Sometimes corrosion is attractive. For example, the vivid colors on titanium jewelry are oxides, with the color difference the result of the oxide thickness and the refracted light. Also, from an artistically oriented viewpoint, the golden-brown appearance of many of the new high-voltage towers made from ASTM A 242 (Corten) steel is more appealing than their bright galvanized cousins, and that “pleasing brown color” is a tightly bonded coating of rust. But most of the time, and in several ways, corrosion presents difficulties that can greatly shorten the life of mechanical equipment.

Typically, as engineers and technical people, we know that steel corrosion involves oxygen atoms uniting with the iron atoms in the steel. We realize this reaction slowly thins the base metal, and that the steel or iron part could eventually crumble into a pile of rust. But there are also times when the very serious dangers from corrosion are almost invisible to the human eye.

Two general classifications for corrosion are:

Dry—at elevated temperatures.
Wet—where liquid is needed to conduct corrosion currents.
An example of dry corrosion is the scale that develops on grates of a barbecue grill. In those situations, the elevated temperatures supply the energy needed for the oxygen to unite with iron. Fortunately, dry corrosion is uncommon in the machinery world because the temperatures needed for it are in the range where specialty alloys and exotic lubricants are usually needed.

To understand wet corrosion and the problems it can create, Figure 1 shows a microscopic view of what happens in a typical corrosion cell. This shows the corrosion of a steel bar, and because of minute differences in the electrical potentials within the bar, the anode area is being attacked while the cathode area is protected. At the anode, Fe+ (iron) ions are released. (An ion is an atom with an electrical charge.) They are off in the liquid, usually water, and will eventually pair up with some oxygen ions to form the various forms of rust that we see all around us.

Only a very short distance along the bar is the cathode, where the hydrogen ions, from the water H2O molecules, are being liberated. As shown in the diagram, most of those ions rapidly find another ion and form hydrogen gas. However, there is always dissociation and some of those hydrogen ions wander off to do later damage.

Going back to that steel bar, as shown in Figure 2, the internal electric charges flow from the cathode to the anode, and then, to complete the corrosion circuit, the ionic currents flow back through the liquid. From this figure we can see that there has to be liquid present to conduct the currents that result in the corrosion.

Unfortunately, that doesn’t mean the part has to be dripping in water, and we’ve all had cases where a relatively clean piece of steel has emerged from a long seclusion in desk drawer with a light coating of rust. It turns out that, due to intermolecular forces, corrosion can begin at only 60 percent relative humidity.

It’s also interesting that, the more conductive the liquid, up to a point, the more rapid the corrosion. That’s because a more conductive liquid, such as salt water, can more effectively carry the corrosion currents.

The problems that corrosion can cause are three-fold:

Material loss—There is a loss of material and contamination of the product surface. Usually this is readily visible and, although it can be expensive to correct, it is readily detectable.
Hydrogen damage—This type of corrosion damage goes by various names such as hydrogen embrittlement, hydrogen cracking, and stress corrosion cracking, and there are some differences in the exact mechanisms. However, the basic cause is those free hydrogen ions can result in unexpected and undetected fractures.
Reduced fatigue strength—Corrosion continually reduces the fatigue strength of the metal. The more severe the corrosion, the faster the fatigue strength is decreased, and that continual reduction in fatigue strength essentially goes on forever.
(It’s important to realize that corrosion isn’t the only source of the free hydrogen atoms that result in hydrogen cracking. Some of the other processes include steam leaks, plating processes, acid cleaning, welding, etc. Still, corrosion is unique in that it continually generates hydrogen and doesn’t have the recognition and controls of the other processes.)

Scientists have been trying for years to understand exactly how hydrogen and the other chemicals, especially sulfur, interact to cause cracking and the reduction in fatigue strength, and the primary source of the problem appears to be those hydrogen atoms, their need to form hydrogen molecules, and the relative sizes of different atomic structures.

A hydrogen atom, with only one electron, is tiny compared to the rest of the atoms in our universe. A simple example of how a hydrogen atom can cause the cracking is to think of it as the size of a golf ball and the structure of iron and steel as being made up from neatly stacked atoms the size of bowling balls. The single golf ball, i.e., a hydrogen ion, can easily move in between the bowling balls with no problem. But then it meets another hydrogen ion and the atomic forces cause it to form molecular hydrogen, H2, the gas we’re familiar with. However, the hydrogen molecule is about five times larger than the hydrogen atom and that causes stress within that assembly of “neatly stacked bowling balls.”

H2 molecules tend to group together at irregularities within the metal’s structure. Then, as more and more hydrogen molecules are formed, the stress inside the steel structure increases, and the material’s ability to withstand external stresses decreases.