Sodium Molybdate Corrosion Inhibitor Supplier South Africa

What Is Sodium Molybdate?

Sodium molybdate (Na₂MoO₄·2H₂O) is the sodium salt of molybdic acid — a white crystalline solid highly soluble in water. In water treatment chemistry, sodium molybdate is used as an anodic corrosion inhibitor for ferrous metals (carbon steel) in closed recirculating water systems, cooling water, and process water systems. Molybdate inhibitor mechanism: MoO₄²⁻ ions migrate to anodic sites on the steel surface where iron dissolution is occurring, react with Fe²⁺ ions to form an insoluble iron molybdate (FeMoO₄) passivation film, and effectively blocking further metal dissolution. This anodic passivation mechanism makes molybdate a cathodic synergist — it works best in combination with cathodic inhibitors (zinc, benzotriazole) in formulated corrosion inhibitor blends.

Application in Closed Loop and Cooling Water Systems

Sodium molybdate is the preferred corrosion inhibitor in closed loop chilled water and hot water systems because it does not biodegrade, does not form scale precipitates at high concentration, and offers very low mammalian toxicity (LD₅₀ > 4 g/kg oral). In closed systems (HVAC chilled water, heating circuits, closed cooling water), molybdate is maintained at 50–200 mg/L MoO₄²⁻ combined with an organic corrosion inhibitor (azole for copper protection) — this combination provides comprehensive corrosion protection for both mild steel and copper alloy components. In open recirculating cooling systems, molybdate is used where molybdate concentration can be maintained at cycles of concentration (50–100 mg/L target in recirculating water) despite blowdown dilution. Molybdate is compatible with azoles, HEDP, and zinc but incompatible with calcium ions at high concentration (calcium molybdate scale risk above ~150 mg/L Mo in hard water).

Non-Toxic Green Inhibitor and Environmental Profile

A key advantage of sodium molybdate over conventional corrosion inhibitors (chromate, nitrite, zinc) is its environmental and health profile. Chromate inhibitors (historically the most effective) are acutely toxic, carcinogenic, and heavily regulated under environmental legislation — they have been replaced by molybdate in most new systems. Nitrite inhibitors are unstable (oxidise to nitrate under chlorine dosing, support bacterial activity) and regulated in some effluent streams. Zinc is effective but requires careful management to meet discharge limits. Molybdate is non-toxic, non-carcinogenic, biodegradation-resistant, and accepted in effluent discharge (though it is an essential micronutrient — very high Mo discharge to environmentally sensitive water is managed). Terracore Chemicals supplies sodium molybdate dihydrate in 25 kg bags.