Inline measurement of acid strength and iron salts in the pickling bath

A pickling bath is a tank or container filled with an acidic solution used to remove oxide deposits, rust, and other impurities from the surface of steel and other metals. This bath is a key component of the pickling process and ensures a clean and smooth surface before further processing such as galvanizing, coating, or painting.

Components of a pickling bath

1. Acidic solution

   • The type of acid used depends on the material to be processed and the desired cleaning effect:

     • Hydrochloric acid (HCl) - Fast-acting, commonly used for pickling steel

     • Sulfuric acid (H₂SO₄) - More economical but slower, often heated for better efficiency

     • Nitric acid-hydrofluoric acid (HNO₃ + HF) - Used for stainless steel to remove oxides and passivate surfaces

     • Phosphoric acid (H₃PO₄) - Used when a protective phosphate layer is required

2. Inhibitors (optional)

   • Additives that reduce acid attack on base metals while allowing scale and rust removal

3. Temperature control system

   • Some pickling baths are heated to improve reaction efficiency, especially when Pickling with sulfuric acid

4. Rinsing and neutralization (optional)

   • After pickling, the steel is often passed through a Rinse bath with water and sometimes an alkaline solution (e.g., sodium carbonate) to neutralize any acid residues

5. Ventilation and fume extraction

   • Acid baths produce fumes that can be hazardous, which is why industrial facilities are often equipped with fume scrubbers or ventilation systems

How a pickling bath works

1. Steel or metal parts are immersed in the pickling bath

2. The acid reacts with the oxides and dissolves them, cleaning the surface

3. The material is rinsed with water, to remove acid residues

4. If necessary, it is neutralized with an alkaline solution

5. The cleaned steel is dried and sent for further processing (e.g., galvanizing, painting)

Types of pickling baths

• Static pickling baths: For batch processing of steel parts, sheets, or coils

• Continuous pickling baths: Used in high-speed steel production lines where the metal continuously runs through multiple acid baths

• Electrolytic pickling baths: Here, electric current is used along with acid to enhance the removal of scale

Ecological aspects

• Acidic waste must be treated before disposal

• Spent acid can be regenerated (especially hydrochloric acid)

• Fume extraction systems are required for worker safety

Pickling is a chemical process used to remove impurities such as oxides (scale), rust, and other contaminants from the steel surface. It is commonly used in steel production and manufacturing to ensure a clean, smooth surface for further processing such as galvanizing, painting, or coating.

Step-by-step pickling process

1. Pre-cleaning (optional)

   • Before pickling, the steel can be mechanically cleaned or degreased to remove oils, dirt, and loose scale

2. Acid bath (pickling)

   • The steel is usually immersed in a bath with an acidic solution:

     • Hydrochloric acid (HCl) - Fast and efficient, commonly used in modern pickling plants

     • Sulfuric acid (H₂SO₄) - More cost-effective but slower and requires heating

     • Mixed or organic acid baths - For special applications with lower environmental impact

   • The acid reacts with the oxides and scale, dissolving them from the surface

3. Rinsing

   • After pickling, the steel is thoroughly rinsed with water to remove acid residues

4. Neutralization (optional)

   • Some processes include a neutralization step with an alkaline solution (e.g., sodium carbonate or sodium hydroxide) to prevent acid carryover

5. Passivation (optional)

   • A passivation layer (e.g., a phosphate or chromate treatment) can be applied to prevent immediate rusting

6. Drying and final processing

   • The cleaned steel is dried and can be further processed, e.g., by galvanizing, painting, or cold rolling

Types of pickling

• Batch pickling: Steel is processed in batches, which are typically used for coils, sheets, and finished parts

• Continuous pickling: Used for high-speed processing of steel strips in large production lines

• Electrolytic pickling: The removal of oxides is improved with the help of electric current

Summary

• In pickling, hot-rolled steel is immersed in a solution called pickling liquor to remove surface impurities and iron oxide scale

• The solution used is usually hydrochloric acid, but for high-carbon steels, additional acids such as nitric acid, sulfuric acid, or hydrofluoric acid may be required

• Pickling removes 1-3% of the steel mass, which complicates processing and painting

