If water is left to stand in a tube for an extended period, the chemical characteristics of the water change as the mixed scale and corrosion products are deposited. In addition any loose scale that is not well adhered to the wall will not be flushed away and air dissolved in the water will form bubbles, producing air pockets. These processes can lead to a number of problems mainly on horizontal tube runs. Particles of scale that do not adhere to the walls and are not washed away tend to fall into the bottom of the tube producing a coarse porous deposit. Air pockets that develop in horizontal runs disrupt the formation of protective scales in two areas: the water lines at the sides, and the air space at the top of the tube. 
In each of the areas that the scale has been disrupted there is the possibility of the initiation of Type 1 pitting. Once pitting has initiated, then even after the tube has been put back into service, the pit will continue to develop until the wall has perforated. This form of attack is often associated with the commissioning of a system. Once a system has been commissioned it should be either put immediately into service or drained down and dried by flushing with compressed air otherwise pitting may initiate. If either of these options is not possible then the system should be flushed through regularly until it is put into use.