FRETTING CORROSION
8.7 FRETTING CORROSION
Fretting corrosion is another phenomenon that occurs because of mechanical stresses, and, in the extreme, it may lead to failure by fatigue or corrosion fatigue.
FRET TING CORROSION
Figure 8.20. Weight loss of mild steel versus mild steel by fretting corrosion in air [87].
It is defi ned as damage occurring at the interface of two contacting surfaces, one or both being metal, subject to slight relative slip. The slip is usually oscillatory as, for example, that caused by vibration. Continuous slip, as when one roll moves slightly faster than another roll in contact, leads to similar damage. Wear corro- sion and friction oxidation are terms that have also been applied to this kind of damage.
Damage by fretting corrosion is characterized by discoloration of the metal surface and, in the case of oscillatory motion, by formation of pits. It is at such pits that fatigue cracks eventually nucleate. The rapid conversion of metal to metal oxide may, in itself, cause malfunction of machines, because dimensional accuracy is lost, or corrosion products may cause clogging or seizing. The corro- sion products are exuded from the faying surfaces, which, in the case of steel, are
largely composed of α Fe 2 O 3 plus a small amount of iron powder [87] . In the case
EFFEC T OF STRESS
of nickel for continuous - slip experiments, the products are NiO and a small amount of nickel; for copper, they are Cu 2 O with lesser amounts of CuO and copper [88] . Fretting corrosion is frequently the cause of failure of suspension springs, bolt and rivet heads, king pins of auto steering mechanisms, jewel bearings, variable - pitch propellers, shrink fi ts, contacts of electrical relays, connecting rods, and many parts of vibrating machinery. It may cause discoloration of stacked metal sheets during shipment. One of the fi rst examples of fretting corrosion was recognized when automobiles were shipped some years ago by railroad from Detroit to the West Coast. Because of vibration, the ball - bearing races of the wheels became badly pitted by fretting corrosion, so that the automobiles were not operable. Damage was worse in winter than in summer, but could be avoided if the load on the wheels was relieved during shipment.
Fretting corrosion occurs on airframe structural surfaces that move against each other in a corrosive environment. High vibration levels and other types of mechanical stress result in small relative movement between parts of operational aircraft systems [89] . Another example of fretting corrosion can occur with rail- road car wheels that are shrink - fi tted onto their axles. If failure occurs causing the wheel to come off the axle, it can cause derailment and other damage [90] . Electrical contacts made of electroplated gold coatings can fail by fretting cor- rosion. Relative motion between the contacts removes the gold plate, and atmo- spheric corrosion of the substrate increases the contact resistance to intolerable levels [91] . Fretting corrosion is also a problem in nuclear reactors, particularly on heat - exchanger tubes and on fuel elements, where fl uid fl ow generates vibra- tions [92] .
Laboratory experiments [87] have shown that fretting corrosion of steel versus steel requires oxygen, but not moisture. Also, damage is less in moist air compared to dry air and is much less in a nitrogen atmosphere. Damage increases as temperature is lowered. The mechanism, therefore, is obviously not electro- chemical. Increased load increases damage, accounting for the tendency of pits to develop at contacting surfaces because corrosion products — for example,
α Fe 2 O 3 — occupy more volume (2.2 times as much in the case of iron) than the metal from which the oxide forms. Because the oxides are unable to escape during oscillatory slip, their accumulation increases the stress locally, thereby accelerating damage at specifi c areas of oxide formation. Fretting corrosion is also increased by increased slip, provided that the surface is not lubricated. Increase in frequency for the same number of cycles decreases damage, but in nitrogen no frequency effect is observed. These effects are depicted in Fig. 8.20 . Note that the initial rate of metal loss during the run - in period is greater than the steady - state rate.
