Guide to Removing Rust from Iron Using 32% HCl Acid – Dosage, Procedure & Safety

Rust is the silent enemy of every factory, workshop, and industrial park. Even a thin layer of rust on pipes, tanks, or steel structures can lead to equipment failures, production disruptions, and repair costs amounting to hundreds of millions of dong. It is estimated that downtime costs due to metal corrosion account for 3–4% of GDP in industrialized countries. In this context, 32% HCl acid It has become the number one choice in the industrial rust removal industry — thanks to its rapid reaction speed, high efficiency, reasonable cost, and ease of disposal after use. This article provides a comprehensive technical guide on dosage, procedure, and safety when using 32% HCl to remove rust from steel and industrial metal equipment.

Why is 32% HCl the top choice for industrial rust removal?

Among common industrial acids, 32% HCl (hydrochloric acid) stands out with many advantages over other acids in the same group. For an objective assessment, please see the comparison table below:

Key advantages of 32% HCl in industrial rust removal:

• High reaction speed: HCl dissolves iron oxides (Fe₂O₃, FeO, Fe₃O₄) rapidly at room temperature, without the need for heating like H₂SO₄.
• Does not leave sulfate residue: Unlike H₂SO₄, which forms FeSO₄ precipitate that adheres to metal surfaces, HCl forms FeCl₂/FeCl₃, which dissolves completely in water and is easily washed away.
• Easily neutralized: A dilute NaOH solution of 2–5% is sufficient to completely neutralize excess HCl, without generating any dangerous exothermic reactions.
• Low cost: Industrial-grade 32% HCl is competitively priced, especially when purchased in bulk via IBCs or tank trucks.
• Wide range of applications: Acid pickling of carbon steel, rust removal for pipes, boilers, tanks, and industrial steel structures.

In the steel and metalworking industry, the process acid pickling Acid rust removal using HCl is a global standard according to ASTM A380 and EN ISO 11127. This is the clearest evidence of the effectiveness and reliability of HCl in industrial-scale rust removal applications.

Rust Removal Reaction Mechanism — The Chemistry Behind It

Understanding the reaction mechanism helps engineers control the descaling process more effectively, avoiding excessive corrosion of the base metal and properly treating wastewater.

Main Reaction — Dissolving Iron Oxide

Rust is essentially a mixture of iron oxides, the most common of which are Fe₂O₃ (iron(III) oxide, red rust) and FeO (iron(II) oxide). HCl reacts with these oxides according to the following equation:

Reaction 1 — Red rust removal (Fe₂O₃):

Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O

FeCl₃ (iron(III) chloride) dissolves completely in water, removing red rust from metal surfaces.

Reaction 2 — Black rust removal (FeO):

FeO + 2HCl → FeCl₂ + H₂O

FeCl₂ (iron II chloride) also dissolves completely, treating the black rust layer commonly found on carbon steel after heating.

Side Effects — Need to Be Closely Controlled

Reaction 3 — Corrosion of the base steel (Fe):

Fe + 2HCl → FeCl₂ + H₂↑

This is a dangerous side reaction: after the rust layer is dissolved, HCl continues to attack the underlying steel, producing H₂ gas and thinning the material. This phenomenon is called rust formation. hydrogen embrittlement (Hydrogen brittleness) — dangerous for high-strength steel.

Why are Inhibitors Needed?

An inhibitor is a chemical compound added to an HCl solution to inhibit reaction 3 (Fe + 2HCl) while not affecting reactions 1 and 2 (dissolving iron oxide). Mechanism: The inhibitor adsorbs onto the surface of the base metal, creating a thin protective film that prevents HCl from directly contacting Fe, but does not prevent HCl from reaching the iron oxide layer.

Common inhibitors used in industrial rust removal:

• Na₂SO₃ (sodium sulfite) 0,1–0,5%: Inexpensive, readily available, and effective at room temperature.
• Hexamethylenetetramine (urotropine) 0,5%: Highly effective at temperatures of 50–70°C
• Thiourea 0,1–0,3%: The most potent but toxic, wastewater treatment requires careful handling.
• Commercial products (Rodine, Armohib): Complex mixture, used at a rate of 0,1–0,3% by weight of the solution.

