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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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The quality of the weld is also very important regardless
of which method is used to construct the tank bottom.
Welds containing excess slag and porosity and other defects are susceptible
to a number of problems that can result in leaks.
Weld Slag Inclusions and
Porosity As a tank undergoes uniform corrosion, defects in the
welds such as slag inclusions and porosity will be exposed.
Weld slag will leach out leaving a cavity that becomes a leak path for the
acid to penetrate further into the weld. Over
time further corrosion of the weld may result in an acid leak.
Welds may be subject to severe pitting while the
surrounding plate is left virtually untouched. Severe
pitting in combination with other weld defects may accelerate the formation of acid leaks.
Variation between the weld and plate material can create potential differences that result in galvanic corrosion. If the weld is anodic and the surrounding plate cathodic, the weld will preferentially corrode.
If the opposite occurs, the surrounding plate will corrode
faster than the weld. This results in the
weld standing above the surrounding plate.
Full penetration welds are required to avoid the formation
of crevices between the plates. This is
problem with double welded butt joints and fillet welded shell-to-bottom joints. The interior weld will eventually corrode to the
point where the crevice is exposed. This will
allow acid to enter the crevice and travel to any point along the crevice. The result is the
outside weld will begin to corrode earlier than if the crevice did not
exist. Any acid leaks may occur some distance
from the original point at which the acid penetrated the inner weld making it difficult to
find and repair the leak properly. The
crevice will only be evident when the inner weld has corroded sufficiently to expose the
crevice. At this point only the outside weld
is left to hold the plates together. Cracking of welds are rare but can occur due to the
following:
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2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
