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Principals of Scale Formation

BeforeAfter

SUMMARY CHART OF HOW SCALE
FORMS

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What is Dissolution?
How scale (calcium carbonate) gets into the water stream?
Stage 1: Water becomes acidic
Whenever it rains, water in contact with air will absorb the CO2 gas in the air.

The following chemical reaction takes place:CO2 + H2O

H2CO3

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Stage 2: Acidic water dissolves calcium carbonate

As the acidic rain water reaches thesurface, it passes over andpermeates through rocks such as
limestone, marble and seashells,
forming soluble calcium ions andbicarbonate ions.

CaCO3+H2CO3(calcium carbonate) (carbonic acid)Ca+++2HCO3(in solution)

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Stage 3: Any condition which alters the solubility of calcium
bicarbonate will result in the precipitation of calcium carbonate(scale)There are three (3) major factors that can alter the solubility of calcium
bicarbonate and thus cause scale:A.) Temperature effect on solubility.
A change from cold to hot water will cause scale to form.

The reason?
When the temperature increases,
CO2 evaporates, allowing scale toprecipitate. Heating water also
causes evaporation, leaving
minerals behind.

B.) PH Change on Solubility
Solubility of CaCO3 decreases with an increase to thepH.
The reason?
PH is a measure of the acidity of the liquid. The lower the pH, the higher theacid content. This will dissolve more calcium carbonate.

C.) Pressure Effect on Solubility
A change in water pressure from high to low will cause scale to form. Pressure
drop may occur from internal friction between the molecules of water, external
friction between the water and the walls of the piping system, or rough area inthe channel through which the water flows.

The reason?
At a lower pressure, CO2 evaporates, thusincreasing pH. A higher pH allows for lessscale formation.

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What is Supersaturation ?
Supersaturationat the point of crystallization is the primary causeof scale deposition.
ØDefinitions:
Saturation: The maximum equilibrium concentration of a compound that will
dissolve into a solution under a given set of conditions (temperature, pressure,flow velocity, etc.).
Supersaturation: The solutions that contain higher concentrations of dissolved
solute than their equilibrium concentration.

To simplify "supersaturation", it can best be described as scale-causing ions thatbarely "hang in the water" When calcium and bicarbonate ions are hydrated,molecules are attached to the calcium and bicarbonate ions via ionic bonds,
which are much stronger than the vander Waal force. In a supersaturated
solution, the calcium and bicarbonate ions are 'partially' hydrated by watermolecules. The harder water is, the calcium and bicarbonate ions are hydrated
with much weaker hydration energy. We conclude that in a supersaturated
solution, calcium ions are barely 'hanging in water'.

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Causes of Local Supersaturation• Increase in temperature• Increase in pH• Decrease in pressure• Agitation of the solution• Decrease in flow velocity
Even when the bulk solution is less than fully saturated, scale formation canoccur due to local supersaturation.

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Nucleation Precipitation
You may be wondering when scale forms, why does it stick to surfaces?
The Answer: The electrostatic attraction between the metal surface andscale-causing minerals. Gravity plays no role in scale formation.

The unique characteristics of scale deposits
are its uniformity. Precipitates or crystals
formed in one part of a system and carried toanother part are less adherent than thosecrystals formed on site.

ØThe electrostatic attraction can be described in three theories:A ) Helmholtz Model
When a metal is in contact with an ionic solution such as water with scaleproducing minerals, the metal surface has a high density of electrons, giving ita locally negative charge.
The solvated positive ions such as H+, Ca++ and Mg++ align themselves
along the surface of the metal surface, producing an (electric) double layerno thermal motion of ions considered.

B ) The Gouy-Chapman Model – Diffuse Double-Layer
Due to the thermal motion of the ions in the solution, the population ofpositive charges (i.e., H+, Ca++ and Mg++)
decreases exponentially with increasing distance from the metal surface.
C ) Stern Model - The Previous Two are Combined.
The positive ions closest to metal surface are constrained into a rigidHelmholtz plane while outside the plane, the positive ions are dispersed
as in Gouy-Chapman model. It is this electric potential (or coulombs)
difference that causes the attraction of the scale to surfaces, which
explains their uniform deposition.