface oxide.
§3. The eight forms of corrosion [15:15]
I used to hand out a whole chapter out of a book by Fontana and Greene called Corrosion Engineering. The outline of this 400-page book has sort of been inserted as a classic — it comes out of Ohio State, which has had a big corrosion center for 60 years. Their index has the eight forms of corrosion. And the National Association of Corrosion Engineers came up with this pamphlet, Forms of Corrosion: Recognition and Prevention by C. P. Dillon. It's a different approach — he's a stainless steels expert — but in here he talks about the forms of corrosion.
The eight forms are: uniform general attack, where the surface just eats away. We know that on steels as rust. You have localized attack, which is localized corrosion or pitting or crevice corrosion. It's localized because you get a different oxygen potential inside the crevice or in the pit, and for some reason that surface oxide breaks down. Chlorides will break down the surface oxide, among other things. Stainless steels pit in seawater terribly, but they don't rust generally in seawater.
Then velocity phenomena. Erosion — that's when you lose the copper oxide on the surface, we've talked about that. Cavitation — a big problem in propellers. At certain velocities on a propeller, one side you're creating suction, the other side creating positive pressure. They have to balance across that propeller, but on the suction side you may get to the point where the water vaporizes, and you get little bubbles. When those bubbles collapse, they reach a singularity — just like that wave of water forming, everything coming to a singularity point. You can get 10,000, 20,000 PSI pressures when that bubble collapses.
If you've ever seen an ultrasonic cleaner — the water bath you throw rings into — you're creating cavitation. Even when you can't see the bubbles, you're getting microscopic bubbles that are pounding the surface with 10,000 to 20,000 PSI pressures, and that can mechanically beat away the protective surface oxide. It's a problem for sonar, it's also a problem for propellers — microscopic wear that wears away some of the oxide and causes corrosion.
Intergranular attack — we talked about that, sensitized steels, weld decay, sensitized things. There are other things like exfoliation, where you actually grow between the pancake layers. This occurs in aluminum alloys, but we know how to control it with the proper heat treatments. I've only seen two cases of exfoliation corrosion in my life. One of them was a 1948 seaplane that crashed in Miami harbor about four or five years ago — it was in the news a lot.
The other one I saw was a guy who decided to build his own — what's that propulsion called? — water jet pump. Jet-powered, where you don't have a propeller, you just pump water out of the back end. So this guy up in Maine decided to build his own fishing boat with a water jet pump, and he decided to make it out of aluminum, and he had enough velocity coming through that water — it's seawater — that he ended up with exfoliation. The hull came apart within six months. So changing velocities and things like that, you can set up different potentials, and exfoliation can occur. He could have used something else when he had that problem.
Dealloying attack happens in copper-zinc alloys for example, copper-aluminum alloys, a few other alloys, where one very reactive metal — zinc in brass alloys — can be leached out, leaving behind almost pure copper residue. That's very porous, very weak. It's a very slow process. The only real failure I think from this was a 30-year-old plumbing valve down in Dedham, Massachusetts. It had been leaching away over 30 years and just lost most of its strength, caused a flood, half a million dollars damage.
Stress corrosion cracking — we talked about that, stainless steels, certain alloys that are almost immune to corrosion. Corrosion fatigue is basically the same thing. When you have dynamic stresses and you get a fatigue crack, they can grow much faster.
§4. Brazilian helicopter rotor and the corrosion fatigue diagnosis [21:22]
I once had a problem with a helicopter rotor blade down in Brazil. These guys decided Bell Helicopter was charging too much money to do the work on the rotor blades — a critical element — and Bell recommended you send it back to a Bell-authorized service center. But they're pricey. So these guys decided to do their own repair. They took off the rotors, ground down the paint and any dings in the blade. They kind of blended it smooth, figured they got rid of any stress concentrations, and then they didn't do a very good job painting it again. They went to fly it, and within a couple of weeks the blade flew off, killed a few people.
