Frecuencia conmutación hasta reducir al mínimo el ruido del motor

Mechanically assisted degradation of metals is defined as the degradation which involves both corrosion mechanism and a wear or fatigue mechanism. In this velocity, abrasion, hydrodynamics, etc. play a major role and has a significant effect in the corrosion behavior.

3.4.1 Erosion Corrosion

Erosion corrosion is due to the relative movement between a corrosive fluid and the metal surface. Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys, and usually exhibits a directional pattern. Sometimes passive films are developed on the metal surface, which has high corrosion resistance such as aluminum, lead and stainless steels. Erosion corrosion results when these protective surfaces are damaged. Metals that are soft such as copper and lead are quite susceptible to erosion corrosion. The corrosive media, which cause erosion corrosion, are gases, aqueous additions, organic systems, and liquid metals.

All types of equipment exposed to moving fluids are subject to erosion corrosion. Some of these are piping systems, such as bends, elbows and tees, valves, pumps, blowers, centrifugals, propellers, impellers, agitators etc and equipments subjected to spray.

3.4.1.1 Prevention of Erosion Corrosion

• Choose materials with better resistance to erosion corrosion such as aluminum and stainless steel.

• Avoid sharp elbows and angles in the design of equipment.

3.4.2 Impingement Corrosion

Impingement is a process resulting in a continuing succession of impacts between particles and a solid surface. Impingement corrosion is a form of erosion corrosion associated with impingement action of liquids. It may be accelerated by entrained gas bubbles. More specifically it is caused by the impingement action of water carrying entrained gas bubbles and striking the metal surface at an angle. It is not the result of mechanical erosion of the metal itself but is the result of removal of the film of corrosion products by erosion which is ordinarily protective at lower velocities.

Figure 3-31 Impingement corrosion in a bent tube

Impingement corrosion is usually seen at or near the entrance of the tubes and in bends. It takes the form of pitting or grooving and eventual perforation of the wall at that location while the remainder of the tube shows no sign of corrosion. Parts like pump casings, pump shafts and impellers, nozzles and valve seats, tubes are some of the examples where impingement corrosion can be seen.

3.4.2.1 Prevention of Impingement Corrosion

Design, shape and geometry are some of the aspects that are to be considered.

Inhibitors and coatings are to be applied.

It is desirable to separate out solids, water or gas early in a flow system in order to avoid two phase flow.

3.4.3 Cavitation Corrosion

Cavitation corrosion occurs under conditions of severe turbulent flow and rapid pressure changes. The cavitation process is responsible for the breakdown of the protective surface film on the metal. This depassivation results in an accelerated corrosion and causes gas pockets and bubbles to form and collapse. High flow velocities and the entrainment of solid particles are the main causes of erosion corrosion. Bubble implosions are responsible for the breakthrough of the passive film in cavitation corrosion. These bubbles may result from boiling phenomena or may arise because of the release of dissolved gases from the fluid as a result of pressure drops.

This type of corrosion can be seen at the suction of a pump, at the discharge of a valve or regulator, at pipe elbows and expansions. This form of corrosion will eat out the volutes and impellers of centrifugal pumps. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops

below the vapor pressure of the liquid and air ingress. Resilient coatings and cathodic protection can be considered as control methods.

Large pressure changes should be avoided and surface layers should be hard to prevent from cavitation corrosion.

3.4.4 Fretting corrosion

The rapid corrosion that occurs at the interface between contacting highly loaded metal surfaces which are enhanced through slight vibratory motions. It is caused by the combination of corrosion and the abrasive effects of corrosion product debris often seen in component with moving or vibrating parts. Pits or grooves and oxide debris characterize this damage. Bolted assemblies and ball bearings are most vulnerable cases of fretting corrosion. Contact surfaces exposed to vibration during transportation undergo the effects of fretting corrosion. The metal surface gets exposed to the atmosphere, while the oxide film is removed due to this effect. This rubbing occurs at small amplitudes. While the rubbing motion continues, fatigue cracks are initiated by high shear stresses. As a result of surface initiated fatigue, wear particles break out of the material, become trapped between the surfaces and oxidize. Apart from causing dismounting problems, these oxidized particles prevent free axial displacement and introduce increased load in the bearing, which in severe cases may lead to immediate bearing failure.

Parameters that need to be controlled in fretting corrosion evaluations include corrosive environment, contact load, amplitude and frequency of load fluctuations, temperature and availability of moisture.

Increased surface hardness and use of lubricants are some of the preventive methods to be used for fretting corrosion. Bearing loads has to be reduced on mating surfaces.