Environmental Engineering Reference
In-Depth Information
steels is more resistant than a soft, brittle film as on lead. The nature of the
protective film depends largely on the corrosive itself.
In most metals and alloys corrosion rate increases with increase in velocity,
but a marked increase is experienced only when a critical velocity is reached.
For example, an admiralty brass (70Cu-29Zn-1Sn) showed a corrosion rate
of 1-2 mdd in seawater in the solution velocity range of 1-4 ft/s, and above
300 mdd at the solution velocity of 27 ft/s. However, stainless steels, nickel
alloys such as Monel and Hastelloy, and titanium show a remarkably low corro-
sion rate even at this high velocity. Their critical velocities are of still higher
value.
Turbulence is caused when the liquid flows from a larger area to a small-
diameter pipe as in the case of the inlet ends of tubings in heat exchangers.
Internal deposits in the pipes or any obstruction to the flow inside a pipe by a
foreign body, such as a carried-in pebble, can also cause turbulence. The liquid
hits the wall of the pipe more vigorously and a rapid corrosion ensures.
Impingement, i.e., direct hitting of the corroding fluid on the metal surface,
occurs at bends, elbows, and toes in a piping system and causes intense attack.
Impingement is also encountered on surfaces of impellers and turbines, in areas
in front of inlet pipes in tanks, and in many other situations. The attack appears
as horseshoe-shaped pits with deep undercut and the end pointing in the direction
of flow (Fig. 3.28).
The corrosive effect of high velocity, turbulence, and impingement is further
aggravated when the solution contains solids in suspension and at higher tempera-
tures. Rapid attack on type 316 stainless steel was reported to have been experi-
enced in a sulfuric acid-ferrous sulfate slurry moving at a velocity of 39 ft/s.
The rate of corrosion increased to about 4500 mpy, whereas the laboratory tests
in stagnant solution at room temperature indicated a nil corrosion rate. Lead re-
sists attack by static dilute sulfuric acid at all temperatures up to the boiling point
of the solution, but at high velocity of the solution the corrosion rate increases
steadily with a rise in temperature (Fig. 3.29).
Steam carrying water condensate droplets provides an aggressive medium for
erosion corrosion of steel and cast iron pipes. The impingement of water droplets
at the return bends destroys the protective oxide film and accelerates the attack
on the substrate.
Figure 3.28 Longitudinal section through condenser tube showing impingement at-
tack by seawater in the direction of flow (7 ).
Search WWH ::




Custom Search