Brian Lex
Chem. 405
Project Paper
Corrosion is a very natural and annoying process that occurs everyday, everywhere you go. Look at your car, is there rust? Many of you will answer yes to this question. If not rust on your car, then look at concrete, can you tell where the cement is getting rough and weathered? This is another form of corrosion. Corrosion has many costs and consequences. First of all it costs the people of the United States millions and millions of dollars each year to keep their surroundings as free of corrosion as possible. Along with the economic losses incurred due to corrosion the loss of human life sometimes happens. Localized corrosion often results in the sudden failure of critical metal parts in automobiles, trains, or airplanes causing explosions or wrecks.
Corrosion involves applications in two fundamental areas of chemistry, they are thermodynamics and kinetics. Thermodynamics deals with the energetics of reactions and an approach to equilibrium. Chemical kinetics involves the study of how rapidly equilibrium is attained and the mechanisms of these reactions. An everyday example of corrosion such as rusting of cars is easy to find, a lot of effort is done to try and inhibit this process.
What exactly is corrosion? Corrosion can be defined as the environmental deterioration of any substance. Thus the gradual disappearance of concrete formations such as statues is included in this process. In this paper however it will be mostly concentrated on the conversion of metals or alloys to their compounds (rust) if you will. Corrosion represents a return of the metal to an ore like state. An old rusty vehicle shows this and every time you go to touch it, it just crumbles apart. The compounds that result from corrosion represent a lower energy state than that of the free metal, thus implying that the reactions might be spontaneous, the thermodynamic approach. Fortunately for us though, the process is a very long, slow process. This is the kinetic aspect. Many of the products of corrosion include oxides, carbonates, sulfides, sulfates, hydroxides, and hydrous oxides. In this paper, oxides are the primary basis of discussion.
Most of the corrosion theory is based upon thermodynamics despite some limitations. Thermodynamics is concerned with the initial and final states but says nothing of the time involved for the reactions to occur. A fundamental classification of thermodynamics is that spontaneous reactions must have a negative free energy change (DG). This implies that reactions proceed toward the lowest energy state. Another rule is that free energy change of the reaction may be calculated by subtracting the sum of the free energies of formation for reactants from the sum of free energies of formation for the products. This is given in the following equation: DGrxn = SDGf (products)- SDGf (reactants) This equation is followed by using values from the standard states of course which are 1 atm pressure and 25°C. Now remember that any element in its standard state has energy of formation that is equal to zero.
The general corrosion equation is as follows: xM (c) + y/2 O2 (g)à MxOy (c). M represents a metal, g a gas, and c, a crystalline solid. The normal state of metal is a crystalline solid, this holds true except for mercury, which is a liquid. Oxygen is here as a diatomic gas molecule. We know that silver, copper, and aluminum oxidize very slowly in air. An example of this would be silver eating utensils becoming tarnished over the years of sitting around in grandmas house. It is important to remember that the thermodynamic considerations are only a partial answer to the process of corrosion. Corrosion is basically an electrochemical process. The equation that connects electrochemistry and thermodynamics is DG= -nFE°, n is the moles, F is the Faraday constant, and E is the cell potential. Positive cell energy corresponds to a negative free energy and thus a spontaneous reaction.
To use this electrochemical data, it is easiest to do in half reaction form. There are two types of half reactions, reduction half reactions and oxidation half reactions. It is convenient to use the galvanic cell to best observe these reactions at the electrodes. Oxidation occurs at the anode and reduction occurs at the cathode. If there is a potential difference between them, the stronger oxidizing metal will oxidize and pass into solution while the less active metal is reduced. Here is a sample reaction: oxidation: Fe (c) à Fe2+(aq) + 2 e- E= +.47 V, Reduction Cu2+ (aq) +2e- à Cu (c) E= + .34V. Adding these together we get Fe (c) + Cu2+ (aq) àFe2+ (aq) + Cu (c) E= +.81V. Because the cell potential here is positive, this corresponds to a negative free energy, which indicates that the reaction is spontaneous.
Inhibition when dealing with corrosion means to prevent or hinder the process. When many metals are exposed to the environment, they develop a sort of coat, which will in effect inhibit the metal from further corrosion. Another way to inhibit corrosion is to treat the metal with certain other chemicals, which gives the metal a film that makes it invulnerable to rust. An example of this in every day life would be aluminum cans, these cans are very durable and it takes quite some time for the environment to get through the strong coating that protects the metal surface. If for some reason the surface of the aluminum is scratched, the oxide layer readily reforms and begins to protect the can again. Aluminum is very susceptible to chloride ions in the environment. A chlorine ion will disrupt the continuity between the aluminum and its oxide layer thus allowing the aluminum to be oxidized and become very brittle and pitted. Iron cannot display this property of protective oxidation however and must be treated with Nitric acid to oxidize the surface of the iron into ferric oxide or various chromate solutions, which work very nicely in inhibiting corrosion.
