Environmental Engineering Reference
In-Depth Information
treatments is helpful to prevent fatigue crack propagation initiated by fretting.
Elimination or reduction of vibration would be ideal to reduce relative slip, but
often this measure is not practicable.
3.8.6 Practical Examples
Leakage was detected in a malleable iron elbow after only 3 months in service,
whereas the life expectancy was 12-24 months (Fig. 3.32) [2]. The component
was exposed to alternate steam and cooling water supply for at least 16 h a day.
Investigations revealed a ferritic malleable iron structure, which was less resistant
to erosion corrosion than pearlitic malleable iron. A proper heat treatment im-
proved the performance.
Figure 3.33 illustrates a cavitation damage of a pump impeller made of 55Ni-
21Cr-3Mo-3.5Cu-1W alloy working in a water contain 550 ppm chloride at 44
C
[6]. Closely spaced pits are clearly visible. It may be noted that the material used
belongs to the group of most erosion corrosion-resistant materials. Change in
operating conditions of the pump was recommended as a remedial measure.
°
3.9 CORROSION CRACKING
Many metals and alloys develop cracking under the combined action of stress
and corrosion. Corrosion-induced cracking processes are categorized as:
1.
Stress corrosion cracking,
2.
Corrosion fatigue, and
3.
Hydrogen-induced cracking.
Figure 3.32
Section through the malleable iron elbow showing the area of erosion
corrosion attack.
