Page 182 - CW E-Magazine (4-3-2025)

Special Report Special Report

initiation of crevice corrosion. For that that if a lubricant is used and there is include design changes, dampening any Preventing fretting motions When using lubricants, potential not by pitting. The pitting reaction is
reason, the materials now used for hip damage, the appropriate term used to vibrations, and making certain that all One of the components required for issues may arise in areas with high con- accelerated due to a high concentration of
replacement implants tend to no longer describe the failure is false brinelling. joints are properly tightened. Combining the formation of fretting corrosion is centrations of dirt or dust. Some lubricants ions such as chlorides and high operating
be metal on metal. In addition, design However, the loss of lubricating fi lm movement. Restricting movement pre- tend to retain dust particles that can add temperatures.
changes have been made to reduce the will transform the failure from false bri- vents exposing intact connector metal to to the “roughness” of the interface and
relative motion between components. nelling to fretting corrosion. Figure-5 the atmosphere and hence reduces oxide ultimately contribute to fretting corrosion. Pitting corrosion can be prevented or
shows fretting corrosion of an antifric- debris build-up. One of the most effective minimized by choosing materials with
Dental implants are subjected to tion bearing outer race caused due to ways to achieve this is through the use of Corrosion prevention can also be high pitting resistance that match the
variable amplitude cyclic micro-move- inadequate fi t of the outer race of the high contact forces between the contacts, achieved through the use of gold- or service environment. Keeping service
ments caused by the transmitted loads bearing with the housing in a centri- which provides mechanical stability by silver-plated contacts and terminals. temperatures below the CPT of the chosen
during mastication. Oblique loads can fugal pump. The slippage of the outer ring increasing the friction between them and Gold and silver are both noble metals, material is also an important factor.
promote fatigue. The chemical environ- on the housing caused fretting corrosion helps to counteract the fretting motions. which mean they are naturally resistant to
ment of the mouth is quite complex and leading to the complete damage of the Fig. 6: Fretting corrosion due to external oxidation and corrosion. Though highly Fretting corrosion is a common prob-
vibration induced on a stationary shaft
harsh and can include saliva, bacteria bearing as shown in the fi gure. A patchy However, it should be noted that in- effective, plated contacts can become lem found in electrical terminals and
that can produce microbial induced cor- discolouration of the outer race and the the simulation of fi nite element analysis creasing the contact force could lead to worn after numerous mating cycles, once contacts. There are a number of cost-
rosion (MIC), acidic foods such as soft presence of reddish-brown debris were (FEA) for fretting with electrochemical enhanced wear and tribo-corrosion of the again leaving the connector susceptible effective preventive solutions that greatly
drinks and fl uoride from dental rinses. found on the surface of the failed bearing. calculations for corrosion can be used to contacts over time and effectively reduce to corrosion. Therefore, plated contacts outweigh the cost of replacement or
evaluate the potential of fretting corro- the number of mating cycles before the may require regular maintenance to repair. Determining the fretting driving
Fretting corrosion is a common failure Figure-6 shows a case of fretting sion for a given design. connector is completely worn away. In ensure maximum serviceability. force is vital; however, it is important to
mechanism for tin-plated copper corrosion due to external vibration addition, if the high contact forces are choose an appropriate solution because
electrical contacts. The accumulation of For electrical contacts the most insuffi cient to restrict fretting movement, The common techniques for avoiding an incorrect one may be ineffective or in
oxidation products at the contact spot is effective approach is to avoid the relative the rate of fretting corrosion and contact fretting corrosion may be summarized as some cases actually increase the rate of
caused by the micro-motions of the con- motion or slip in the contact region of degradation may be increased. There- below: fretting corrosion.
tact. Tin is a soft metal and rapidly forms mated connector parts by designing the fore, it is important that the contact force 1. Introducing a lubricating fi lm
a thin and hard oxide. When the hard contact to minimize the micro-motion. being used is appropriate for the fretting between the interactive surfaces. REFERENCES
