Corrosion Inhibition

Instructions for using sodium silicate and sodium sulfite to prevent corrosion in thermal storage systems.

Correction (November 2026): this page previously gave a sodium sulfite dose of 30 grams per cubic metre. That is less than the oxygen in a fresh fill uses up, so it could not leave the target residual. If you dosed at 30 grams per cubic metre, test the sulfite residual and top up as described below.

Sodium silicate (corrosion inhibitor)

Sodium silicate forms a thin, protective silica film on metal surfaces such as copper, brass, and steel. This reduces maintenance costs by minimising corrosion-related damage.

Dosage: 400 grams per cubic metre of water, as sodium metasilicate pentahydrate or as liquid sodium silicate (water glass, about 27 % SiO₂). Either gives about 110 mg/L as SiO₂. Anhydrous sodium metasilicate is stronger (about 49 % SiO₂): use about 230 grams per cubic metre for the same result.

Sodium sulfite (oxygen scavenger)

Sodium sulfite chemically reacts with dissolved oxygen in the water, eliminating it and thus reducing corrosion risk. This is particularly important where air ingress occurs.

First fill: 160 grams of anhydrous sodium sulfite per cubic metre of water.

Top up: to raise the residual by 10 mg/L, add 16 grams per cubic metre.

Where these figures come from

• Each gram of dissolved oxygen uses 7.9 grams of sodium sulfite in theory, and about 10 grams in practice.
• Fresh mains water at 10 °C holds about 11 mg/L of oxygen, so a new fill uses about 110 grams per cubic metre before any residual remains.
• Test kits usually report the residual as sulfite (SO₃), and sodium sulfite is 63.5 % sulfite. A residual of 30 mg/L therefore needs about 47 grams per cubic metre more.
• 110 + 47 ≈ 160 grams per cubic metre. A warmer fill holds less oxygen and needs a little less.

Vented stores

In a vented (open) store, air reaches the water through the vent and the headspace, so sulfite is used up continuously, not only at filling. Test monthly for the first three months to learn how fast your store uses it, then set the test interval from that. Each gram of sulfite used leaves about 1.1 grams of sodium sulfate in the water. If the store needs frequent top-ups, these salts build up: change part of the water and re-dose.

Application process

1. Safety preparation: wear protective equipment (gloves, eye protection)
2. Dissolve and add chemicals to the thermal store
3. Circulate: run pumps for a minimum of 1 hour per cubic metre for thorough mixing
4. Monitor pH (target 9.0-10.0) and sulfite residual (target 20-40 mg/L as SO₃)
5. Top up chemicals when water is added or levels drop

Maintenance

Test monthly for the first three months, then every three months:

• pH level (maintain 9.0-10.0)
• Sulfite residual (maintain 20-40 mg/L as SO₃)
• Visual inspection for corrosion
• Water clarity

Additional considerations

• Keep the pH at or below 10.0. VDI 2035 (heating water) gives 8.2 to 10.0 for systems without aluminium, and at most 9.0 where aluminium is in contact with the water.
• Minimise air ingress to reduce oxygen scavenger consumption
• Use softened or demineralised water where possible
• Store chemicals in cool, dry conditions away from direct sunlight
• Both chemicals are registered under the REACH Regulation (EC) No 1907/2006