Fretting corrosion of stainless steel

Fretting corrosion of stainless steel is corrosion that occurs during cyclic mutual movement of two contacting surfaces and the action of a corrosive environment. It occurs due to the continuous destruction of the oxide protective film at the points of moving contact.

Fretting corrosion affects bolted connections, bearing seating surfaces, leaf springs, gear wheels, couplings, etc.

Fretting corrosion can occur in most metals and alloys where there is vibration between mating surfaces and rotating parts. Stainless steel is particularly susceptible, while brass is resistant. The corrosion product consists of oxidized particles that have been rubbed off the contacting surfaces.

What is fretting corrosion of stainless steel?

Fretting corrosion refers to damage to the irregular surfaces of stainless steel that come into contact with each other. These irregular surfaces, also known as asperities, appear smooth, but under a microscope, sharp grooves and rough pits can be seen.

Fretting is caused by oscillatory sliding and rapid repetitive motion, which results in contact between two surfaces. Although the surfaces are often not intended to touch, even the slightest contact - for example, a movement amplitude of only 3×10 ^-9 m - can eventually lead to fretting corrosion.

This contact can be caused by vibration under load or pressure variations in tightly fitting parts. In the hose industry we often see corrosion on the hose as a result of constant contact with the braid.

Fretting corrosion is basically mechanical damage to metal.

However, when two surfaces rub against each other, new surfaces are exposed. Oxidation occurs and soon oxide waste forms. Iron oxide can form in stainless steel, giving the affected area a red color.

Depending on the material and / or application used, fretting can be abrasive wear, adhesive wear, or both. Abrasive wear occurs when a surface slides over another surface, with the former having a rougher surface than the latter. This results in material loss on the softer surface. Adhesive wear occurs when there is direct friction contact, where both surfaces begin to lose fragments of material. This type of wear can increase roughness and create protrusions. Since the fragments cannot escape contact during fretting, they further contribute to wear.

When it comes to metal hose, we can also call fretting corrosion "braid wear" because the relative movement between the braid and the hose is an example of abrasive wear. The tensile strength of the braid material is higher than that of the hose material - about 100 ksi versus 75 ksi - and thus the hose is "cut" by the stronger wire, which acts like a small knife.

Factors contributing to fretting corrosion of stainless steel

Like many other types of corrosion and wear on stainless steel, the severity of the material(s) depends on many factors including contact load, amplitude, temperature, relative humidity, and inertness of the material(s).

Because fretting corrosion is often mistakenly considered false brinelling, it is important to differentiate between the two types of wear. The main difference is that false brinelling occurs under lubricated conditions, often used to prevent fretting corrosion, while fretting corrosion occurs under dry conditions.

Factors affecting fretting

• Load - the magnitude and position of the load are some of the key factors that contribute to fretting. Uneven loading of mechanical parts can result in concentrated local stress in specific areas on the surface of the stainless metal.
• Environment - fretting corrosion due to oxidation is a consequence of the impact of poor temperature and relative humidity on the mechanisms. High wear rates are likely to occur when metal surfaces are in an aggressive environment or near caustic agents.
• Material properties - ductility, surface roughness and inertia - are some of the properties that influence how surfaces respond under load and contact.
• Movement - small movements from the sliding amplitude and the number of cycles between two metal surfaces affect the amount of fretting wear.
• Surface - fretting wear behavior is largely dependent on material surface treatment, coatings and lubricants.

How to reduce fretting corrosion

Highly dynamic cyclic programs create a greater likelihood of fretting corrosion, especially when similar metals are in contact. For example, a stainless steel hose with a stainless steel braid. In such situations, adding a lubricant can reduce the severity of fretting corrosion.

Another option is to use a dissimilar material or, if the hose and braid material are the same, to introduce a dissimilar material between the other two surfaces. We could do this by adding a layer of "puffy" bronze braid between the stainless steel hose and the stainless steel braid, which acts as a lubricant.

In addition to movement, the external environment plays an important role in the rate of fretting corrosion and must be taken into account when designing the unit. For example, in humid conditions, wear is reduced because the moisture acts as a protective lubricating film layer.

Ultimately, regular maintenance and routine material inspections will protect against all types of corrosion, including fretting, and will extend the life of the hose in service.

Fretting corrosion of stainless steel can be reduced by:

• Avoid fretting corrosion in general: this may include design changes, damping any vibrations and ensuring all connections are properly tightened.
• Fretting corrosion testing of systems: test runs can be used in a controlled environment to address fretting corrosion problems in specific machinery.
• Coatings and lubricants: coatings and lubricants add another layer of friction protection, increasing resistance to fretting corrosion. For example, stainless steel is susceptible to fretting when paired with steel. It can be paired with steel coated with cadmium, indium, lead, tin or silver to reduce fretting. However, with lubricants, they can be squeezed out of the contact area, which will still result in metal-to-metal contact. In some cases, lubricants can have the opposite effect to what is desired, as a lower coefficient of friction can result in more movement.
• Material hardness: hardness usually affects the friction coefficient of a material. Surface and heat treatments can be used to increase the hardness of a material. Another common method is shot peening, which is often used in repairs to increase strength and reduce stress.
• Selecting a stainless steel grade: sometimes it is best to use two materials with different ductility or softness. A soft and hard metal pair has been shown to have less wear damage than two hard metals because the softer metal "flows" rather than "rubs" when in contact.
• Environment: favorable conditions such as a controlled working environment and proper storage reduce unnecessary moisture in the material, reducing oxidation.
• Free metal inserts: in some cases, free metal inserts are used to avoid abrasion or surface contact, such as using thin copper plates for titanium surfaces.
• Rubber: if the application allows, vibrations can be absorbed by rubber, while preventing the formation of slip zones.

Conclusion

Fretting corrosion of stainless steel is a serious problem that can lead to premature failure of metal structures. To effectively combat this phenomenon, it is necessary to take a comprehensive approach to solving the problem, taking into account all the factors affecting corrosion.