Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth water flow, cause localized turbulence and high flow velocities, resulting in erosion corrosion. A combination of erosion and corrosion can lead to extremely high pitting rates.In offshore well systems, the process industry in which components come into contact with sand-bearing liquids, this is an important problem.
High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion.
Materials selection plays an important role in minimizing erosion corrosion damage. Caution is in order when predicting erosion corrosion behavior on the basis of hardness. High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion. Design features are also particularly important.
It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions and obstructions are undesirable. Abrupt changes in flow direction should be avoided. Tank inlet pipes should be directed away from the tank walls, towards the center. Welded and flanged pipe sections should always be carefully aligned. Impingement plates of baffles designed to bear the brunt of the damage should be easily replaceable.
The thickness of vulnerable areas should be increased. Replaceable ferrules, with a tapered end, can be inserted into the inlet side of heat exchanger tubes, to prevent damage to the actual tubes. Several environmental modifications can be implemented to minimize the risk of erosion corrosion. Abrasive particles in fluids can be removed by filtration or settling, while water traps can be used in steam and compressed air systems to decrease the risk of impingement by droplets. De-aeration and corrosion inhibitors are additional measures that can be taken. Cathodic protection and the application of protective coating may also reduce the rate of attack.