The Brazilian transportation authorities and the US authorities and Bell all determined, oh yeah, we've got a problem with the design or something, and they issued an alert service bulletin. I came and I looked at the fracture mechanics, and I said no, you could never grow that crack from corrosion pitting unless it was stress-assisted. Because if you look at the final size — the stress on the rotor blade has like a factor of 10 safety factor in stress, and the final ligament was not very big, so you knew how much stress there was. And you look at the size of the original flaw that everybody was pointing to — they thought it was corrosion pitting — did the fatigue crack even fit? No. It grew through corrosion fatigue, increasing the crack growth rate, not just pitting growing — there was stress corrosion cracking. So they were increasing their specs to look for finer and finer pits, and no — you have to not let people put it back in service without paint on. You've got to protect it from corroding.
So with corrosion fatigue, you get an interaction between the stresses and the corrosion, and cracks grow fast. High-temperature attack, internal attack — basically failures in the scales that form. Rust can actually be very protective.
§5. Rust as protection and the Mass General subway station [24:01]
This is a kind of marine handbook, a corrosion prediction of ship steel in seawater with and without flexing. This is without flex — a clean steel starts rusting very quickly, but after a hundred days or so, the rust basically starts becoming just as protective as paint. With flexing, you get very high corrosion because it's flexing and you're breaking off the rust. One of the things about paint is it's got to be flexible enough to stretch with the metal a little bit. So you can have the same high corrosion rate locally — you can see the spikes in here — but you have to ask yourself if removing rust is good or bad.
They were redoing the subway station down here at Mass General right across the river. The guys doing it were at the firm I used to consult with — civil engineering, back 60 years ago — and they called me up. They said, this is 1910, they built this subway station, the old rhythms, sealants up. They had areas of rust that were half an inch thick. It replaced the beams — in some cases it held the concrete. They go in there and chip out all the rust down to white metal. They called me up, said, Tom, now what do we do? I said, you better plug it up with something, because that rust is keeping it from growing. It's kept it from growing for the last 18 years.
When you grow rust, you're breaking water down into hydrogen or oxygen, and some of that hydrogen can get into the steel and cause hydrogen embrittlement. I've had a couple of cases where people used high-strength bolts at some pier in New Jersey or Connecticut, and they figured, oh, we'll use 140 ksi bolts, different grades of bolts. They put them in there and didn't do anything to give them cathodic protection. They might have zincs, but the same stuff doesn't always cover as well. Within a year or two these things are breaking — hydrogen embrittlement.
Hydrogen embrittlement usually occurs in a couple of weeks if the hydrogen was in there to begin with, but as it corrodes, the hydrogen gets in there — maybe in small amounts, but it still helps that crack grow. We actually call it static fatigue — we do static fatigue tests in welding. You take a threaded rod like a bolt, weld over the end of the bolt, so you have a natural notch no matter how deep your weld metal goes. You've got a notch right there at the fusion line because it's a threaded notch. You put a load on it, a static load, sixty, seventy percent of the proof strength of the bolt, and you wait for it to fail. Today's about 72 hours to look for cracks. We measure the number of hours to see how severely brittle it is. It's called the implant test, because you implant a steel bolt in a hole in a steel pipe, weld over the tip, planting the bolt into the steel.
There's a simpler test for regular bolts that come out of heat treatment where they may add zinc. You have a plate, you torque them down to the proper torque, on a plate with a bunch of threaded holes you put eight or 12 of them in at proper torque, you come back three days later. The electroplating operation will introduce hydrogen. They're laying down positive zinc, positive cadmium, whatever you're plating, and that will cause the hydrogen to go with it right into the steel. It's not big new science to know that positive ions all go to one location, and hydrogen positive ions go the same way. So if you're going to do plating, you're going to have to bake out your steel.
§6. The Canadian helicopter spline case and professional duty [28:54]
I had a case — this was a high-speed spline that goes from the engine to the transmission of a helicopter. A three-quarter-inch diameter high-strength steel tube. To prevent wear on the splines, just inserting a spline, male into female, this female spline, when they made it, they wanted to nitride the surface of the steel — make it very hard, very corrosion-resistant. They had a spec that after they took it out of the nitriding bath, they had to bake it for 24 hours, or four hours, to get rid of any hydrogen that might be there.
