Ortho-Polyphosphate Corrosion Inhibitors

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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.

           olyphosphates (and when           sion protection. This film forming            active player in promoting this film

  P        combined with a zinc source
           is sometimes referred to as
           bimetallic phosphates) have
           been used in treating drinking
                                             action can be the result of a number of
                                             mechanisms with the two most preva-
                                             lent being either anodic or cathodic
                                             deposition. Without going into great
                                                                                           forming process, other factors undoubt-
                                                                                           edly play a role as well. Examples of
                                                                                           these other factors likely include differ-
                                                                                           ences in oxidation potentials, corrosion
water supplies since the 1950s.              detail, it is important to note that the      byproducts, concentrations and chemi-
Orthophosphates (specifically zinc           deposition of a protective film is            cal form of the phosphate, concentra-
orthophosphates) were first used in the      dependent essentially on a chemical pre-      tions of other organic or inorganic com-
late 1960s by Long Beach, CA. As             cipitation process in the immediate area      pounds, or a combination of factors
polyphosphates will revert to the            of the corroding surface. The environ-        which, when present simultaneously,
orthophosphate form over a period of         ment         within       which        this   trigger the precipitation/film forming
time, the use of ortho/polyphosphate         precipitation/film forming process takes      process.
blends has actually been going on for as     place is typically different in some way
long a time as polyphosphates have been      from the bulk water environment. It is        Selection of Ortho/Poly Ratio
used in treating potable water. It was       these differences that “trigger” the pre-         This is probably the most difficult and
not, however, until the 1980s that some      cipitation that leads to the deposition of    fickle step to undertake in the proper
vendors began intentionally adding           the protective barrier that stifles further   selection of the inhibitor. This is because,
orthophosphate to a polyphosphate            corrosion.                                    as previously mentioned, there are many
source to market a blended product.             An example of the above would be           factors that effect the film forming
Today a full range of phosphate prod-        illustrated by the pH change that occurs      process that are not fully understood. As
ucts with or without the addition of zinc    in cathodic and anodic sites on a cor-        all waters differ from one another in their
is available.                                roding metal. The solubility of various       respective chemical makeup, it is virtual-
                                             phosphate salts and compounds is often        ly impossible to predict the effects of one
Composition and                              pH dependent. As the phosphate passes         ratio against another without experimen-
                                             through the cathodic or anodic sites          tation. As it is known that both the ortho
Mode of Action                               where the pH differs from that of the         and poly forms of phosphate will form
   As the name implies, an                   bulk water supply, the phosphate under-       protective films under certain conditions,
ortho/polyphosphate blend is a product       goes a change that results in the precipi-    it cannot be determined which form
containing concentrations of an              tation of a phosphate containing com-         should be maximized without a fair
orthophosphate and polyphosphate             pound, which, hopefully, will attach          amount of study.
source. As is the case with all phosphate    itself to the active anode or cathode.            Having said this, there are several the-
based inhibitors, the properties of corro-   Some of these compounds may form at           ories regarding the role that each form
sion protection can be directly attrib-      higher and others lower pHs than the          plays in the film forming process. One
uted to the film forming action of the       bulk water pH, which will determine           popular theory rests on the known fact
inhibitor on the surface of the material     whether the inhibitor is acting as a          that polyphosphate will act to sequester
being protected. The more uniform and        anodic or cathodic corrosion inhibitor.       cations in aqueous solutions while
tenacious the film, the better the corro-    While the pH is probably the most             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 con-
centrations of polyphosphate being con-
sumed in this process, the orthophos-
phates will have the opportunity to react
in the anodic or cathodic sites to form a
protective film. In the absence of suffi-
cient polyphosphate to sequester these
reactive cations, the orthophosphate
concentrations are consumed by form-
ing premature precipitates with these
cations rendering their resulting concen-
trations 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 max-
imize polyphosphate concentrations. It
is doubtful that the presence of
orthophosphate will be of any value in
sequestering any metals in solution.

   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