The mechanism of corrosion is not well understood, but it is pretty clear that electrochemical principles are involved. The first step must involve an oxidation of some metal and with that a reduction must also be occurring. Although this theory may not be complete it is pretty obvious that corrosion prevention is taking place everywhere you go. If it weren’t happening, there would have to be an alternative to iron in society as a whole. Iron rusting requires two things to be present along with the iron, they are water and oxygen, and two things that are very abundant on this place we call earth. When iron corrodes, the rate is controlled by a cathodic reaction, which is slower than an anodic reaction. The first step of corrosion involves the formation of Fe2+ ions this process would be unable to continue if there were no means of removing the electrons, but we have water molecules, which contain H+ atoms so the process must go on. The hydrogen atoms could be expected to form H2 molecules, but in the rusting process, no hydrogen gas is present. The ferrous ions react with the oxygen to form rust and reproduce aqueous protons. The oxide film that protects the iron dissolves in acidic environments and rusting proceeds more rapidly than before because there is a brand new surface to be devoured. This mechanism accounts for other observations that may be made dealing with rusting. For example if you connect a galvanized iron pipe to a copper pipe, the iron pipe rusts faster than ever because the copper provides a means by which electrons can be removed. This replaces step one from above. In addition, the H+ ions are less tightly bound to the copper than they are to the iron, this accelerates step three from above. Rusting is accelerated by the presence of catalysts also known as electrolytes. For instance, steel ships rust more rapidly in seawater than freshwater because of the salt in the water. The chloride ion, which is present in salt, prevents formation of an oxide coat as discussed earlier. Perhaps a more available example to you is when in the winter when there is a lot of salt on the roads, your car suddenly begins to turn orangeish red in places. If the salt used in salting the roads is hygroscopic, which means it absorbs water, the moisture retained by the ions accelerates the final step in the corrosion mechanism. Iron rusts much more rapidly in acidic environments and hydrogen gas is made. The source of this acidity is formed in many ways. Air pollutants such as sulfur and nitrogen oxide form acidic salts in water and thus accelerate the rusting process. Acid rain with a pH as low as 4 has been observed which as you can see from reading this would cause great problems not only dealing with rusting, but also other things which I won’t get into now.
Oxidation of metals occurs in areas where oxygen is denied. Thus steel generally corrodes near a defect in the finish underneath the paint. The paint is on there to protect the steel from the dangers of the atmosphere. Where the oxygen supply is limited under the paint, ferrous ions may diffuse before encountering enough oxygen to be oxidized to Iron (III) and form the hydrated ferric oxide. All this indicates that the rust forms away from the spot where the metal is oxidized. Hence a small spot of rust on the fender of your car is evidence of a more serious rust problem underneath the paint. Electrochemical corrosion of metals as opposed to oxide film formation, requires water. Electrolytes expedite rusting by functioning as charge carriers, but the mechanism as discussed earlier is also valid for atmospheric rusting processes. Iron will usually not rust until a relative humidity of approximately 60% is observed. But in the presence of a salt, sodium chloride for example, rusting is observed at approximately 40% humidity.
The mechanism for corrosion seems to be a very accurate one but one should be skeptical when trying to decide if the mechanism is truly correct or if it is just mostly correct. Technological advances everyday would allow the mechanism to become more developed to try and prove some other theory as true. As long as a mechanism can explain something that has happened, we accept it as true, and as long as we keep observing new things, we will have a need for new mechanisms.
A practical aspect of corrosion that affects my life directly is the rusting of the chrome running boards on my truck. The corrosion has begun and now I will explain why. Chromium is more electropositive than iron but because of the oxide film that forms in aqueous solutions, the chromium is more noble than the iron. Now the chromium acts as the cathode and the current now accelerates the formation of rust on the iron. If the chromium on the running boards becomes scratched, which it has, the steel beneath the chrome. This results in many orangeish blemishes on the running board.
In order to prevent corrosion without failure, there are a few conditions that would need to be implicated. First of all corrosion cannot occur in a vacuum environment. The second condition that would work would be a very dry environment, such as deserts where there is no humidity, just heat. These seem like sensible procedures to take, but they are not very practical. It would be impossible to keep a vehicle for instance in a constant vacuum. Well you could do it, you just wouldn’t be able to use it. The most used way of preventing corrosion is by using inhibitors, at almost all carwashes this cycle is included in the price of a wash. This works because the inhibitor used is anodic and this forms an insoluble layer on the anode. This process limits the amount of oxygen that can get through to the metal. The most commonly used inhibitors here are compounds that contain borates, silicates, or phosphates. This procedure of protecting the metal only works if the surface becomes completely covered. If not, then pitting will occur and the metal will become brittle just like the aluminum can discussed earlier. The use of protective coatings could also be used. These coatings could be either metallic or nonmetallic. These coatings work much the same way as the inhibitors do. Nonmetallic coatings include oxide films, sulfates or phosphates, which are used by dipping the iron into these solvents. Other good coatings include organic things such as grease, oil, tar, and even Vaseline, which forms a very good barrier to the metal from the environment.
Corrosion prevention is a very costly process with exact numbers that could not be found. It (corrosion) is everywhere you go, from the car you drive, to the bridges you drive across, even just the pipes your bath water runs through may be corroded if they are not made of PVC or copper. The next time you are out and about, take a look around you and notice the little things that are happening. The next time you go to a carwash, take a look at the menu and see what is all available. If the rust inhibitor costs an extra dollar, it may be worth your while to just get it to be sure that your vehicle looks the best that it can.
1. corrosion-doctors.org/Concrete/Problem.htm Concrete corrosion. 4/23/01
2. www.corrosionsource.com/hotopics/97-151-1.htm#Overview, Inhibitor performance 4/19/01
3. www.hghouston.com/mtechart.html Ferrous metallurgy. 4/19/01
4. Corrosion, Journal of chemistry 1976 volume 49, no. 1
5. www.corrosionsource.com/hotopics.htm Thermal mishandling of high alloyed stainless steel
6. www.corrosionsource.com/hotopics/09041998.htm Development of a non-toxic corrosion inhibitor