But they decided they needed to know at this aircraft manufacturer whether they had the proper thickness of the nitriding case. So they would go through the whole process, they'd cut a piece off one of them — destroy one part — cut it, send it to the met lab, and three days later they would get the analysis back to tell them if they had enough nitriding. And if they had, they would then do their hydrogen evolution heat treatment. That's sort of like preheating the part after the damage is done.
When I saw this — it turns out this was a Canadian case — the Canadian member of the team just could not see this, because the aerospace company basically said, hey, ITAR, traffic and trafficking in arms, just like the Soviets. So they hired me because I was a US citizen and could read confidential documents. But the Canadian member couldn't. So I read the process spec, and they had this three-day delay, and this is one where I had to call up the attorneys and say, look, you better tell the attorneys for this aerospace company they've got a problem that could kill some more people. This is why they were getting embrittlement cracking on their splines and losing power to their transmission in their helicopters.
My duty as a professional engineer is — the primary duty is to public safety. People could die. Even though I was involved in some sort of lawsuit between the helicopter company that lost a couple of employees and the aerospace company that made this engine, I couldn't wait until three years from now when they try to solve the problem. But I couldn't just go and contact the attorneys on the other side — that's against the bar rules. So I told the attorneys, I said, this is serious, you have to tell them, you've got to get back to me and let me know when you told them they have a serious problem. And they told them. The attorneys on the inside didn't like that at that point. I've had this type of situation two or three times in my career where you can't just wait until everything is over to tell people they have a situation. You have to.
§7. Seawater, oxygen, and the warehouse sprinkler system [32:33]
There are some things you want to know about seawater corrosion. First of all, this is the corrosion rate of AISI 1010 steel, or other steels, as a function of oxygen — corrosion rate in millimeters per year.
This is depth of the ocean. At the surface, you have a lot more oxygen present, and your corrosion rate of your steel basically tracks the amount of oxygen in the water.
As I always say, the reason we have iron piping in our boilers in our homes is because you shouldn't be getting any oxygen in there. It's a closed system. Once you consume the oxygen that's there, there's no makeup water to bring in new oxygen. But our potable water piping is full of oxygen and there's corrosion. So we have to use copper with better corrosion resistance, higher cost, or plastic nowadays, with the problems with flexible piping in many cases. I always say, if the fish can't breathe, the water is probably safe. But if the fish can breathe you've got to do something — you can kill all the fish in your bottled water boiler.
One time I got a two-inch diameter pipe, maybe one and a half inch, completely occluded with rust. This had been in a big warehouse, and they told me that all the pipes — this is a huge warehouse — everything was occluded. They were going to have to replace all the steel piping in their fire protection system. They wanted to do some tests. I called them up and said, you guys have a leak in your system? Oh yeah. They had a leak, a stream of water that they had just running into a drain. They'd had it for a couple of years and they didn't fix it. I said, well you're going to have to replace all your piping because of that stream of water — the makeup water is bringing oxygen into the system and corroding it. You were supposed to have a closed system that didn't have a lot of makeup water. You can tolerate a little makeup water — you always have a little bit of oxygen — but you don't just let a leak go. They just didn't bother to fix this thing. I said, well, too bad, you've got to replace all the pipes. An awful lot of iron oxide.
So that's why with a nuclear reactor support, when you shut it down you make sure you get rid of the oxygen. Startup of commercial reactors — they basically boil the water in a separate system before they turn the reactor on, to get the oxygen out. They do something other than hydrazine — they do belts and suspenders, three of them, to suck the oxygen out before the criteria are met. The whole plant can be there. I don't know or care how you do it exactly — they do that for commercial reactors.
