Caustic Soda Resistant Centrifugal Fan

Overview

Sodium hydroxide is a strong alkali whose corrosion behaviour differs from acids: it attacks amphoteric metals such as aluminium and zinc, and hot concentrated lye causes stress corrosion cracking in carbon steel and austenitic stainless steel. The fan wetted parts are made from 304, 316L, nickel alloy or FRP / PP non-metallic materials, and high-stress carbon steel members are avoided.

Material Selection by Concentration, Temperature and Corrosion Mechanism

Alkali corrosion follows two main lines: dissolution of amphoteric metals and stress corrosion cracking. Selection starts by fixing concentration, temperature and whether evaporation crystallisation or high temperature is present.

Location	Material / process	Basis
Ambient dilute lye	304 / 316L stainless steel	Alkali resistant at low temperature and concentration; no amphoteric metal
Higher temperature or concentration	316L or duplex	Resists uniform corrosion; avoids high-stress carbon steel
Hot concentrated lye	Nickel alloy or FRP / PP	Avoids stress corrosion cracking and hot alkali attack
Impeller	Stainless steel, nickel alloy or PP / FRP, integrally formed	Wetted parts in direct contact with alkali mist
Sealing	Fluoroelastomer or EPDM seal	Resists alkali and stops mist leakage

Construction and Process Measures

Location	Material / process	Basis
Stress control	Avoid high-stress carbon steel and control weld residual stress	Prevents stress corrosion cracking in hot lye
No amphoteric metal	Aluminium, zinc and galvanised parts kept off wetted areas	Alkali reacts with amphoteric metals
Weld treatment	Post-weld grinding and residual stress relief	Reduces caustic embrittlement zones
Drain and wash	Low point drain and wash connection	Limits lye evaporation crystallisation and residue
Motor isolation	Anti-corrosion baffle between motor and casing	Protects motor insulation and isolates alkali mist

Applications

Chlor-alkali electrolysis, pulp and paper, alumina production, textile mercerising, detergent and soap saponification and water treatment pH adjustment, wherever sodium hydroxide or alkali mist is handled.

Lye concentration rises on evaporation and crystallised deposits increase local attack, so liquid pooling and dead corners must be avoided.

Hot concentrated lye risks stress corrosion cracking in carbon steel and austenitic stainless steel, so material and welding stress are assessed together. Where acid is also present, the lye selection rules 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
ISO 12944-5	Paints and varnishes - Corrosion protection of steel structures by protective paint systems - Part 5	Coating system and durability class
GB/T 3854	Barcol hardness test for reinforced plastics	Hardness acceptance of FRP parts
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
GB/T 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing grade

Please provide the sodium hydroxide concentration range, medium temperature, whether evaporation crystallisation occurs and the air flow. Size and motor power are calculated for the actual duty.

FAQ

Why are aluminium and galvanised parts not used in a sodium hydroxide fan?

Aluminium and zinc are amphoteric metals that dissolve in sodium hydroxide, and zinc coatings are attacked in the same way. Wetted parts use 304, 316L, nickel alloy or FRP / PP instead.

Why is high-stress carbon steel avoided in hot concentrated lye?

Hot concentrated lye causes stress corrosion cracking in carbon steel and austenitic stainless steel, and high-stress members crack first. Nickel alloy or non-metallic materials are used with controlled weld residual stress.

What is the maintenance focus for a sodium hydroxide fan?

Welds, impeller roots and low point liquid pockets. Evaporation crystallisation deposits corrode first, so establish the interval during the first year through visual inspection and thickness measurement.

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