mp dec2012 - coating flange face
TRANSCRIPT
46 MATERIALS PERFORMANCE December 2012 NACE International, Vol. 51, No. 12
C L B L O G
Continued from page 45
ture, and 3) acidic vapor sources. A
proÅled surface will hold more soda than
a smooth surface. High humidity and
rainfall will supply more moisture than a
desert environment. Carbon dioxide
(CO2) forms carbonic acid (H
2CO
3) on
contact with moisture, but acidic fumes
from chemical processes may be more
problematic.
If the vessel surface has a sharp angu-
lar proÅle acceptable for the coating to be
applied, no additional work is required.
The problem is to verify the proÅle depth
and shape. Depth is easy. ProÅle shape
is more difÅcult. An additional concern
would be verification that the soda-
blasted surface is free of soluble chloride,
nitrate, and sulfate salts.
Coating flange face
QI need to apply an in-
terior epoxy lining to
steel pipe for a water
authority project. The
pipe is carbon steel with a bolted Æange
connection. My instructions are to coat
only to the welded corner of the pipe wall
to the Æange intersection. When bolted
into place, the gasket between the pipe
Æange and a mating Æange leaves a small
area of uncoated steel at each joint. I
believe the coating should be extended
onto the Æange face to provide protection
there. Engineers counter that any coating
on the Æange face will affect the gasket
seal. Which practice is more acceptable?
AJust tell the engineer to have
plenty of spare Æanges on hand
when he has to go back and re-
pair the corroded Æanges when
they start to leak! I hope it’s not my drink-
ing water.
AIf the water is potable, I hope the
coating is certified for use for
potable water. Also, the small
uncoated area will corrode
quickly because of the small anodic area
present and the high current density that
will develop.
ACoating Æange faces has always
been a hot discussion between the
end users and contractors. Some
end users are not willing to go for
such an option while others push for it.
Some important factors, which must be
considered while dealing with flange
faces, are:
• If you are dealing with a Æat face,
normally pressures are low and
rubber or non-asbestos compressed
gaskets are used. Even if you have
an uneven surface, the gasket can
provide the cushion to hold the
pressure. It is common to extend
the coating on the face.
• If you are dealing with a raised face
or ring-type joint Æanges, normally
pressures are quite high. In this
NACE International, Vol. 51, No. 12 December 2012 MATERIALS PERFORMANCE 47
case, termination of the coating can
be carried out in two different ways.
One is by a rebate groove, which
may extend to a length of 3 to 5 mm
on the Æange face and with a depth
of 500 to 1,500 µm, depending upon
the coating material used. The sur-
face can be ground manually back
to the original dimensions once the
application is dry. It covers the initial
crevice where you may expect corro-
sion. The second option is normally
suitable for new installations, where
the Æange is applied with the coat-
ing and machined to the dimensions
prior to the fabrication required. I
have used similar applications on
oil and gas separators at >1,000 psi
(6.8 MPa) with no leaks afterward.
• The most important point that must
be considered is the type of coating
system; if it does not have sufÅcient
compressive strength and Æexibility,
it will crack during the bolt ten-
sioning. Fibers or thin mat can be
combined to enhance the properties
of a coating system. Rubbers, once
applied on water boxes and similar
areas, are good for low pressures
and temperatures. The same mate-
rial applied on a high-pressure and
high-temperature area may deform
over a period of time, creating a bare
spot that may lead to a leak.
AMy preference would be to carry
the coating over the Æange. The
gasket should have enough com-
pressibility to give an effective seal.
When we were making polysulÅde rubber
in one of our reactors, we had to do exactly
that. When we Årst lined the reactor, we
stopped the coating at the Æange edge. We
had signiÅcant corrosion, even though we
were using an exotic alloy tank. We re-
lined the reactor and lid. We brought the
coating over the Æange face for both the
lid and the tank. When we sealed it with a
gasket 2.25-in (57-mm) wide by 0.25-in
(6.4-mm) thick, we had no further corro-
sion. When we shut down the operation
after 23 cycles, there was no corrosion
anywhere.