Phosphoric Acid Resistant Centrifugal Fan

Overview

Phosphoric acid is a medium-strength tribasic inorganic acid with little oxidising power, so it does not promote the passive film on stainless steel the way nitric acid does. Selection is made directly from concentration, temperature and impurity content. 316L is common in ambient dilute acid, while hotter or stronger acid moves to 2205 duplex, 904L or non-metallic materials. Wet-process acid often carries fluoride and sulphate impurities that change the corrosion behaviour.

Material Selection by Concentration, Temperature and Impurities

Because phosphoric acid is not oxidising, stainless steel cannot rely on passivation and its resistance depends on the grade itself and the duty window.

Location	Material / process	Basis
Ambient dilute acid	316L stainless steel	Resistant and cost-effective at low concentration and temperature
Medium temperature or higher concentration	2205 duplex or 904L	Better pitting and uniform corrosion resistance
Hot concentrated acid or fluoride impurities	PVDF / PP / FRP non-metallic	Avoids localised attack and fluoride erosion
Impeller	316L, 2205 or PP / PVDF, integrally formed	Wetted parts in direct contact with the medium
Sealing	PTFE or fluoroelastomer seal	Resists phosphoric acid and stops acid mist leakage

Construction and Process Measures

Location	Material / process	Basis
Weld treatment	Post-weld pickling, passivation and grinding	Reduces weld sensitisation and crevice attack
Fasteners	316L or coated bolts	Prevents fasteners corroding first
Drain and wash	Low point drain and wash connection	Limits acid and impurity residue
Motor isolation	Anti-corrosion baffle between motor and casing	Protects motor insulation
Fluoride handling	Non-metallic lining isolation	Wet-process fluoride attacks metal and glass phases

Applications

Wet- and thermal-process phosphoric acid production, phosphate and compound fertiliser plants, metal phosphating treatment, pre-electroplating treatment and food-grade phosphoric acid dosing, wherever phosphoric acid or its mist is handled.

Wet-process phosphoric acid often carries fluoride and sulphate impurities, and condensed acid mist is more aggressive, so liquid pooling and dead corners must be avoided.

Phosphating baths run warmer with fluctuating concentration, so selection must be checked against the actual bath composition and temperature. Where hydrochloric or hydrofluoric acid is also present, the rules for phosphoric acid no longer apply.

Acceptance and Verification

Standard	Title	Purpose
HG/T 20679	Design code for external anti-corrosion of chemical equipment and piping	Anti-corrosion design for chemical media
GB/T 4237	Hot rolled stainless steel plate, sheet and strip	Stainless impeller and casing material
GB/T 20878	Stainless and heat-resisting steels - Designation and chemical composition	High temperature and corrosion resistant grades
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
GB/T 10178	Industrial fans - Performance testing in situ	On-site performance verification

Please provide the phosphoric acid concentration range, medium temperature, whether fluoride impurities are present and the air flow. Size and motor power are calculated for the actual duty.

FAQ

Why cannot phosphoric acid rely on passivation like nitric acid?

Phosphoric acid is a medium-strength reducing acid, so it does not promote passivation of stainless steel and resistance depends on grade, concentration and temperature. 316L suits only ambient dilute acid; hotter or stronger acid moves to 2205 duplex or PVDF / PP non-metallic materials.

Is material selection the same for wet- and thermal-process phosphoric acid?

No. Wet-process acid often carries fluoride and sulphate impurities, and fluoride attacks metals and glass phases, so PVDF, PP, FRP or higher alloys are preferred. Thermal-process acid has fewer impurities and a wider metal selection window.

What is the maintenance focus for a phosphoric acid fan?

Welds and impeller roots. Crevices and weld slag residues corrode first, so establish the inspection interval during the first year through visual inspection and thickness measurement.

Related pages: Anti-corrosion fan range  Sulphuric acid resistant centrifugal fan  Hydrochloric acid resistant centrifugal fan