§8. Hot corrosion and the Mexican airline engines [36:13]
This is an example of hot corrosion at high temperatures, not aqueous systems. This is what a turbine blade is supposed to look like — this came out of a commercial airliner engine. This is hot corrosion by sulfur and oxygen — sulfur and oxygen are the two things that create problems. Here's the plot — and you have to notice there's a break here. This is 200 up here, there's a break from a hundred to 180. This is the corrosion rate, it goes up by a factor of a hundred in a very narrow range. Usually you're operating well up here. It's too hot for the sulfur oxide to condense on the surface. If you look at where it corroded, it's all kinds of bubbles — those are sulfide bubbles or nickel sulfide bubbles. Over here the kinetics are too slow, but it can still occur. But in a very narrow range of a hundred degrees centigrade or so, you have really appreciable corrosion.
What we usually do at start-up — as you start the engine, you don't power it up right away, you let it warm up for a couple of minutes before you take off. This happened to be a Mexican airline. Fuel was high, and they were basically trying to save energy — they were starting their engines and going to full power for takeoff without warming up their engines properly. Instead of takeoff power getting them up out of this range where you don't have corrosion, they're getting a couple of minutes in this range — the corrosion range — because they hadn't preheated their engines. Little changes sometimes make big differences.
§9. Principles of corrosion and Tufte on PowerPoint [38:27]
This is my one-page principles of corrosion. Everything you need to know about corrosion: most metals except gold exist as oxides and sulfides, that's what we mine out of mountains. Metals possess a varying nobility — either immunity or protective oxide scales. That protective oxide is what we're dealing with. Anything that destroys that gives you pitting, general corrosion, stress corrosion cracking, whatever. It's those protective scales that are making things stable — chromium oxide or aluminum oxide. The reason we use aluminum bronze for propellers is because the aluminum forms a very protective oxide. Free oxygen or sulfur is required for corrosion — if fish can breathe.
Moisture provides the electrical pathway for chemical charge transfer. We store aircraft — aged aircraft — out in the desert outside of Tucson, because it's dry. Moisture causes corrosion. We have acres and acres of hangars to store all these old aircraft — 1952 B-52s or 747s or whatever — we just store them in the desert. Corrosion occurs less in the desert.
So moisture is a problem. Localized corrosion is due to electrical differences like crevice corrosion or exfoliation or pitting. We have little copper areas in aluminum alloys and you get cathodes and anodes. Velocity phenomena are due to mechanically breaking up oxide scale. Dealloying is selective leaching. Stress corrosion cracking, we talked about, and hydrogen, you know that by now. So the next three minutes — any other questions?
One last thing. I mentioned this to you before, with the students who do the presentations. I often hand out this paper by Tufte, who's a professor at Yale, who has made millions of dollars by teaching people how to communicate better. He despises PowerPoint. He wrote this paper — it should be on your Stellar site — where he took PowerPoint with auto-formatting, put in some prose, and let it make up its own PowerPoint slides based on this sort of artificial intelligence. He'd say it's artificial. This is the Gettysburg Address in six PowerPoint slides.
So here's the PowerPoint of the Gettysburg Address. Title, author, organizational overview, 1887. Dedication, a portion fitting, unfinished work, great task — review of key objectives, critical success factors not on the agenda. And summary: new nation, Civil War, dedicate field, dedicated, new birth of freedom. This is what PowerPoint can do for great prose.
So think about this. I'm not trying to make you guys feel bad, but you need a little help on your PowerPoints. The best speakers use no overheads. They just keep you interested with what they're saying. PowerPoint is a crutch where we just read back what's already on a slide, because we haven't really got a lot to say. Some people use things that are too small to see — I've done that, I just did that a couple of times here. But if a good presentation has a slide that was too small to read, why are you even showing it? Some people put something up for two seconds — why bother to show it? I would say fifty percent of your slides really didn't need to be shown.
In many cases it's the old story — I can't remember who said it, but the person was asked if they would give a talk on such-and-such, and they said, I can give an hour-long talk right now, if you want a fifty-minute one, give me a week. It's a lot harder to prepare a short presentation than a long one. I brought it up now rather than to an individual, because they might take offense. Quietly off-handed.