Thomas Industrial Library

12.5 CORROSION WEAR

Corrosion occurs in normal environments with virtually all materials except those termed noble, i.e., gold, platinum, etc. The most common form of corrosion is oxidation. Most metals react with the oxygen in air or water to form oxides. Elevated temperatures greatly increase the rate of all chemical reactions.

Corrosion wear adds to the chemically corrosive environment a mechanical disruption of the surface layer due to a sliding or rolling contact of two bodies. This surface contact can act to break up the oxide (or other) film and expose new substrate to the reactive elements, thus increasing the rate of corrosion. If the products of the chemical reaction are hard and brittle (as with oxides), flakes of this layer can become loose particles in the interface and contribute to other forms of wear such as abrasion.

Some reaction products of metals such as metallic chlorides, phosphates, and sulfides are softer than the metal substrate and are also not brittle. These corrosion products can act as beneficial contaminants to reduce adhesive wear by blocking the adhesion of the metal asperities. This is the reason for adding compounds containing chlorine, sulphur, and other reactive agents to create EP (extreme pressure) oils. The strategy is to trade a slow rate of corrosive wear for a more rapid and damaging rate of adhesive wear on metal surfaces such as gear teeth and cams, which can have poor lubrication due to their nonconforming geometry.

Corrosion Fatigue

The phenomenon variously called corrosion fatigue or stress corrosion is not yet fully understood, but the empirical evidence of its result is strong and unequivocal. When a part is stressed in the presence of a corrosive environment, the corrosion process is accelerated and failure occurs more rapidly than would be expected from either the stress state alone or the corrosion process alone.

Static stresses are sufficient to accelerate the corrosion process. The combination of stress and corrosive environment has a synergistic effect and the material corrodes more rapidly than if unstressed. This combined condition of static stress and corrosion is termed stress corrosion . If the part is cyclically stressed in a corrosive environment , a crack will grow more rapidly than from either factor alone. This is called corrosion fatigue . While the frequency of stress cycling (as opposed to the number of cycles) appears to have no detrimental effect on crack growth in a noncorrosive environment, in the presence of corrosive environments it does. Lower cyclic frequencies allow the environment more time to act on the stressed crack tip while it is held open under tensile stress, and this substantially increases the rate of crack growth.

Fretting Corrosion

When two metal surfaces are in intimate contact, such as press-fit or clamped, one would expect no severe corrosion to occur at the interface, especially if in air. However, these kinds of contacts are subject to a phenomenon called fretting corrosion (or fretting ) that can cause significant loss of material from the interface. Even though no gross sliding motions are possible in these situations, even small deflections (of the order of thousandths of an inch) are enough to cause fretting. Vibrations are another possible source of small fretting motions.

The fretting mechanism is believed to be some combination of abrasion, adhesion, and corrosion.[8] Free surfaces will oxidize in air, but the rate will slow as the oxides formed on the surface gradually block the substrate from the atmosphere. Some metals actually self-limit their oxidation if left undisturbed. The presence of vibrations or repeated mechanical deflections tends to disturb the oxide layer, scraping it loose and exposing new base metal to oxygen. This promotes adhesion of the “cleaned” metal asperities between the parts and also provides abrasive media in the form of hard oxide particles in the interface for three-body abrasion. All of these mechanisms tend to slowly reduce the solid volume of the materials and produce a “dust” or “powder” of abraded/oxidized material. Over time, significant dimensional loss can occur at the interface. In other cases, the result can be only a minor discoloration of the surfaces or adhesion similar to galling. All this from a joint that has no designed-in relative motion and was probably thought of by the designer as rigid and static! Of course, nothing is truly rigid, and fretting is evidence that microscopic motions are enough to cause wear. Figure 12-6 shows fretting on a shaft where a hub was press-fitted.[5] Fretting is sometimes encountered on roller-follower studs that have no relative motion versus the follower arm.

Some techniques that have proven to reduce fretting are the reduction of deflections (i.e., stiffer designs or tighter clamping) and the introduction of dry or fluid lubricants to the joint to act as an oxygen barrier and friction reducer. The introduction of a gasket, especially one with substantial elasticity (such as rubber) to absorb the vibrations has been shown to help. Harder and smoother surfaces on the metal parts are more resistant to abrasion and will reduce fretting damage. Corrosion-resistant platings such as chromium are sometimes used. The best method (impractical in most instances) is to eliminate the oxygen by operating in a vacuum or inert-gas atmosphere.