Anti-Corrosion Phosphate Dosing Equation
Phosphate dosing controls corrosion and scale in boilers, cooling towers and potable distribution. Orthophosphate (PO₄³⁻) forms a passive film on iron; polyphosphate sequesters calcium and prevents scale; zinc-phosphate blends do both. Boiler dose is set to maintain 30–60 mg/L PO₄ in the boiler water; cooling-tower dose is 2–8 mg/L recirculating residual.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Caustic Soda Dosing Calculator — pH Raise · Neutralisation
- Lime Dosing Calculator — pH Correction · Alkalinity Boost
- RO Antiscalant Dosing Calculator — RO · Desalination · Membrane Protection
- All Chemical Engineering Calculators →
Frequently Asked Questions
2–8 mg/L PO₄ residual in recirculating water for orthophosphate-based programs; 6–15 mg/L total inorganic phosphate for poly + ortho blends. Apply enough makeup-rate dose to balance the bleed-off losses (cycles of concentration matters).
Coordinated (high-pH) phosphate: maintains Na:PO₄ ≥ 2.6 to avoid free caustic — used in industrial boilers up to 100 bar. Congruent (lower Na:PO₄ < 2.6): used in older lower-pressure boilers but risks acid-phosphate corrosion. Modern utility boilers use coordinated or all-volatile treatment (AVT) with no phosphate.
EPA secondary MCL = 0.5 mg/L total phosphate as P in finished drinking water. Higher residuals encourage biofilm regrowth in distribution mains and feed algae blooms in receiving waters. Always dose to the lowest residual that controls red-water iron leaching (typically 0.5–2 mg/L PO₄).
No — phosphate sequesters hardness ions (Ca²⁺, Mg²⁺) only at low concentration (<300 mg/L hardness). Lime softening removes the ions completely. Phosphate is a maintenance program; lime is bulk-removal. They are often used together: lime softens, then phosphate stabilises the residual.
Yes in some surface-water-discharging utilities — phosphate is a eutrophication driver, and many EU and US states are tightening WWTP discharge limits to <1 mg/L P. Cooling-tower programs are migrating to all-organic (phosphino-carboxylic acid) inhibitors, though phosphate remains the best for mild-steel boilers.