Civil Engineering Reference
In-Depth Information
ferent formulations of bimetallic ortho- or polyphosphate, which differ with respect to
the zinc-to-phosphate ratio and solubility. It has been claimed that bimetallic phos-
phates perform better than individual phosphates at lower dosages, particularly in more
mineralized and harder waters. It has also been postulated that zinc forms carbonate
compounds and block the cathodic corrosion cell sites, while phosphate forms com-
pounds at the anodic sites. The higher zinc concentrations are thought to reduce the
time to establish corrosion control by rapid film formation at the cathode.
Zinc-based compounds have historically been used to alleviate corrosion problems
on iron surfaces and are often used in connection with red water-related corrosion
problems. Although many utilities use a combination of zinc sulfate and phosphoric
acid in their lead and copper programs, the mechanism of zinc action on lead and
copper surfaces has never been quantified. Most rigorous corrosion-control pilot pro-
grams that have compared zinc-orthophosphate compounds versus simple orthophos-
phates have shown no significant benefit from zinc additives at operational dosage
levels.
The zinc used in the bimetallic corrosion inhibitors is an environmental concern
because it contributes to the overall zinc load on wastewater treatment facilities. The
prescribed discharge limitation of zinc into the wastewater collection systems of many
municipal jurisdictions is often in the range of 0.1 to 1 mg / L in order to prevent
excessive zinc concentration in the effluent and to protect the aerobic biological pro-
cesses of secondary wastewater treatment plants. The zinc will also concentrate in the
wastewater sludges and may limit land disposal application. For these reasons, main-
taining the zinc concentration at less than 0.25 mg / L in the water distribution system
is ordinarily an objective. Bimetallic polyphosphate or zinc orthophosphate solutions
are usually effective in the range of 0.1 to 0.3 mg / L.
Phosphate Blends
The majority of commercial drinking water corrosion inhibitors
contain a blend of orthophosphate and polyphosphate compounds. The predominant
polyphosphate component in most blends is either tripolyphosphate, hexametaphos-
phate, or both. In many cases, mixtures of ortho- and polyphosphates are used to
combine corrosion protection with hardness stabilization or red-water suppression.
The fraction of orthophosphate typically ranges from 5 to 40 percent. High pro-
portions of orthophosphate provide more corrosion protection, while high proportions
of polyphosphate enhance sequestering. The dosage requirements are primarily depen-
dent on the pH and the calcium concentration in the water. Initiation of corrosion
control with blended polyphosphates is often slow; low dosages are used to avoid soft
scale formation and discolored water. Positive corrosion control may require three to
six months of dosing.
Although extraordinary claims of effectiveness relative to lead and copper corrosion
control have been made for many blended phosphate formulations, the preponderance
of the evidence suggests that simple orthophosphate is the active constituent and its
effectiveness is independent of its source.
Silicates
Silicate corrosion inhibitors come in both dry chemical and liquid silicate
solutions. In dry form, the silicates are Na
2
SiO
3
,Na
6
SiO
7
, and Na
2
Si
3
O
7
, with variable
waters of hydration. They are produced by fusing sodium carbonate with silica sand.
Under heat and pressure, soluble silicates are produced, and these vary in their pro-
portions of sodium oxide (Na
2
O) and silica (SiO
2
). Both dry solid and soluble sodium