The golden rule: If the expected soaking time exceeds 20 minutes or the HCl concentration is above 10%, an inhibitor must be used to protect the steel substrate.

Table of HCl Concentration and Dosage by Application

Choosing the appropriate concentration of HCl is crucial for effective rust removal and metal safety. The table below summarizes the technical specifications for specific industrial applications:

Materials that should NOT be treated with HCl for rust removal.

This is a crucial point that many technicians overlook:

• Aluminum (Al) and aluminum alloys: Al + 3HCl → AlCl₃ + H₂↑ — a strong reaction, producing a large amount of H₂ gas, posing a risk of explosion.
• Copper (Cu) and copper alloys (brass, copper): HCl forms CuCl₂ — a toxic copper salt that is difficult to treat in wastewater.
• Zinc (Zn) and galvanized surfaces: Zn + 2HCl → ZnCl₂ + H₂↑ — destroys the protective coating.
• 304/316 Stainless Steel (Austenitic SS): HCl causes pitting corrosion on stainless steel surfaces and should not be used under any circumstances.
• High-strength steel (HY-80, 4340): Risk of hydrogen embrittlement; consult a metallurgist before use.

How to Calculate the Amount of 32% HCl Needed

The formula for calculating the amount of 32% HCl needed to create a rust removal solution at the desired concentration is as follows:

V(HCl 32%) = [C_đích × V_dung dịch] / 32

Ví dụ: Pha 100 lít dung dịch HCl 10%:
V(HCl 32%) = (10 × 100) / 32 = 31,25 lít HCl 32%
Nước thêm vào = 100 - 31,25 = 68,75 lít

Important note: Always pour acid into water (never pour water into acid) to avoid a sudden exothermic reaction.

EHS Standard 6-Step Rust Removal Process

The process below complies with industrial EHS (Environmental, Health & Safety) standards, meeting the inspection requirements of industrial parks and ISO 14001 and OHSAS 18001 certification bodies.

Step 1: Assess the Level of Rust and Choose the Concentration

Before starting, the engineer in charge must:

• Identify the type of material: carbon steel, cast iron, or alloy? (See the table of materials that should not be treated with HCl above)
• Assess the degree of corrosion according to ISO 8501-1 standards (Sa 1 — mild corrosion, Sa 2 — moderate corrosion, Sa 3 — severe corrosion).
• Measure the material thickness, if possible (using an ultrasonic thickness gauge), to determine the maximum allowable corrosion threshold.
• Choose the HCl concentration according to the table above.
• Develop a Chemical Safety Plan for each shift.

Step 2: Dilute the Acid Properly

MANDATORY RULE: ALWAYS pour acid into water, NEVER pour water into acid.

Mixing procedure:

1. Pour the required amount of clean water into the mixing container (HDPE or PP, do not use metal).
2. Stir the water gently.
3. Slowly pour 32% HCl into the water — at a slow rate, ensuring the bath temperature does not exceed 40°C.
4. Stir well after pouring.
5. Measure the concentration using a densimeter or titration meter before use.

Reason: When water is poured into concentrated acid, a large amount of heat is released instantly, which can cause it to boil over, splashing acid onto people and the surrounding environment.

Step 3: Add Inhibitor (If Needed)

• Dissolve the inhibitor in a portion of water first, then add it to the prepared HCl solution.
• Stir well to ensure the inhibitor is completely dispersed.
• Allow the solution to stabilize for 5–10 minutes before immersing the metal in it.
• Dosage of Na₂SO₃: 0,1–0,5% by weight of solution

Step 4: Soak or Spray — Control the Time

• Static immersion method: Immerse the equipment/part in the acid bath. Monitor visually every 5–10 minutes. Stop immediately when all the rust has come off.
• Circulatory method (boiler/tube): Use an acid-resistant pump to pump HCl solution through the system. Measure the inlet and outlet pH every 30 minutes.
• Spray pickling method: For large structures, immersion is not permitted. Ensure nozzles and piping are made of acid-resistant materials (HDPE, PVC, PVDF).
• Do not exceed the recommended time — if the rust is not completely removed, increase the concentration or temperature instead of extending the soaking time.

