Carbonate Scale Minerals

Among the most frequently encountered corrosion scale compounds in distribution systems are the carbonates of Ca, Cu, and Pb.

Common carbonate minerals found as corrosion products in drinking water distribution systems
Name, chemical formula	Scale image (Click for larger image)	Photomicrograph (SEM)
Cerussite  PbCO3
Hydrocerussite  Pb3(CO3)2(OH)2 l
Plumbonacrite Pb5O(OH)2(CO3)3
Malachite   Cu2(CO3)(OH)2
Calcite   CaCO3

These substances share the property that their solubility decreases if either pH or alkalinity is raised, hence the strategy of pH adjustment for corrosion control. For each component of a corrosion scale, we can quantify its solubility by using the Saturation Index, which compares the measured concentrations of the consituents of the mineral to corresponding equilibrium concentrations. For example, for the dissolution of CaCO₃

CaCO3 à Ca++ + CO3 =

Ksp = (a_{Ca, equil})(a_{CO3, equil}) and IAP = (a_Ca,meas)(a_{CO3, meas})

When IAP = K, the ratio IAP/K = 1, the log (IAP/K) = 0, and we say the system is at equilibrium, or in the case of dissolving solids, the solution is saturated with respect to the compound of interest.

Because it is necessary to calculate activity coefficients and extent of complexing for each substance that is present in significant amounts, the calculation of degree of approach to equilibrium (or to saturation for solids) requires the iterative solving of an extensive set of simultaneous equations. Several computer codes have been developed to do these calculations. See the page Approach to Equilibrium

  Degree of Saturation of Carbonates in Distribution Systems

  Most lead and copper surfaces in distribution systems are coated with oxide, carbonate, or phosphate minerals, often in successive layers. Among the most common are the Pb carbonates cerussite and hydrocerussite. Copper and brass surfaces host copper oxides and the carbonate malachite plus a variety of copper sulfates. The zinc component of brass can yield the carbonate smithsonite, but this phase is fairly soluble and is not usually seen on pipe fittings, but can occur in faucets and on faucet aerators.

  Most drinking water at the tap is undersaturated with any of the lead carbonates at the Pb action level (0.015 mg/L). Therefore water reacting with, say, a Pb service line, will, given time, dissolve these minerals to yield Pb concentrations above the action level. That is, the waters are corrosive to Pb scales at the action level.

The table below shows some examples from systems with a range of alkalinity values.