I identified limits with my own knowledge within
the subject area of corrosion, I felt improving in this subject area would
broaden, deepen and improve my engineering capability. I completed and
successfully passed a distance learning Master of Science course module in
Material and Corrosion Science.
Based on a
previous factual failure scenario the coursework involved investigating the
failure of a heat exchanger (HE) in an aggressive environment after a short
This device consisted
of a tube bundle held securely with a tube-sheet with baffles for support,
which transfer gas and reduces the temperature using filtered seawater as a
cooling medium. The HE had suffered several premature
failures despite tubing material upgrades from copper-nickel alloy to high
performance nickel alloys, in an attempt to prevent early failures caused by
corrosion. The aim of
the coursework was to investigate the failures, conclude, and make
recommendations to avoid such failures in the future.
A root cause analysis was performed using a
5-why’s technique. The
investigation highlighted the cause of failure mainly
related to crevice corrosion, crevices developed between the tube and tube-sheet,
the positive metal ions from the tubing were released into the crevice
and coupled to the seawater chloride ions. Due to limited
seawater circulation within this area the minimum temperature for corrosion to
develop, the Critical
Crevice Corrosion Temperature (CCCT) was exceeded. This eventually
led to a significant reduction in tube wall thickness, until eventually holes
formed. Internal tubing oxidisation damage occurred
due to the removal of the natural
protective film, likely as a result of damage from the tubing
to tube plate roller expansion assembly process as a result of operator inaccuracies.
on the findings I offered the following recommendations to increase operating
service life: 1. Ensure the
tubing manufacturing process of hydraulic expansion is used instead of roller
expansion to reduce the chances of crevices developing. 2. Change to a material grade with higher CCCT
properties to resist against crevice corrosion. 3. Improve the baffle plate
design to improve circulation and 4. Install a pre-water refrigerant upstream
cooling the water inlet to ensure the CCCT is not exceeded.
has benefited and improved my engineering ability both in my existing job
function and for the future.