torrnsinn The theory, causes, and mechanism of fretting corrosion and also preventive measures are described bg M w s 6. Fontasa
F
corrosion is the corrosion that occurs a t contact areas between metals under load and subject to vibration and actual slip at these surfaces. In other words, fretting corrosion is the attack on one or both metals because of chafing under load. The corrosion products are usually oxides aad quite often the rate of attack is rapid. This phenomenon has also been called false Brinelling and friction oxidation : the former, because of the formation of indentations or pits, and the latter, because of the nature of the oxides formed. The development and use of high powered machinery, precision mechanisms, and highly stressed components, as well as decreased tolerances in machined and fitted parts, have resulted in increased accent on this type corrosion. Numerous failures have occurred in engine components, automotive parts, rail tie plates, bolted parts, and other machinery. Additional research and study of fretting corrosion are needed because its mechanism is not clearly understood, and a positive and general solution of the problem has not been developed. The basic requirements for the occurrence of fretting corrosion are as follows: The metal interface must be under load; vibration must prevail at the interface so that one metal is striking or rubbing the other; and a small degree of slip must exist between the two metal surfaces. The results of this type of attack are: removal of metal or pitting in the area of contact; production of an oxide debris-in iron or steel ferric oxide is the usual product; galling, seizing, cracking, or fatigue of the metal-the latter because the pits act as Severe stress-raisers; loss of dimensional tolerances in accurately fitted parts; loosening of bolted or clamped surfaces; and destruction of ball and roller bearings. General loosening of parts because of metal removal can also result in other type failures. An outstanding example of fretting corrosion involves the shipment of automobiles by rail or boat for relatively long distances. Serious corrosion was found between the axles and the front wheel bearings when the cars were held in position by chocking the wheels. Supporting the cars on the axles so the wheels hung free decreased the load and severity of attack, but the corrosion was not completely eliminated. Normal operation of an automobile does not show this difficulty because the slip between the surfaces is too great (complete revolutions). An older problem concerns the bolted tie plates on railroad rails. Frequent tightening is required because the parts are not lubricated and fretting corrosion proceeds rapidly. Aircraft engines and parts are particularly subject to fretting corrosion because of unusual vibrations and relatively high stresses. Examples are connecting rods, pins, splines, couplings, and shafts. An excellent example of fretting corrosion was observed in the writer’s laboratory in connection with a recording dilatometer. The equipment contains a rather extensive linkage system on ball bearings and a vibrator is used to avoid sticking and to ensure continuous transmission of the change in dimension of the RETTING
specimen to the recording pen. Very rapid attack occurred on the bearings which interfered with proper functioning of the dilatometer. Perhaps the first recorded observation in the literature on fretting corrosion is that by Eden, Rose, and Cunningham in England in 191 1. These men were conducting fatigue tests on clamped metal specimens. They noted a considerable volume of reddish-brown oxide at the interface between a steel specimen and the holder. They were surprised to observe also that liberal oiling of this interface did not eliminate the effect although it was somewhat reduced. Theory and mechankm
G. A. Tomlinson in 1927 published a paper entitled “Rusting of Steel Surfaces in Contact” [Proc. Roy. SOC. (London), 115, Series A, 472-83 (1927)] and Max Fink in 1930 published, “Wear Oxidation, a New Component of Wear” [Trans.Am. SOC.Steel Treating, 18, 1026-34 (1930)]. These men presented theories for the mechanism of fretting corrosion. Tomlinson devised a machine that would vibrate metal surfaces together under an applied force with small amounts of slip. He believed that intermolecular forces at the interface result in the pulling or jerking away of small particles of one or both metals from the parent material. These small steel particles are then quickly oxidized to form ferric oxide. The common electrochemical corrosion mechanism for metal removal is not involved. Tomlinson also showed that fretting