Nitric Acid Resistant Centrifugal Fan

Overview

Nitric acid is a strongly oxidising acid, so its corrosion behaviour differs from that of hydrochloric or sulphuric acid. Within a suitable concentration and temperature window, 304L and 316L stainless steel passivate and can be used, while dilute or hot acid requires titanium or non-metallic materials. Selection starts by fixing the concentration range, temperature limit and whether nitrogen oxides are present.

Material Selection by Concentration and Temperature Window

The oxidising nature of nitric acid promotes a passive film, which is why stainless steel can be used. When concentration or temperature enters the aggressive range, that film breaks down and attack accelerates.

Location	Material / process	Basis
Ambient, moderate concentration	304L / 316L stainless steel	Stable passive film in oxidising conditions
Higher temperature or fluctuating concentration	316L, duplex or titanium	Upgrade when the passive film becomes unstable
Concentrated or hot acid	Titanium or PTFE / PVDF lining	Avoids passive film breakdown and intergranular attack
Impeller	Stainless steel or titanium, integrally formed	Wetted parts in direct contact with the medium
Sealing	PTFE or fluoroelastomer seal	Resists nitric acid and stops acid mist leakage

Construction and Process Measures

Location	Material / process	Basis
Weld treatment	Post-weld pickling and passivation	Restores the passive film at stainless welds
Fasteners	Stainless or coated bolts	Prevents fasteners corroding first
Drain and wash	Low point drain and wash connection	Limits acid residue
Motor isolation	Anti-corrosion baffle between motor and casing	Protects motor insulation
Shutdown protection	Wash and keep dry when stopped	Reduces attack during standstill

Applications

Nitric acid production and storage, nitration reaction off gas, metal pickling, laboratory ventilation and electronics etching mist, wherever nitric acid or nitrogen oxide gases are handled.

Nitric acid vapour is often accompanied by nitrogen oxides, which become more aggressive on condensation, so stagnant areas and dead corners must be avoided.

Where hydrochloric or hydrofluoric acid is also present, the rules for nitric acid no longer apply and the mixed acid has to be assessed separately.

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 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing grade

Please provide the nitric acid concentration range, medium temperature, whether nitrogen oxides are present and the air flow. Size and motor power are calculated for the actual duty.

FAQ

Why can stainless steel be used for nitric acid?

Nitric acid is a strongly oxidising acid, so within a suitable ambient concentration window it promotes a stable passive film on 304L and 316L stainless steel. If the concentration is too low or the temperature rises, the film breaks down and titanium or non-metallic materials are required.

How does nitric acid selection differ from hydrochloric acid?

Hydrochloric acid attacks the passive film through chloride ions, so stainless steel is generally unsuitable. Nitric acid is oxidising, so within the right concentration and temperature window stainless steel can be passivated and used; the two rules must not be mixed.

What is the maintenance focus for a nitric acid fan?

Welds and impeller roots. Areas that were not fully pickled and passivated after welding corrode first, so establish the interval during the first year through visual inspection and thickness measurement.

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