Chemical Engineering August 2020 - 38

tion, plus a far less permeable diaphragm,
the problem can be largely
eliminated in most situations.
Gaseous applications
FIGURE 2. Once hydrogen permeates the diaphragm, it becomes trapped in the fill fluid, gradually building
up pressure until it distorts the diaphragm, as shown in these two pictures
The more technical description is
catastrophic membrane deformation,
which requires replacement of
the complete transmitter.
Aqueous applications
A common situation where this happens
involves water, a product normally
considered benign. The driving
force causing ions to migrate
from water through a diaphragm is
independent of pressure. The action
is galvanic, which causes this problem
to appear in some unexpected
applications, such as wastewateror
biogas-treatment plants.
Picture this situation: a plant's
maintenance department installs an
access ladder into a large sump pit
where a pressure instrument is installed
nearby to measure level. The
frugal engineer chooses a hot-zinc
galvanized ladder rather than stainless
steel, hoping to minimize corrosion
while saving a few dollars.
After operating for two months,
the sump is drained for cleaning
and the stainless-steel instrument
diaphragm distends due to hydrogen
permeation. This negates the
cost saving on the ladder, so the
maintenance manager wants to
know why it happened. After all, it's
just water. This isn't a hydrogen application,
is it?
It became a problem due to galvanic
interaction, or " battery effect, "
between the components.
The zinc on the ladder acts as the
anode and the stainless-steel diaphragm
of the pressure instrument
is the cathode. The water present
in the tank is electrolyte.
The result of this galvanic process
36
is a reduction of water molecules
into ions at the cathode (Figure
3). Stripped of their electrons and
driven by galvanic action, extremely
small H+ cations can diffuse through
a 316 stainless-steel or Alloy C diaphragm.
As described earlier, once
on the other side of the diaphragm,
the H+ ions capture electrons and
recombine into H2 molecules in the
fill fluid. This type of diffusion is independent
of process pressure, but
the diffusion rate
can increase with
higher temperature.
The solution for
this type of situation
calls for a two-layer
coating to effectively
seal the diaphragm
(Figure 4).
First a layer of
gold (Au), between
15 and 25 µm, is
added to the full
diaphragm surface.
Gold has a very
low diffusion coefficient,
increasing
the hydrogen
permeability resistance
up to
Aqueous
solution
Water
dissociation
Ho
H2O
H+
H+
OH-+H+
Ho
Proton diffusion
through steel
diaphragm
FIGURE 3. In aqueous solutions, galvanic action can cause water molecules
to dissociate and diffuse through the diaphragm
one
million times. This
is supplemented
by an additional
coating of rhodium
(Rh) on top of the
gold. Rhodium
promotes recombination
of H2 in the
solution, reducing
its ability to diffuse.
With fewer cations
present in the soluAqueous
solution
H2O
H+
2H++2ē
OH-+
H+
H2
H+
Fill
fluid
Hydrogen
recombination
2Ho
H2
The discussion so far has focused
on aqueous solutions with dissociating
water molecules, but diaphragm
permeation can also take
place with gaseous hydrogen where
no liquid is involved, although the
drivers are different.
Gaseous molecular hydrogen can
dissociate into single atoms, and
even H+ ions in situations when the
pressure and temperature are high
enough (Figure 5). Ions deposit on
the pressure instrument diaphragm
and can be forced through into the
fill fluid, where they pick up electrons
and recombine into molecular
hydrogen, just like in aqueous
applications.
The precise points where temperature
and pressure become a factor
are difficult to define specifically, howFill
fluid
Rhodium
316L/AlloyC
Gold
FIGURE 4. Adding a double coating of gold and rhodium reduces permeability
and promotes reformation of molecular hydrogen
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Chemical Engineering August 2020

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