oxide layer gets broken, it can be pressed Plating the mating contact surfaces with movement expected. 2. Phosphate coatings may be used in 1. Van Delinder, Ed., Corrosion Basics,
into the soft and ductile tin matrix where it gold is an expensive option but even that conjunction with lubricants as these NACE International, (1984),
can accumulate. It has been speculated does not eliminate the potential of fretting Other means of preventing or mini- coatings are porous providing oil 2. Schweitzer, P.A., Ed., Corrosion
that it is the micro-slip action between corrosion. mizing fretting motions include using reservoirs. Gold- and silver-plated Engineering Handbook, Marcel
the contacts that creates the fretting, extra clamps or brackets to reduce con- contacts and terminals may be used Dekker (1996).
which combined with the oxidation With medical implants, design nector movement; using seals to dampen in special cases. 3. Chakravorty, S.K., Case studies on
causes the thick insulating layer at the changes that increase the stability of the vibrations and prevent air/oxygen intru- 3. Gaskets may be used to absorb major accidents due to corrosion,
contact interface and the substantial interface can minimize fretting corro- sion; or by using engineering methods to vibration and exclude oxygen. Alkali Bulletin, Vol. XLIII, No:5,
increase in contact resistance and ultimate sion. Materials changes include cross- adjust the relative movements between 4. Proper fi tting and increased load pp:1-5.
failure of the contact. Fig. 5 : Fretting corrosion of bearing outer linked polyethylene and more abrasion- the contacting surfaces. help reduce slip between mating 4. Fontana, M.G., and Green, N.D.,
race resistant ceramics. surfaces consequently reducing ‘Corrosion Engineering’, McGraw
Compressor blades of a gas turbine induced/transmitted on a stationary shaft Preventing fretting corrosion fretting corrosion(7, 8). Hill Book Company, (1967).
undergo centrifugal forces during opera- lying on the plain (sliding) bearing of a Anti-fretting coatings for turbine If fretting movements cannot be re- 5. Uplenchwar, A.M., National
tion and in combination with vibratory pump in idle condition. Vibration trans- blades have also been applied and found stricted, measures can be taken to prevent CONCLUSION Conference on Total Corrosion
load can cause a fretting motion between mitted from external sources (nearby to reduce fretting corrosion. Lubrica- corrosion associated with these micro- Though barely visible, pitting cor- Management, FICCI, June 2008,
the dovetail joint of the compressor running machines) caused damage to the tion, where feasible, is another approach scopic movements. The most common rosion can have serious implications New Delhi.
blade and the disc. The debris that is bearing. Pitting and pick-up were seen for minimizing the potential of fretting and effective method is to apply a lubri- because the cavity it bores below the 6. Laque, F.L., Marine Corrosion – Causes
trapped between the contacts areas can on bearing surface (bottom load bearing corrosion. cant between the contacting surfaces. surface can be both wide and deep, thus and Prevention, John Wiley & Sons
then be oxidized at elevated tempera- area) due to slippage of the shaft on the Lubricants provide protection in two seriously affecting the load carrying (1975).
ture, resulting in fretting corrosion. bearing bottom surface area. To avoid Methods to control fretting corrosion ways: fi rst, by reducing the friction bet- capacity of the structure. 7. Chakravorty, S.K., ‘Forms of corro-
this failure, it is recommended to operate in electrical devices ween the contacting surfaces, thus mini- sion in industry and its prevention –
For wind turbines the oscillating the idle pump regularly. There are a number of measures that mizing the rate of contact wear over time; Metals at risk are those that depend Part-I & II, Alkali Bulletin, July
movement between the rolling element can be taken to mitigate the effects of and second, by acting as a barrier that on a passive oxide fi lm for their corro- 2018, and August 2018.
and raceway of the blade bearing can Prevention of fretting corrosion fretting corrosion, which fall into two prevents air and moisture from coming sion protection, such as aluminium 8. Chakravorty, S.K., Tribocorrosion
give rise to fretting corrosion or other Prevention of fretting corrosion approaches: fretting movement preven- into contact with the electrical terminal alloys, nickel and SS. Carbon steels nor- in process plants, Chemical Weekly,
types of tribocorrosion. It can be noted starts with preventing fretting. This can tion and corrosion prevention. or interface. mally fail due to uniform corrosion and December 19, 2017.

182 Chemical Weekly March 4, 2025 Chemical Weekly March 4, 2025 183

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