hromite powder is widely used in foundry coatings (also called mold washes or refractory coatings) to improve the surface quality of metal castings.

1. Role of Chromite in Foundry Coatings

Chromite powder (FeCr₂O₄) is applied as a refractory coating on sand molds and cores to:

• Prevent metal penetration (burn-on defects).

• Reduce sand fusion (sand sticking to the casting).

• Improve surface finish (smoother castings).

• Withstand high temperatures (especially for steel and high-alloy castings).

• Minimize veining and erosion from molten metal flow.

2. Key Properties of Chromite for Foundry Use

• High Melting Point (~2,180°C / 3,956°F) – Resists steel and iron pouring temps.

• Low Thermal Expansion – Reduces cracking and veining in molds.

• Chemically Inert – Minimizes reactions with molten metal.

• High Density – Helps block metal penetration into sand molds.

• Good Thermal Conductivity – Promotes even cooling.

3. Typical Foundry Coating Formulations

Chromite is mixed with carriers and binders to form a sprayable or brushable coating:

• Base Refractory (40–60%): Chromite powder (sometimes blended with zircon or alumina).

• Carrier Liquid (30–50%): Water or alcohol (e.g., isopropanol).

• Binder (5–10%): Sodium silicate, colloidal silica, or organic resins.

• Additives (0–5%): Suspension agents (e.g., bentonite), wetting agents, or graphite.

Example Coating Recipe (for steel casting):

• 50% Chromite powder

• 40% Water

• 8% Colloidal silica (binder)

• 2% Bentonite (anti-settling agent)

4. Application Methods

1. Spraying (most common for large molds).

2. Brushing/Dipping (for smaller cores and intricate molds).

3. Flow Coating (for uniform coverage).

After application, the coating is dried with torches or air before pouring metal.

5. Advantages Over Other Refractory Coatings

Best Used For:

• Steel castings

• High-alloy metals (e.g., manganese steel).

• Large castings requiring thermal stability.