Iron is frequently used in water distribution systems, and its corrosion is of concern.
While structural failure as a result of iron corrosion is rare, water quality problems
(e.g., “red water”) can arise as a result of excessive corrosion of iron pipes. The corrosion
of iron is a complex process that involves the oxidation of the metal, normally
by dissolved oxygen, ultimately to form a precipitate of iron(III). This leads to the formation
of tubercules on the pipe surface. The major water quality factors that determine
whether the precipitate forms a protective scale are pH and alkalinity. The
concentrations of calcium, chloride and sulfate also influence iron corrosion. Successful
control of iron corrosion has been achieved by adjusting the pH to the range
6.8–7.3, hardness and alkalinity to at least 40 mg/litre (as calcium carbonate), oversaturation
with calcium carbonate of 4–10 mg/litre and a ratio of alkalinity to Cl- +
SO4
2- of at least 5 (when both are expressed as calcium carbonate).
Silicates and polyphosphates are often described as “corrosion inhibitors,” but there
is no guarantee that they will inhibit corrosion in water distribution systems. However,
they can complex dissolved iron (in the iron(II) state) and prevent its precipitation
as visibly obvious red “rust.” These compounds may act by masking the effects of corrosion
rather than by preventing it. Orthophosphate is a possible corrosion inhibitor
and, like polyphosphates, is used to prevent “red wate