corrosion would not occur unless mechanical slip is present and that the magnitude of slip may be very small and therefore difficult to eliminate in many cases. He also observed that rough surfaces are less susceptible than smooth ones to fretting corrosion. Fink used two steel disks rotating against each other around their periphery in an Amsler wear-testing machine so that 1% slip occurred. Fink concluded that the mechanism consisted first of an oxide forming on the metal and then the removal of this oxide film by the chafing action. He also found that the disks showed practically no weight loss when nitrogen instead of air made up the environment. J. 0. Almen [Mech. Eng., 59, 415-22 (1937)l also showed reduced attack when tests are conducted in vacuum. The two theories are probably not in conflict if it is considered that the investigations were made under quite different conditions. Tomlinson’s conditions perhaps approach more closely the conditions usually found in actual cases bf fretting corrosion. One or a combination of the two mechanisms is likely to be involved in any given case of fretting corrosion. In a second paper, “Investigation of the Fretting Corrosion of Closely Fitting Surfaces,” by Tomlinson, Thorpe, and Gough [Proc. Inst. Mech. Engrs., 141, 223-49 (1939) 1, the earlier principles of Tomlinson were more firmly established. (Continued on page 64 A ) 63 A
Corrosion A flat cylindrical specimen was vibrated between two others, as in a sandwich, with spring loading. No correlation was found between load and corrosion, and if vibration ceased corrosion also stopped. A small amount of lubricant reduced corrosion but a slight increase in load re-established the original rate. Experiments were also conducted with a sphere between two metal plates (somewhat similar to a ball bearing). No corrosion occurred without slip, but when slip was present attack was noted at the periphery of contact between the ball and the plate. Sakmann and Rightmire in “An Investigation of the Fretting Corrosion of Closely Fitting Surfaces” [Natl. Advisory Comm. Aeronaut., Tech. Note 1492, 57 (1948)l also showed the necessity of the presence of slip for fretting corrosion. They also studied combinations of different metals and alloys; their results are summarized in Table I. Tomlinson in his work found brass to be one of the best and stainless steel one of the worst materials as far as susceptibility to fretting corrosion is concerned. Other investigators indicate that cast materials are superior to wrought. TABLE I.
FRETrINQ CORROSION RESISTANCE OF VARIOUS MATERIALS UNDER DRYCONDITIONS”
(Vibration of a loaded sphere on a flat surface) Low Resistawe Medium Resistance High Resistance Flat FlaT Flat Sphere surface Sphere surface Sphere surface Steel Steel Cadmium Steel +ad Steel Nickel Steel Zinc Steel Silver plate Steel Aluminum Steel Copper elloy Steel Silver plate A1 plate AI-Si alloy Steel Copper plate AI Parkolubrited Steel Antimony plate Steel Nickel plate AI nteel Tin Steel Silver plate AI Zinc-plate steel Steel Iron plate AI Iron-platej steel A1 From Sskmann and Rightmire.
In general most metals depend on surface films for resistance to corrosion (and sometimes wear) of any type or form. Mechanical wearing or removal of these films would tend to activate the metal and accelerate corrosion. In this sense fretting corrosion could be related to erosion corrosion. If actual pulling of the metal particles is involved, tension and/or shear stresses are operative and fretting corrosion could be related to stress corrosion. It is well known that it is possible to weld materials by spinning or vibrating two surfaces together under high loads and speeds where sufficient heat of friction is developed to actually fuse the metals together. The writer observed such a case where welding unintentionally occurred in a deep well pump. Welding by fusion or sustained high temperatures over substantial areas would not be regarded as fretting corrosion in the usual sense of this term. Preoentton
Fretting corrosion can be minimized or practically eliminated in many cases by means of one or a combination of the following: lubrication with low viscosity, high tenacity oils; increasing the load to eliminate vibration and slip; use of rough surfaces to increase friction; use of gaskets to absorb vibration; increasing the hardness of one or both surfaces-that is, soft steel is more susceptible than hard steel; and use of coating or surface treatments, including shot peening, to achieve one of these effects. Aside from other considerations, a proper lubricant tends to exclude air and thus reduce the tendency toward fretting corrosion. 64A