• The remaining pickling liquid, known as pickling sludge, is often neutralized with a base and disposed of, but it can also be used to recover useful products like hydrochloric acid and iron oxide

The monitoring of acid strength in a pickling bath is crucial for maintaining efficiency, controlling metal dissolution rates, and ensuring product quality. There are several methods to determine the strength of an acid solution:

Sound velocity measurement (ultrasound method)

• Principle: Measures the sound velocity in an acidic solution to determine the concentration
• Method: An ultrasonic sensor sends a sound wave through the liquid; the travel time is recorded and compared with calibration data
• Best suited for: Continuous real-time monitoring of acid concentration in industrial pickling baths
  
• Advantages:
  ✔️ Non-destructive and fast - No sample extraction required
  ✔️ Highly precise - No interference from dissolved metal ions
  ✔️ Real-time monitoring - Suitable for automated process control
• Disadvantages:
  ❌ Requires initial calibration for each type of acid and temperature (included in LiquiSonic)

pH measurement

• Principle: Measures the concentration of hydrogen ions in the solution
• Method: A pH meter or indicator strips indicate the acidity level
• Best suited for: Quick, rough estimation of acid content
  
• Advantages:
  ✔️ Simple and fast - Requires minimal equipment
  ✔️ Low cost - Simple pH strips or meters are affordable
• Disadvantages:
  ❌ Not accurate for strong acids - pH value does not indicate exact acid concentration
  ❌ Influenced by Temperature and impurities
  ❌ Extremely high maintenance and material costs (ongoing costs)
  ❌ Unreliable measurements, often worn out and need calibration after a few days

Measurement of specific gravity

• Principle: The concentration of an acid correlates with its Density (g/cm³)
• Method: With a Hydrometer the specific gravity of the solution is measured, which is then compared with reference tables
• Best suited for: Quick field tests of H₂SO₄ and HCl baths
  
• Advantages:
  ✔️ Fast and easy - No chemical reagents required
  ✔️ Non-destructive - No need to neutralize or discard samples
• Disadvantages:
  ❌ Does not distinguish between free acid and dissolved metal salts
  ❌ Temperature-dependent, requires corrections

Acid titration

• Principle: Uses a Neutralization reaction with a known base (e.g., NaOH) to determine the content of free acid
• Method: A measured sample is titrated with sodium hydroxide (NaOH) and an indicator to determine the acid concentration
• Best suited for: Accurate testing of acid concentration in the laboratory
  
• Advantages:
  ✔️ Highly precise - Determines the exact concentration of free acid
  ✔️ No interference from dissolved metal content
• Disadvantages:
  ❌ Time-consuming - Requires laboratory equipment and qualified personnel
  ❌ Required sample - Destructive testing method
  ❌ No inline measurements and therefore time delay and blind operations
  ❌ Operator error possible and employee training required

Measurement of conductivity

• Principle: Measures the electrical conductivity of the solution, which increases with acid concentration
• Method: A conductivity meter provides a measurement that correlates with acid strength
• Best suited for: Continuous monitoring of HCl and sulfuric acid solutions (with appropriate calibration)
  
• Advantages:
  ✔️ Fast and easy - Provides real-time measurements
  ✔️ Non-destructive - no sample loss
• Disadvantages:
  ❌ Affected by dissolved metal ions and requires frequent calibration
  ❌ Not suitable for all acids (e.g., oxidizing acids)
  ❌ No distinction between acid and metal content

• Pickling is essential for preparing metal surfaces for further processing, such as cold working, welding, or painting
• Pickling helps remove mill scale and iron scale from hot-rolled steel, improving surface quality and reducing the risk of defects
• Pickling is widely used in various industries, such as construction, automotive, and manufacturing
• Pickling can also be used to remove surface contaminants from other metals such as stainless steel and aluminum
• The pickling process can help improve the corrosion resistance of metal surfaces

• Monitoring the acid concentration in the pickling bath is crucial for the effectiveness of the pickling process
• The pickling bath should be regularly cleaned and descaled to prevent the buildup of iron oxide and other contaminants
• The use of automatic monitoring systems can help optimize the pickling process and reduce waste