Civil Engineering Reference
In-Depth Information
recommended during initiation of a corrosion-inhibitor program of any kind in a mu-
nicipal water supply.
Dehydrated
(
Condensed
)
Polyphosphates
This group of phosphate compounds
represents a veritable soup of long- and short-chain polymerized molecules, even in-
cluding some cyclic compounds. The most common compounds in this group are
sodium tripolyphosphate (Na
5
P
3
O
10
), sodium hexametaphosphate (polyphosphate mix-
ture) (NaPO
3
)
6
, and other sodium and potassium polyphosphates. All polyphosphates
are formed from the combination of soda ash, caustic soda, or potassium hydroxide
with phosphoric acid. Many different formulations are possible; differences stemming
largely from heating and crystallization conditions during manufacture. Although a
variety of vendor claims are made about the homogeneity and stability of their product,
analysis of commercial polyphosphate formulations has shown that these products
contain many different phosphate groups, and that the composition will change during
long-term storage. The AWWA standard for purchase of sodium tripolyphosphate and
sodium hexametaphosphate is AWWA Standard B503-89.
Polymerized forms of phosphoric acid are strong metal-complexing agents and are
capable of solubilizing metal oxides and actually dissolve some forms of corrosion
scale. Polyphosphates prevent formation of slightly soluble scales of calcium carbonate
and calcium sulfate, and sequester and stabilize iron and manganese to prevent red
water. The reduction in scale formation can be attributed to the adsorption of the
polyphosphate on crystal faces, thus arresting the growth of the crystal. Increased
corrosion and metal release rates may occur because of the complexing and seques-
tering properties of polyphosphates, leading to a higher solubility of metal salts or to
the formation of less protective layers.
There is no evidence that pure polyphosphates decrease the solubility of the native
corrosion scales that form on lead and copper surfaces. Hence, their effectiveness as
corrosion inhibitors on these surfaces is in doubt. In most utility studies, the evidence
suggests that orthophosphate, not polyphosphate, is the active form of the corrosion
inhibitor for lead and copper. There have been reported successes using inhibitor for-
mulations composed of polyphosphates, but often these formulations are a combination
of ortho- and polyphosphates. There is little evidence to suggest that pure polyphos-
phates play a role in corrosion inhibition. If there is effective metal corrosion protec-
tion, it may be due to reversion to the orthophosphate form.
The effectiveness of polyphosphates as inhibitors of iron and steel corrosion has
been observed, but studies have not demonstrated conclusively how and why the in-
hibitors work. Divalent ions, calcium in particular, seem to be needed with the poly-
phosphates for effective inhibition of steel. The ratio of calcium ion concentration to
polyphosphate concentration should be at least 0.2:1, and preferably 0.5:1 or more.
Formation of many different minerals in deposits or protective scales is possible during
the corrosion of iron, and undoubtedly is affected by the inhibitor formulations.
Bimetallic
(
Zinc-Containing
)
Phosphates
The bimetallic phosphates combine zinc
in concentrations of 5 to 25 percent (by weight) with ortho- or polyphosphates. They
were developed and first used as corrosion inhibitors about 1950.
50
These are generally
proprietary formulations, but are available from many producers. To maintain the zinc
in solution, they are usually provided in a sulfuric or hydrochloric acid solutions and
require stainless steel or plastic tanks, pumps, and valves. Many suppliers offer dif-
