How to clean the inner and outer walls of the fermentation tank?

### Surface Treatment for Fermentation Tanks: Pickling, Passivation, and Blue Spot Inspection

#### **1. Purpose of Passivation**

The passivation process creates a colorless, dense, and passive film on the stainless steel surface after pickling, enhancing its corrosion resistance.

#### **2. Pickling Solution Formula**

- **Nitric Acid (1.42% RH):** 20%

- **Hydrofluoric Acid:** 5%

- **Water:** 75%

**Preparation Steps**:

- Slowly add nitric acid and hydrofluoric acid to water while stirring thoroughly.

#### **3. Pickling Procedure**

1. **Surface Preparation**:

- Protect carbon steel surfaces with acid-resistant paint or paraffin.

- Polish weld scars and scratches on stainless steel until smooth.

- Clean with an alkaline solution (e.g., soapy water) to remove oil and dirt, then rinse thoroughly.

2. **Pickling Process**:

- Use a mop, fine sandpaper, or brush to apply the pickling solution to the surface, keeping it moist.

- Scrub the surface with a stainless steel wire brush until it becomes silvery white, paying extra attention to welds and heat-affected zones.

3. **Rinsing and Drying**:

- Rinse with clean water.

- Scrub with soapy water, rinse again, and dry using compressed air or air-drying.

#### **4. Passivation Solution Formula**

- **Nitric Acid (1.42 wt%)**: 5%

- **Potassium Dichromate:** 2%

- **Water:** 93%

**Preparation Steps**:

- Slowly add nitric acid and potassium dichromate to water while stirring. **Warning**: Never add water to nitric acid to avoid hazards.

#### **5. Passivation Procedure**

- Apply the passivation solution using a mop, gauze, or wool.

- Keep the surface moist for 2–3 hours, ensuring even coverage.

#### **6. Blue Spot Inspection for Passivation Quality**

- **Test Recipe**:

- Potassium Ferrocyanide: 10 g

- Nitric Acid: 30 g

- Distilled Water: 960 g

- **Inspection Method**:

- Apply the test solution to the cleaned, passivated surface.

- After 30 seconds, check for blue spots.

- **Acceptance Criteria**: No blue spots should appear, with a maximum allowance of 5 blue spots per square meter. Inspect suspicious areas closely.

By following these steps, the fermentation tank surface can achieve optimal corrosion resistance and quality.