What Does Impingement Corrosion Mean?
Impingement corrosion is a physico-chemical form of degradation of a metallic or nonmetal surface and its material properties. It is caused by the sustained impact of fluid or solid particles in a high-velocity flow against a surface.
Impingement corrosion is significant because it affects critical equipment in industries, such as petrochemical, power plants, desalination and other process industries.
Corrosionpedia Explains Impingement Corrosion
Impingement corrosion is generally caused by a high-velocity, flow induced physico-chemical removal of protective surface layers, resulting in accelerated wear. The impingement angle of the particles and the presence of fluid velocity in excess of a certain critical value determine the onset of this corrosion for a given material surface. Impingement damage and corrosive wear are mostly interrelated.
Studying impingement erosion is geared toward:
- Monitoring and controlling surface deterioration
- Selecting corrosion-resistant coating to resist impingement
- Designing improvements
- Minimizing catastrophic failures of boiler tubes, condensers, heat exchangers and other critical equipment
The mechanical surface that is damaged by the impacting particle flow is caused by disruptive shearing forces and abrupt pressure changes on the material surface, which is sometimes coated with a protective layer. Deterioration is increased by solid particles and gas bubbles entrapped in the fluid, as well as the corrosive nature of the fluid in the working environment that reacts with the surface.
The surface morphology affected by impingement corrosion may appear in the form of shallow pits, horse shoe patterns or patterns related to local flow directions. The relative movement of a corrosive fluid and the metal's surface is responsible for impingement corrosion and damage to the surface. In the case of boiler tubes, for example, the turbulence due to initial pitting on the internal surface can result in accelerated corrosion and eventual tube leakage. Faulty manufacturing of tubes, such as burrs at smooth tube ends, can result in flow turbulence and high impingement velocities, thus causing severe impingement damage.
Impingement corrosion and cavitation are mechanically occurring degradations. Any equipment components directly exposed to fluid flow above a certain velocity can suffer impingement erosion. Similarly, cavitation, as a special case of impingement corrosion, is caused by the collapse of gas bubbles in a liquid near a solid surface.
Laboratory tests used to detect impingement corrosion and other types of surface damage are designed to estimate and predict impingement damage intensities on material specimens. These tests simulate working conditions and are used for ranking substrate materials and anti-corrosive coatings according to damage intensity.