Introduction

Friction corrosion is another type of localized corrosion. Friction corrosion is the loss of metal due to relative motion between a corrosive liquid and a metal surface.

The fluid movement is usually very rapid and causes mechanical abrasion of the metal surface. In this type of corrosion, dissolved metal ions do not remain on the metal surface due to movement on the surface, and solid corrosion products are also peeled off and mechanically separated from the surface. Friction corrosion is sometimes also called impact corrosion.

The appearance of fretting corrosion is grooved, wavy, or in the form of spherical holes and an uneven surface. Most of the damage caused by this type of corrosion occurs in short periods of time. Most metals and alloys are susceptible to this type of corrosion and its resistance depends on the type of surface layer. In metals such as aluminum, lead, and stainless steel, fretting corrosion occurs when these surface layers are damaged or worn away. It is noteworthy that metals that have low hardness and are easily damaged or mechanically worn quickly, such as copper, lead, and zinc, are highly susceptible to abrasive corrosion.

In various industries, abrasive corrosion occurs most frequently in equipment such as shear devices, impellers, agitators, heat exchanger tubes, pumps, elbows, tees, etc., which are in contact with moving fluids.

The most important factors affecting abrasive corrosion are:

1. Surface layers: Surface protective layers that form on some metals and alloys play an important role in the resistance of the metal to abrasive corrosion. The denser the surface protective layers are, the greater the resistance, and soft, brittle, and stressed surface layers have less protective power.
2. Fluid velocity: One of the factors affecting abrasive corrosion is the speed of the fluid. Often, as the speed of the fluid increases, surface layers are peeled off and abrasive corrosion increases.
3. Fluid turbulence: Sometimes, when the fluid passes through the inlets of pipes, grooves, or changes in the cross-sectional area of ​​the flow path, the turbulence of the fluid flow increases, which increases abrasive corrosion.
4. Impact: The impact of the fluid on the surface walls of the metal increases the rate of abrasive corrosion.
5. Galvanic effects: Galvanic corrosion can affect the abrasive corrosion of two similar metals connected together in a moving system. The effect of galvanic corrosion may be zero when the fluid is stationary, but it increases sharply when the fluid is moving.
6. Metal type: The resistance of metals and alloys to fretting corrosion depends on the chemical composition, hardness, and corrosion resistance of the alloy. In general, the chemical composition of a metal determines its corrosion resistance. If the metal in question is active and has a high affinity for the environment, it can be resistant to corrosion when it is able to form a strong, adhesive, and protective shell. Adding an element to an alloy often increases its resistance to fretting corrosion. For example, adding iron to a copper-nickel alloy increases its resistance to seawater, or adding molybdenum to 304 stainless steel increases its resistance to fretting corrosion.

Methods for controlling fretting corrosion

Select materials appropriately and systematically: One of the most economical ways to prevent fretting corrosion is to choose the right alloy.

• Proper design: In this regard, some points can be mentioned in the field of equipment design. Increasing the diameter of the pipes to reduce the fluid velocity, increasing the radius of the bends, increasing the thickness of the metal in critical points and vulnerable areas, and finally choosing a more resistant material at the fluid inlet.
• Changing the environment: The spread of abrasive corrosion can be prevented to some extent by applying environmental changes such as removing oxygen, removing solid particles suspended in the fluid, reducing temperature, etc.
• Using cathodic protection.
• Using corrosion inhibitors.
• Using coatings that are more resistant than the metal itself.