Feed water treatments are designed to prevent the formation of calcareous deposits and the occurrence of corrosion. Despite the two phenomena (scale and corrosion) being separated, the problem is to be considered unique and interconnected because the limestone deposits often facilitate the emergence of new corrosive phenomena.

ENCRUSTATIONS: WHAT ARE THEY?

Encrustations are deposits formed in the pipes, in the heat exchangers, in the filling packs of the evaporative tower, and whenever there is hard water with the increase of its temperature. The hardness of the water indicated the concentration of calcium (Ca) and magnesium (Mg), that are responsible for the calcareous formations, together with the concentration of bicarbonates (alkalinity).

Hardness is a parameter that can be controlled using softening systems with ion exchange resins, that can replace calcium and magnesium with sodium (which does not form deposits), however; alkalinity can be reduced with appropriate products, especially via purges, both in towers and in boilers. Tables are available that define the control limits for hardness and alkalinity to prevent fouling; compliance with these limits must however be associated with the use of effective anti-scaling products.

In addition to the hardness and alkalinity of water, other factors that cause fouling are:

• Higher water temperatures that decrease the solubility of the precipitated calcium salts.
• The presence of suspended material.
• A low water flow rate which favours sedimentation of deposits.

WHAT INCONVENIENCES CAUSE ENCRUSTATIONS?

As the encrustations increase, energy losses increase, due to the fact that the limestone prevents heat exchange by dispersing heat and energy. This economic problem becomes a worrying safety issue in boilers. In steam generators high temperatures and high pressures are reached; if the heat exchange between the flame (and the fumes) and the water is prevented by the presence of deposits, the pipes overheat, with the possibility of explosion.

IS IT POSSIBLE TO PREVENT ENCRUSTATIONS?

One can intervene in different ways depending on the system considered:

• Sanitary water: using appropriate anti-scaling agents for drinking water, added with a proportional system.
• Water in closed circuits (heating and cooling): it is advisable to use softened water or, if it is not available, raw water added with appropriate inhibitory products.
• Cooling water: it is advisable to use raw water, softened with anti-fouling products, and to properly control the drains. The anti-scaling chemicals work by modifying the structure of calcium carbonate in a non-adherent and, above, all irregular shape, such as not to grow the crystals that form the deposit.
• Boiler water: it is necessary to sweeten or demineralize feed water, to add dispersants and check the drains.

CORROSION: WHAT IS IT?

Corrosion is an electrochemical process in which metal (e.g. iron) is transformed into its oxide (e.g. iron oxide = rust). For corrosion to occur it is necessary that, as in a pile, there is an anode (the part of the system that corrodes), a cathode (that part of the system responsible for oxidation, which is reduced) and an electrolyte (water). The metal oxidizes, i.e. dissolves (anode) and develops an electric current (electrons), which, via the metal, goes to the cathode where the reduction reaction takes place. The result of this process is the consumption of metal and the formation of oxide.

The most important factors influencing corrosion are:

• Oxygen and other gases dissolved in water: the presence of oxygen is essential for the oxidation-reduction reaction (corrosion) to proceed, therefore eliminating oxygen means eliminating the main cause of corrosion.
• Conductivity (salinity) and suspended solids: the higher the conductivity of the water, i.e. the concentration of dissolved salts, the higher the process of migration of electrons and ions, therefore the greater the corrosion rate. Among the most dangerous dissolved salts we mention chlorides and sulphates. The presence of suspended solids (sand, limestone or iron deposits) has an erosion effect on the metal which accelerates the corrosion of the system; moreover, the suspended solids can sediment and adhere to the metal, causing dangerous deposits.
• Alkalinity and acidity: acid or neutral water (pH ≤ 7) attack the metal and remove the oxide film that protects the metal; alkaline waters (pH> 8.5) favour the formation of a protective film. Amphoteric metals (aluminium) are the exception because they tend to corrode at very high pH.
• Water speed: very high water velocity values ??(greater than 2 meters per second) increase corrosion, as they increase the oxygen transport on the metal surface and the removal of the protective oxide film. High speed values ??also increase erosion. On the other hand, very low water velocity values ??(less than 0.5 meters per second) cause sedimentation, deposit formation and, consequently, corrosion. The optimal speed is 1-2 meters per second.
• Temperature: the increase in temperature increases the corrosion rate. In steam generators, where high temperatures are reached, the danger of localized corrosion is very high.
• Bacteria: the presence of bacteria, therefore the formation of silt, causes corrosion. There are also anaerobic bacteria (living in the absence of oxygen) that attack the iron causing corrosion.

WHAT ARE THE METHODS USED TO PREVENT CORROSION?

First of all, it is necessary to use suitable materials, i.e. metals that resist corrosion (copper, aluminium, stainless steels). Moreover, it is always important, if possible, to work in conditions of pH ≤ 8.5 and, above all, to use an effective chemical treatment based on corrosion inhibitors. Corrosion inhibitors are chemicals that reduce or block the oxidation reaction, responsible for corrosion, by acting on the cathode, the anode or both.

An effective treatment program is based on the use of valid products, checked by competent technicians who, through the analysis of the chemical parameters of water can verify the effectiveness of the treatment and make any corrections to ensure the promised results.

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