Failure Analysis
- Forensic Engineering ANALYSIS
OF A LEAKING OIL COOLER FROM A DIESEL ENGINE
A leaking oil cooler
that led to damage of a large diesel engine was
analyzed to determine the cause of the leak.
The heads on each end of the boost cooler were cut
off so that the tube openings and tube sheets could
be observed during leak testing. The shell
side of the boost cooler was pressurized with water
to examine for evidence of leakage. Two pin-holes
were identified in the tube sheet.
The leaking tube sheet was cut away
from the boost cooler. A close-up view of
the pin-holes are displayed in this image.
The surface at the edge of the pin-hole
is presented at a higher magnification using an
SEM. The surface exhibits erosion damage and
evidence of intergranular corrosion.
An axial cross-section was taken
through a relatively non corroded area of the tube
sheet and is displayed at low magnification using
an optical microscope. The cross-section
exhibits two braze layers that bridge the gap between
two adjacent tubes.
The cross-section through the edge
of one of the pin-holes is displayed at low magnification.
Note that the outer tube sheet braze layer and most
of the inner layer is missing.
Another axial cross-section was
taken through a shallow (partial) pit in the tube
sheet surface.
The pit location from the previous
image is presented here at a higher magnification.
The dark gray area in the outer braze layer is oxidation
corrosion products of the outer braze layer.
EDS analysis of corrosion products found down in
the inner layer identified chlorine in the corrosion
products.
CONCLUSIONS:
Results indicate that the
boost cooler exhibited two pin-hole leaks in one
of the tube sheets due to corrosion of the braze/solder
filler used in tube sheet construction.
The top braze layer exhibited general uniform
corrosion. Once the inner copper braze layer
was exposed, corrosion accelerated resulting in
pitting. High concentrations of chlorine
were found in the corrosion deposits at the pin-holes.
Chlorinated flux, used during brazing, was identified
as the source of corrosive material. The
use of a non chlorinated flux was recommended.
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