Boiler water chemicals include all chemicals that are used for the following applications:

• Oxygen scavenging;
• Scale inhibition;
• Corrosion inhibition;
• Antifoaming;
• Alkalinity control.

Coagulants

When referring to coagulants, positive ions with high valence are preferred. Generally aluminium and iron are applied, aluminium as Al2(SO4)3- (aluin) and iron as either FeCl3 or Fe2(SO4)3-. One can also apply the relatively cheap form FeSO4, on condition that it will be oxidised to Fe3+ during aeration.

Coagulation is very dependent on the doses of coagulants, the pH and colloid concentrations. To adjust pH levels Ca(OH)2 is applied as co-flocculent. Doses usually vary between 10 and 90 mg Fe3+/ L, but when salts are present a higher dose needs to be applied.

Corrosion inhibitors

Corrosion is a general term that indicates the conversion of a metal into a soluble compound.

Corrosion can lead to failure of critical parts of boiler systems, deposition of corrosion products in critical heat exchange areas, and overall efficiency loss.

That is why corrosion inhibitors are often applied. Inhibitors are chemicals that react with a metallic surface, giving the surface a certain level of protection. Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film.

There are five different kinds of corrosion inhibitors.

These are:

1. Passivity inhibitors (passivators). These cause a shift of the corrosion potential, forcing the metallic surface into the passive range. Examples of passivity inhibitors are oxidizing anions, such as chromate, nitrite and nitrate and non-oxidizing ions such as phosphate and molybdate. These inhibitors are the most effective and consequently the most widely used.
2. Cathodic inhibitors. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc or magnesium, may be precipitated as oxides to form a protective layer on the metal.
3. Organic inhibitors. These affect the entire surface of a corroding metal when present in certain concentration. Organic inhibitors protect the metal by forming a hydrophobic film on the metal surface. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface.
4. Precipitation inducing inhibitors. These are compounds that cause the formation of precipitates on the surface of the metal, thereby providing a protective film.The most common inhibitors of this category are silicates and phosphates.
5. Volatile Corrosion Inhibitors (VCI). These are compounds transported in a closed environment to the site of corrosion by volatilisation from a source. Examples are morpholine and hydrazine and volatile solids such as salts of dicyclohexylamine, cyclohexylamine and hexamethylene-amine. On contact with the metal surface, the vapour of these salts condenses and is hydrolysed by moist, to liberate protective ions.

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