Acid Phosphate Corrosion

What Does Acid Phosphate Corrosion Mean?

Acid phosphate corrosion is one type of corrosion that occurs in boiler feedwater systems in the inlet tube sheets. Boilers, which generally use congruent phosphate pH treatment to prevent caustic corrosion, may in turn be subject to acid phosphate corrosion when boiler water is concentrated beneath deposits. This is the result of a localized concentration of phosphate and the high temperature of the metal tube.

Corrosionpedia Explains Acid Phosphate Corrosion

To avoid any caustic corrosion, demineralization of boiler feedwater systems must be performed or the waterfeed has to be evaporated or condensated to prevent the spread of caustic corrosion attack on the outside layer of the boiler or beneath the boiler by controlling the pH content of the phosphate.

The pH value of phosphate can be controlled by adding phosphate buffers in the boiler feedwater, which basically reduces large pH changes due to the development of highly caustic concentrations. However, an excess caustic concentration is also a problem because it combines with disodium phosphate to form trisodium phosphate. To prevent a caustic buildup under the deposit or in a crevice, sufficient disodium phosphate needs to combine with the free caustic to form trisodium phosphate.

The reaction to form trisodium phosphate to control the pH value is:

Na₂HPO₄ + NaOH —-> Na₃PO₄ + H₂O

where, Na₂HPO₄ is disodium phosphate

NaOH is sodium hydroxide

Na₃PO₄ is trisodium phosphate

H₂O is water

The water that is formed from the reaction contains phosphate, and it tends to form deposits near the inlet tube or in places where it leaks and leads to acidic phosphate corrosion of the boiler tubes and nearby metal corrosion. Visually it is difficult to differentiate between caustic corrosion and acid phosphate corrosion, but by analyzing the salt deposits at the site and by conducting certain tests you can reveal the difference.

Some of the factors that may lead to acid phosphate corrosion in boiler feedwater systems include:

• When the mole ratio of Na/PO₄ has been lowered.
• The concentration of phosphate is increased more than the threshold limit.
• The operational temperature is high.