8.7.1 Mechanism of Fretting Corrosion
When two surfaces touch, contact occurs only at relatively few sites, called asperi- ties, where the surface protudes. Relative slip of the surfaces causes asperities to
FRET TING CORROSION
rub a clean track on the opposite surface, which, in the case of metal, immediately becomes covered with adsorbed gas, or it may oxidize superfi cially. The next asperity wipes off the oxide; or it may mechanically activate a reaction of adsorbed oxygen with metal to form oxide, which in turn is wiped off, forming another fresh metal track (Fig. 8.21 ). This is the chemical factor of fretting damage. In addition, asperities plow into the surface, causing a certain amount of wear by welding or shearing action, through which metal particles are dislodged. This is the mechanical factor. Any metal particles eventually are converted partially into oxide by secondary fretting action of particles rubbing against themselves or against adjacent surfaces. Also, the metal surface after an initial run - in period is fretted by oxide particles moving relative to the metal surface rather than by the mating opposite surface originally in contact (hence, electrical resistance between the surfaces is at fi rst low, then becomes high and remains so).
An equation for weight loss W of a metal surface undergoing fretting corro- sion by oscillatory motion has been derived [93] (Appendix, Section 29.7 ) on the basis of the model just described, which accounts reasonably satisfactorily for data of Fig. 8.20 :
W = ( kL 12 0 / − kL 1 ) + k lLC 2
where L is the load, C is the number of cycles, f is the frequency, l is the slip, and k 0 , k 1 , and k 2 are constants. The fi rst two terms of the right - hand side of the equa- tion represent the chemical factor of fretting corrosion. These become smaller the higher the frequency, f , corresponding to less available time for chemical reaction (or adsorption) per cycle. The last term is the mechanical factor inde- pendent of frequency, but proportional to slip and load. It is found that either the mechanical or chemical factor may dominate in accounting for damage depending on specifi c experimental conditions. In nitrogen, the mechanical factor alone is operable, the debris is metallic iron powder, and W is independent of frequency, f .
Fretting corrosion is not a high - temperature oxidation phenomenon. This is demonstrated by increased damage at below - room temperatures; by less
Figure 8.21. Idealized model of fretting action at a metallic surface.
EFFEC T OF STRESS
damage at high frequencies, for which surface temperatures are highest; by the fact that oxide produced in fretting corrosion of iron is α Fe 2 O 3 and not the high - temperature form, Fe 3 O 4 ; and, fi nally, by steel being badly fretted in contact with polymethacrylate plastic, which melts at 80 ° C, and hence the surface could have reached temperatures of this order, but not higher [94] .
The effect of moisture when adsorbed on the metal surface may be that of a lubricant. In addition, hydrated α Fe 2 O 3 is probably less abrasive than the anhydrous oxide. At low temperatures, damage is greater presumably because O 2 can adsorb more rapidly or more completely than at high temperatures. More fundamental research is needed to help clarify these details of the general mechanism.
8.7.2 Remedial Measures
1. Combination of a Soft Metal with a Hard Metal. At suffi ciently high loads, soft metals serve to exclude air at the interface; furthermore, a soft metal may yield by shearing instead of sliding at the interface, thereby reducing damage. Tin - , silver - , lead - indium - , and cadmium - coated metals in contact with steel have been recommended. Brass in contact with steel tends to
be less damaging than steel versus steel. Combinations of stainless steels tend to be worst.
2. Design of Contacting Surfaces to Avoid Slip Completely (e.g., grit blasting, or otherwise roughening the surface). Intentional design to completely avoid slip is not always easy to accomplish, because damage is presumably caused by relative movement approaching the order of atomic dimensions. Increased load is effective in this direction if it is high enough to prevent slip; otherwise, damage is worse.
3. Application of Cements (e.g., rubber cement to the faying surfaces). Cements exclude air from the interface.
4. Use of Lubricants. Low - viscosity oils, particularly in combination with a phosphate - treated surface, can be helpful in reducing damage if the load is not too high. Low - viscosity oils diffuse more readily to the clean metal surface produced by oscillatory slip. Molybdenum sulfi de is effective as a solid lubricant, particularly if baked onto the surface, but the benefi cial effects tend to be temporary because the lubricant is eventually displaced by surface movement.
5. Use of Elastomer Gaskets or Materials of Low Coeffi cient of Friction. Rubber absorbs motion, thereby avoiding slip at the interface. Polytetra- fl uoroethylene (Tefl on) has a low coeffi cient of friction and reduces damage. Because of their relatively poor strength, materials of this kind are expected to be effective only at moderate loads.
6. Use of Cobalt - base Alloys. These are effective especially in the presence of liquid water or aqueous solutions (see Chapter 24 ).
REFERENCES