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Nguyen
Bich
Technical Director
CorrOcean Canada, Inc.
Calgary, Alberta, Canada
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Henry
the VIII and Equipment Replacement In Kind
Henry
the VIII wanted Sir Thomas Moore to deliver an angry message
to the King of France. Fearing for his life, in case the message
was not well received, Sir Thomas requested the King to reconsider
his mission. "Do not worry", Henry the VIII assured
his loyal subject, "if the King of France cuts off your
head, I will make sure every Frenchman in London will be one
head shorter." Sir Thomas replied; "I am grateful
for Your Majesty's gesture but I am afraid none of these heads
would fit my shoulders."
In addition to a good mechanical fit requirement, from the
corrosion prevention viewpoint, replacement in kind must also
avoid potential unpleasant surprises later. In one gas plant,
a 6-in piping connected to a large vessel was replaced after
15 years of service. The replacement lasted only 18 months.
The general comments were: "They don't make steel like
they used to!"
When a
new and smaller component is joined to an older and much larger
structure, accelerated corrosion could result due to the unfavorable
galvanic coupling between the two structures. In the gas plant
case, the older vessel had been covered with naturally protective
corrosion product scales. The new piping surface was fresh
metal and anodic to the old vessel. High velocity fluid flow
in the piping did not allow the protective film to form quickly.
The very large cathodic area of the vessel accelerated corrosion
of the smaller anodic piping.
What
can we do to avoid similar failures:
- When
a small part is to be joined to a much larger structure,
the smaller component must be at least as corrosion resistant
as its larger brother. If that is not feasible, isolate
them with gaskets.
- If
a replacement part is of the same material, it might be
necessary to pre-expose it to the environment to allow sufficient
build-up of naturally protective scales.
- Try
to minimize large cathode/small anode area ratio in the
design of equipment.
Trivia:
You are
the proud owner of the "ODD COUPLE" boat of which
one half of the hull is carbon steel and the other half copper.
You only have enough money to paint one half of the hull.
Which side should you paint? The corrodible steel side or
the corrosion-resistant copper side?
Note:
Sir Thomas was actually beheaded by Henry the VIII on July
6, 1535 but for a different reason.
Copyright©
2001 for
this article
All rights reserved
, Nguyen Bich
Please
send your feedback on this article directly to Bich by clicking
here.
To learn more about CorrOcean,
please visit www.corrocean.com.
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