Fretting corrosion is based on physicochemical processes that occur at the molecular level. At the first stage, electrochemical destruction predominates. In the contact zone of metals (or metal with non-metal), oxides are formed, due to which mechanical wear is activated. These two processes are closely interconnected and affect the strength characteristics of the assembly. The phenomenon of fretting has been studied by researchers for more than a century, but its forecasting is still poorly developed.
Description
Fretting corrosion is one of the varieties of spontaneous destruction of the metal. This process occurs at the interface of tightly contacting metal-metal or metal-non-metal pairs. Its characteristic feature is the presence of vibrational displacements of small amplitude. Not only carbon steels are subject to fretting corrosion, but also corrosion resistant.
For the occurrence of this phenomenon, a cyclic amplitude of only 0.025 ฮผm is sufficient. Its maximum value can be 200-300 microns. Externally, the destruction is manifested in the appearance of small ulcers, nats, tears, colored spots, powdery plaque on the contact surface.
Oxide-like corrosion products of steel parts have a different color - from reddish to dark brown. It depends on the brand of material and operating conditions. They cannot leave the contact area due to the small amplitude of the oscillations of the mutual displacement of the surfaces, as a result of which their abrasive effect is enhanced.
The most negative consequence of this phenomenon is the fatigue failure of parts. The ability to absorb cyclic loads in nodes is reduced to 5 times.
Wear Features
Fretting corrosion has the following differences from other types of wear and tear :
- Damage to the metal occurs in a reciprocating motion.
- Localization of damage - only in the contact zone of parts.
- Low moving speed in a rubbing pair.
- The destruction of oxide films occurs mainly due to tangential (tangential) forces.
- Rupture of welding bridges when setting surfaces leads to detachment of atoms and the appearance of fatigue cracks.
- Detached metal particles oxidize rapidly in air.
- Corrosion products are actively involved in the further process of wear.
Causes and mechanism of the phenomenon
Simplified fretting corrosion process can be represented as follows:
- Movement and deformation of surfaces.
- The oxidation of metal.
- The destruction of oxide films.
- The discovery of pure metal.
- Its setting with the contacting surface.
- Destruction of gripping bridges.
- Increased oxygen concentration in the open areas.
- Repetition of the corrosion cycle, a gradual increase in caverns.
As a result of the abrasive action of the detached particles, there is also an increase in temperature in the contact zone (in some cases, up to 700 ยฐ C). A white layer is formed, consisting of altered metal structures.
The following main causes of fretting corrosion are distinguished:
- Dynamic loads of small amplitude in fixed joints.
- Aggressive environment.
- Temperature factor.
The nature of the corrosion process depends on what stage it is at. At the initial stage, the predominance of oxidative reactions was recorded due to electrochemical interaction. The slowdown of this process is facilitated by the use of chemical compositions that weaken the effect of the aggressive environment. What corrosion inhibitors are described below.
The stress state of the material has three components - the compression force directed perpendicular to the contact surface, alternating tangential stresses and friction force. Wear during fretting corrosion is a fatigue failure. Small cracks merge over time and pieces of metal break off.
Structural units
The fretting corrosion characteristic of assemblies that are nominally motionless. Most often, the destruction of the metal is observed in the following types of compounds:
- Bolted.
- Rivets.
- Slotted.
- Contact electric.
- Castle.
- Toothed Hirt.
- Flanged.
- With press fit (bearings, discs, wheels, couplings on shafts, axles and wheel hubs).
- The supporting surfaces of the springs and others.
Fretting corrosion of bolted joints is caused by wear of the threaded part and the appearance of leaks in the gap. This is facilitated by a decrease in the tightening during operation, self-loosening of the joints due to vibration loads. However, an increase in the tightening force is not a guarantee of reduced fretting corrosion, since in this case contact welding of surfaces may occur. As a result, the operation of the threaded connection will occur under adverse conditions of tensile stresses.
Destruction rate
The fretting corrosion rate depends on several tens of factors. The most important of them are:
- Ambient atmosphere (in the air, corrosion is faster). This phenomenon is also observed in vacuum, nitrogen and helium.
- Amplitude and frequency of oscillatory movements (friction speed). The relationship between the fracture rate and amplitude is almost linear.
- Pressure (load) in the contact zone and other operating conditions. With a significant load, the depth of damage increases.
- Hardness of the base metal and protective coatings of parts, roughness of the contacting surfaces.
- Technological factors (the method of obtaining the workpiece, residual stresses, precision machining and rigidity of the unit when assembled).
- Properties of oxide products resulting from wear.
- Temperature. In most cases, its negative values โโcontribute to higher corrosion activity. Positive temperatures favorably affect the health of the unit only to a certain critical value. With overheating, the rate of destruction increases.
- Abrasion resistance of wear products.
Methods of struggle
Ideal ways to deal with this phenomenon does not exist. To reduce it, take the following measures:
- Decrease in relative displacement due to increase of friction forces. Increased roughness, pressure, or reconfiguration of parts. The first method is most effective if one of the elements is non-metal. The friction force can also be increased by coating the copper, tin or cadmium electrolytically.
- If the elimination of vibration is not possible, then the opposite method is required - a decrease in the friction force through the use of phosphate, lead or indium coatings, as well as the introduction of lubricants. As a part of the latter, it is recommended to use additives of corrosion inhibitors. This method transfers the slip to the intermediate medium.
- Increasing the hardness of one of the parts (heat treatment, mechanical hardening). This measure allows you to reduce the mutual adhesion of the mating surfaces and reduce wear.
Oil and grease-based lubricants effectively reduce contact wear. Most often they use their consistent types - substances that at a temperature of 25 ยฐ C represent a thick, ointment-like material. Its retention on surfaces is facilitated by phosphate and anode metal coatings.
What are corrosion inhibitors
When the material is destroyed by the type of fretting wear, mainly contact type inhibitors are used. They slow down corrosion in an aggressive environment, and the principle of their action is based on the formation of sparingly soluble compounds with metal ions.
Contact inhibitors include chromates, nitrites, benzoates, phosphates and other compounds. Filling the gap with plastic materials between mating parts not only protects them from corrosion, but also contributes to sealing. Contact inhibitors include Vital, SIM, M-1 and others. A list of inhibitors and recommendations for their use can be found in GOST 9.014-78.