Alkalinity

In most countries total alkalinity is the first factor that has to be examined.

The presence of alkalinity is necessary as to control corrosion tendencies and to participate to scale inhibition programs.

Magnesium requires alkalinity for its control, as to be precipitated into the boiler by the following reactions:

Mg 2++OH-----> Mg (OH)2  (Broucite)

3Mg2++2 SiO32-+2 OH------> 2Mg SiO3*Mg (OH)*H2O    ( Serpentine)

Calcium is precipitated under phosphate treatment by the reaction:

   Ca2++6CO32- +2 OH-  ------à 3 Ca3(PO4)2 * Ca(OH)2

While in all organic:

                                    Ca2+ +CO32-       -----> CaCO3

Under insufficient alkalinity, in phosphate treatment the reactions will be

                 Mg2+             + SiO3 2-  ------> Mg SiO3          

                  Ca2+            + SiO32-  ------> Ca SiO3        

                     3 Ca2+  + 2 PO43- ------> Ca3 (PO4)2         

              3 Mg2+ + 2 PO43- ------> Mg3 (PO4)2

which are hard and very difficult removable scales.

In all organic treatment the reactions will be:

                        Mg2+      + SiO3 2-  ------> Mg SiO3          

                  Ca2+            + SiO32-  ------> Ca SiO3        

                          Ca2+ +SO42--------> CaSO4*2 H2O

Resulting also to hard and non-removable scale.

While an alkalinity presence is necessary, the increase of its value over a certain limit will induce foaming and carry over problems and caustic attack of the boiler metals.

Caustic in excess is reacting with the iron producing Metaferric Sodium and native Hydrogen:

Fe + 2 NaOH à Na2FeO2 +2[H]

The [H] attacks sementite producing methane gas.

Fe3C+4[H] -----> 3 Fe+CH4

Sementite is a crystallographic substance, in very low quantity into the metal crystals and is responsible for the mechanical properties of the steel.

Its deterioration from caustic attack is proportionally deteriorating the pressure resistance of the steel, while is not leading to significant metal mass loss. The danger of explosion is obvious, so even under low pressure conditions we have to observe low caustic alkalinity excess.

According to the ASME STANDARDS for boiler pressure below 14 bar limit the total alkalinity to 700 ppm as CaCO3.

Normally there is not limit in total alkalinity only, but in the relationship of the different alkalinity measurements, related to the specific products in use.

If the chemicals supplier has not defined another ratio, we suggest to keep the following relation:

200 < 2 * pA - mA < 400

pA and mA in ppm as CaCO3.

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