Explosion-Proof Motor Selection Guide for Chlor-Alkali Chemical Plants

The chlor-alkali industry produces chlorine (Cl₂), hydrogen (H₂), and caustic soda (NaOH) through electrolysis of brine. However, the process creates an extremely aggressive corrosive environment for electric motors.

In 2023, a chlor-alkali plant in Shandong experienced catastrophic motor failure after only 8 months of operation. The gray cast iron housing had severely corroded through, allowing corrosion debris to fall into the stator winding and cause a turn-to-turn short circuit. The entire electrolysis line was shut down for 36 hours. Investigation revealed: the motor housing was standard cast iron with no anti-corrosion treatment, and the terminal box gaskets had completely failed within 3 months in the chlorine atmosphere.

1. Strongly Oxidizing Chlorine (Cl₂)

   • Forms hypochlorous acid (HClO) and hydrochloric acid (HCl) in humid conditions

   • Aggressively attacks copper, iron, and aluminum

   • 2.5* denser than air, accumulates near ground level

2. Caustic Soda Mist (NaOH)

   • Mist concentration from electrolytic cells: 5%–15%

   • Strongly attacks aluminum and zinc coatings (aluminum dissolves in alkali forming sodium aluminate)

   • Crystalized solids block cooling air passages

3. Coexisting Hydrogen Risk

   • Hydrogen is a byproduct of chlor-alkali electrolysis

   • Zone 2 hydrogen areas exist in electrolysis buildings

   • Motors must meet both IIC (hydrogen) and corrosion protection requirements

4. High Temperature + High Humidity Overlay

   • Ambient around electrolytic cells: 35–55°C, 80%–95% RH

   • Electrolyte evaporation drives humidity near saturation

5. Salt Spray Deposition

   • Sodium chloride particles from brine spread with steam, forming conductive salt films

   • Salt films absorb moisture, reducing insulation resistance and causing creepage flashover

6. Galvanic Corrosion

   • Dissimilar metals form galvanic cells in electrolyte environments

   • Example: cast iron housing vs. copper winding accelerates corrosion

Additional corrosion marking: Recommend Corrosion Protection C5-M (per ISO 12944).

• Primer: Inorganic zinc-rich (zinc content ≥80%, DFT 60–80 μm)

• Intermediate: Epoxy micaceous iron oxide (DFT 80–100 μm), blocks chloride ion penetration

• Topcoat: Polyurethane or fluorocarbon (DFT 40–60 μm), UV and chemical resistant

• Total DFT: ≥200 μm (ISO 12944 C5-M requirement)

• Adhesion: Pull-off test ≥ 8 MPa

• O-rings: Perfluoroelastomer (FFKM, e.g., Kalrez or Chemraz), resists chlorine and caustic

• Cable entry: Triple seal — outer EPDM dust seal, middle PTFE anti-corrosion gasket, inner FFKM main seal

• Bearing seal: Magnetic labyrinth seal + nitrogen purge (0.1–0.3 bar), prevents corrosive gas ingress

• Drain valve: Hastelloy C276, spring-return type, corrosion-resistant

• Magnet wire: Corrosion-resistant polyimide/polyamide-imide (PI/PAI) composite coated wire

• Impregnation: Chemical-resistant epoxy resin vacuum pressure impregnation (VPI), fully fills voids

• Slot liner: Polyethylene naphthalate (PEN) film, acid/alkali and high temperature resistant

• End winding lacing: PTFE lacing tape, non-hygroscopic and non-hydrolyzing

• Air path optimization: Positive pressure ventilation using filtered and dried instrument air

• Air intake: HEPA filter (H13 grade) + activated carbon adsorption layer, removes Cl₂ and acid gases

• Air outlet: One-way check valve, prevents backflow of corrosive gas

• Pressure monitoring: Differential pressure switch with alarm/shutdown at low setpoint

Third-party lab simulation of chlor-alkali electrolysis environment (5 ppm Cl₂ / 10 mg/m³ NaOH mist / 55°C / 95% RH, 2,000-hour continuous operation):