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olyphosphates (and whencombined with a zinc sourceis sometimes referred to asbimetallic phosphates) havebeen used in treating drinking

water supplies since the 1950s.Orthophosphates (specifically zincorthophosphates) were first used in thelate 1960s by Long Beach, CA. Aspolyphosphates will revert to theorthophosphate form over a period oftime, the use of ortho/polyphosphateblends has actually been going on for as

long a time as polyphosphates have beenused in treating potable water. It wasnot, however, until the 1980s that somevendors began intentionally addingorthophosphate to a polyphosphatesource to market a blended product.Today a full range of phosphate prod-ucts with or without the addition of zincis available.

Composition andMode of Action

As the name implies, anortho/polyphosphate blend is a productcontaining concentrations of anorthophosphate and polyphosphatesource. As is the case with all phosphatebased inhibitors, the properties of corro-sion protection can be directly attrib-uted to the film forming action of theinhibitor on the surface of the materialbeing protected. The more uniform andtenacious the film, the better the corro-

sion protection. This film formingaction can be the result of a number ofmechanisms with the two most preva-lent being either anodic or cathodicdeposition. Without going into greatdetail, it is important to note that thedeposition of a protective film isdependent essentially on a chemical pre-cipitation process in the immediate areaof the corroding surface. The environ-ment within which thisprecipitation/film forming process takesplace is typically different in some way

from the bulk water environment. It isthese differences that trigger the pre-cipitation that leads to the deposition ofthe protective barrier that stifles furthercorrosion.

An example of the above would beillustrated by the pH change that occursin cathodic and anodic sites on a cor-roding metal. The solubility of variousphosphate salts and compounds is oftenpH dependent. As the phosphate passesthrough the cathodic or anodic sites

where the pH differs from that of the

bulk water supply, the phosphate under-goes a change that results in the precipi-tation of a phosphate containing com-pound, which, hopefully, will attachitself to the active anode or cathode.Some of these compounds may form athigher and others lower pHs than thebulk water pH, which will determine

whether the inhibitor is acting as aanodic or cathodic corrosion inhibitor.

While the pH is probably the most

active player in promoting this filmforming process, other factors undoubt-edly play a role as well. Examples ofthese other factors likely include differ-ences in oxidation potentials, corrosionbyproducts, concentrations and chemi-cal form of the phosphate, concentra-tions of other organic or inorganic com-pounds, or a combination of factors

which, when present simultaneously,trigger the precipitation/film formingprocess.

Selection of Ortho/Poly RatioThis is probably the most difficult and

fickle step to undertake in the properselection of the inhibitor. This is because,as previously mentioned, there are manyfactors that effect the film formingprocess that are not fully understood. Asall waters differ from one another in theirrespective chemical makeup, it is virtual-ly impossible to predict the effects of oneratio against another without experimen-tation. As it is known that both the orthoand poly forms of phosphate will form

protective films under certain conditions,it cannot be determined which formshould be maximized without a fairamount of study.

Having said this, there are several the-ories regarding the role that each formplays in the film forming process. Onepopular theory rests on the known factthat polyphosphate will act to sequestercations in aqueous solutions whileorthophosphates will not. It is also

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Since the late 1980s, a new type of corrosion inhibitor has becomeincreasingly popular in treating many water supplies. These inhibitors,classified as ortho/polyphosphate blends, represent a blend of twoprevious technologies that have been established over a much longerperiod of time. What follows is a bit about the history ofortho/polyphosphates, their mode of action, and applications wherethey may be likely to yield results.

Ortho-PolyphosphateCorrosion InhibitorsOrtho-PolyphosphateCorrosion Inhibitors

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known that this sequestration propertyof polyphosphates has synergisticeffects, which means that a smallamount of polyphosphate will sequestera large amount of reactive cations.

According to this theory, only enoughpolyphosphate should be added to the

water supply to ensure that adequatesequestration takes place. An overdoseof polyphosphate will act to strip theprotective orthophosphate film so itsconcentrations should be minimized.Once the reactive cations (i.e., calcium,iron, and manganese) have beensequestered, which results in the con-centrations of polyphosphate being con-sumed in this process, the orthophos-phates will have the opportunity to reactin the anodic or cathodic sites to form aprotective film. In the absence of suffi-cient polyphosphate to sequester these

reactive cations, the orthophosphateconcentrations are consumed by form-ing premature precipitates with thesecations rendering their resulting concen-trations to be too low to form protectivefilms at the actively corroding sites.Essentially, the polyphosphate plays therole of the sacrificial lamb in allowingthe orthophosphate to remain availableto do its job.

Of course, if the application of theinhibitor is for sequestration purposes,the answer is easy. Choose the inhibitor

with the best bang for the buck to max-imize polyphosphate concentrations. Itis doubtful that the presence oforthophosphate will be of any value insequestering any metals in solution.

ConclusionsOrtho/polyphosphate corrosion

inhibitors can often times be the bestchoice in selecting an inhibitor to solvea corrosion related problem. They aremore environmentally friendly than

some of the other alternatives as theycontain no zinc, which can sometimesbe considered objectionable. As theycan, however, contribute to the growthof algae in open reservoirs, their useshould be applied with caution whereopen reservoirs exist.

The preceding is courtesy of SweetwaterTechnologies, www.sweetwatertech.com.Part 5 of this series will discuss potableorganic polymerstypes and applications.

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