Nature of Metal or Alloy

The nature and properties of the protective fihns that form on some metals or alloys are very important from the standpoint of resistance to erosion-corrosion. A hard, dense adherent and continnons film would provide better protection than one that is easily removed by mechanical means or worn off. A brittle film that cracks or breaks up under stress may not be protective. 

The velocity of the environment plays an important role in erosion-corrosion Table 2.2 shows the effect of velocity on a variety of materials and alloys exposed to seawater. These data show that the effect of velocity can range from nil to extremely great. 

Increases in velocity result in increased attack, particularly if substantial rates of flow are involved. Table 1.2 lists several examples exhibiting little effect when the velocity is increased from 30-120 cm/sec but destructive attack will be at 820cm/sec. This high velocity is below the critical value for other materials listed at the bottom of the table. 

Many erosion-corrosion failures occur because turbulent flow conditions exist. This type of failure occurs in the inlet ends of tubing in condensers and similar shell-and-tube heat exchangers and is designated inlet-tube corrosion.  

Many failures are directly attributed to impingement. The vertical and horizontal runs of pipe were unaffected, but the metal failed where water was forced to turn its direction of flow. 

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