Ortho-Polyphosphate Corrosion Inhibitors - Government ...

Ortho-Polyphosphate
Corrosion Inhibitors
Since the late 1980s, a new type of corrosion inhibitor has become
increasingly popular in treating many water supplies. These inhibitors,
classified as ortho/polyphosphate blends, represent a blend of two
previous technologies that have been established over a much longer
period of time. What follows is a bit about the history of
ortho/polyphosphates, their mode of action, and applications where
they may be likely to yield results.
P
olyphosphates (and when
combined with a zinc source
is sometimes referred to as
bimetallic phosphates) have
been used in treating drinking
water supplies since the 1950s.
Orthophosphates (specifically zinc
orthophosphates) were first used in the
late 1960s by Long Beach, CA. As
polyphosphates will revert to the
orthophosphate form over a period of
time, the use of ortho/polyphosphate
blends has actually been going on for as
long a time as polyphosphates have been
used in treating potable water. It was
not, however, until the 1980s that some
vendors began intentionally adding
orthophosphate to a polyphosphate
source to market a blended product.
Today a full range of phosphate products
with or without the addition of zinc
is available.
Composition and
Mode of Action
As the name implies, an
ortho/polyphosphate blend is a product
containing concentrations of an
orthophosphate and polyphosphate
source. As is the case with all phosphate
based inhibitors, the properties of corrosion
protection can be directly attributed
to the film forming action of the
inhibitor on the surface of the material
being protected. The more uniform and
tenacious the film, the better the corrosion
protection. This film forming
action can be the result of a number of
mechanisms with the two most prevalent
being either anodic or cathodic
deposition. Without going into great
detail, it is important to note that the
deposition of a protective film is
dependent essentially on a chemical precipitation
process in the immediate area
of the corroding surface. The environment
within which this
precipitation/film forming process takes
place is typically different in some way
from the bulk water environment. It is
these differences that “trigger” the precipitation
that leads to the deposition of
the protective barrier that stifles further
corrosion.
An example of the above would be
illustrated by the pH change that occurs
in cathodic and anodic sites on a corroding
metal. The solubility of various
phosphate salts and compounds is often
pH dependent. As the phosphate passes
through the cathodic or anodic sites
where the pH differs from that of the
bulk water supply, the phosphate undergoes
a change that results in the precipitation
of a phosphate containing compound,
which, hopefully, will attach
itself to the active anode or cathode.
Some of these compounds may form at
higher and others lower pHs than the
bulk water pH, which will determine
whether the inhibitor is acting as a
anodic or cathodic corrosion inhibitor.
While the pH is probably the most
active player in promoting this film
forming process, other factors undoubtedly
play a role as well. Examples of
these other factors likely include differences
in oxidation potentials, corrosion
byproducts, concentrations and chemical
form of the phosphate, concentrations
of other organic or inorganic compounds,
or a combination of factors
which, when present simultaneously,
trigger the precipitation/film forming
process.
Selection of Ortho/Poly Ratio
This is probably the most difficult and
fickle step to undertake in the proper
selection of the inhibitor. This is because,
as previously mentioned, there are many
factors that effect the film forming
process that are not fully understood. As
all waters differ from one another in their
respective chemical makeup, it is virtually
impossible to predict the effects of one
ratio against another without experimentation.
As it is known that both the ortho
and poly forms of phosphate will form
protective films under certain conditions,
it cannot be determined which form
should be maximized without a fair
amount of study.
Having said this, there are several theories
regarding the role that each form
plays in the film forming process. One
popular theory rests on the known fact
that polyphosphate will act to sequester
cations in aqueous solutions while
orthophosphates will not. It is also
48 ■ GOVERNMENT ENGINEERING ■ SEPTEMBER–OCTOBER 2006 www.govengr.com

known that this sequestration property
of polyphosphates has synergistic
effects, which means that a small
amount of polyphosphate will sequester
a large amount of reactive cations.
According to this theory, only enough
polyphosphate should be added to the
water supply to ensure that adequate
sequestration takes place. An overdose
of polyphosphate will act to “strip” the
protective orthophosphate film so its
concentrations should be minimized.
Once the reactive cations (i.e., calcium,
iron, and manganese) have been
sequestered, which results in the concentrations
of polyphosphate being consumed
in this process, the orthophosphates
will have the opportunity to react
in the anodic or cathodic sites to form a
protective film. In the absence of sufficient
polyphosphate to sequester these
reactive cations, the orthophosphate
concentrations are consumed by forming
premature precipitates with these
cations rendering their resulting concentrations
to be too low to form protective
films at the actively corroding sites.
Essentially, the polyphosphate plays the
role of the sacrificial lamb in allowing
the orthophosphate to remain available
to do its job.
Of course, if the application of the
inhibitor is for sequestration purposes,
the answer is easy. Choose the inhibitor
with the best bang for the buck to maximize
polyphosphate concentrations. It
is doubtful that the presence of
orthophosphate will be of any value in
sequestering any metals in solution.
Conclusions
Ortho/polyphosphate corrosion
inhibitors can often times be the best
choice in selecting an inhibitor to solve
a corrosion related problem. They are
more environmentally friendly than
some of the other alternatives as they
contain no zinc, which can sometimes
be considered objectionable. As they
can, however, contribute to the growth
of algae in open reservoirs, their use
should be applied with caution where
open reservoirs exist.
GE
The preceding is courtesy of Sweetwater
Technologies, www.sweetwatertech.com.
Part 5 of this series will discuss potable
organic polymers—types and applications.
www.govengr.com GOVERNMENT ENGINEERING ■ SEPTEMBER–OCTOBER 2006 ■ 49