Step 5: Rinse and Neutralize

1. First rinse with clean water: Using a high-pressure water jet, rinse for 2–3 minutes to remove most of the acid and FeCl₂/FeCl₃.
2. Soak in clean water: Soak in a clean water bath for 5 minutes (or circulate clean water through the system).
3. Neutralize with 2% NaOH: Spray or soak the solution 2% NaOH Spread across the entire surface. Measure the surface pH using a pH strip — target pH 6–8.
4. Second rinse: Rinse thoroughly with clean water one last time to remove excess NaOH.
5. Drying: Use clean compressed air or heat drying to remove moisture and prevent re-rusting immediately.
6. Protective coating: Apply epoxy paint, protective oil coating, or galvanizing within 4 hours of rust removal.

Step 6: Wastewater Treatment Before Discharge

Wastewater from the descaling process contains FeCl₂, FeCl₃, excess HCl, and has a low pH — it must not be discharged directly into the environment. See details in section [link]. Wastewater Treatment under.

Chemical Safety — PPE and EHS Required

32% HCl is a hazardous chemical classified as Category 8 (corrosive) according to the GHS and ADR classifications. Compliance with EHS regulations is not only a legal requirement but also protects the lives of workers and plant assets.

Minimum Required PPE

• Chemical gloves: PVC gloves with a thickness of ≥0,4mm or neoprene. Do not use natural rubber (latex) gloves as they decompose quickly due to HCl.
• Chemical goggles: The type with a closed rim, not an open top.
• Gas masks: Acid filter (cartridge type P100 + acid gas OV/AG) or full-face mask when working in confined spaces.
• Chemical-resistant safety shoes: Rubber or PVC shoes are not allowed; leather shoes are not permitted.
• Protective clothing (apron or coverall): PVC or Tyvek material, full body coverage.
• Emergency eyewash station: Must be within a 10-second walk of the work area where acid is being handled.

Work Environment Requirements

• Ventilation: Hydrochloric acid vapor (TLV-TWA: 1 ppm, OSHA PEL: 5 ppm) causes severe respiratory irritation. Local exhaust ventilation (LEV) or outdoor work is required.
• No smoking, no fire: Hydrogen gas produced from the reaction of Fe + HCl can accumulate and cause explosions.
• Separate the work areas: Barriers, warning signs, and restrictions on unauthorized access to the rust removal area.
• Prepare the neutralizing materials in advance: Always have a bucket of NaOH or lime powder on hand to deal with spills.

Spill Handling

• Small overflow (<5 liters): Complete PPE team → Absorb with absorbent clay (do not use sawdust or cloth) → Neutralize with lime powder or NaOH → Collect in hazardous waste container
• Large overflow (>5 liters): Evacuate the area → Activate the emergency response procedure → Prevent acid from flowing into regular drainage systems → Call a hazardous chemical disposal contractor if necessary
• Acid gets into the eyes: Rinse immediately with an eyewash station for at least 15 minutes → Seek immediate medical attention.
• Acid gets on the skin: Remove acid-contaminated clothing → Rinse with clean water for 15 minutes → Seek medical attention
• Inhaling acid fumes: Move to a place with fresh air immediately → Perform artificial respiration if breathing stops → Call emergency services

SDS/MSDS Reference

Before using 32% HCl, it is mandatory to read the Safety Data Sheet (SDS) according to GHS standards, especially the following sections:

• Section 7 — Handling and Storage
• Section 8 — Exposure Controls / PPE
• Section 13 — Disposal Considerations

SDS conforming to GHS/REACH standards is available from reputable suppliers. Contact Loc Thien to receive a complete SDS in Vietnamese.

Wastewater Treatment After Rust Removal — Standard QCVN 40:2011

Wastewater from the HCl rust removal process is classified as hazardous waste according to Circular 02/2022/TT-BTNMT. Improper treatment not only violates the law but can also result in fines of 100-200 million VND under Decree 45/2022/NĐ-CP.

Composition of Wastewater After Rust Removal

• FeCl₂ and FeCl₃ (concentration 10–150 g/L depending on the degree of rust removal)
• Excess HCl (normal pH 0,5–2,0)
• Inhibitor and its breakdown products
• Dissolved metal impurities from the material are derusted.

Standard Wastewater Treatment Process

1. Neutralize pH: Add NaOH or Ca(OH)₂ (lime) to the wastewater tank and stir well. The target pH is 8,5–9,5 for maximum precipitation of Fe(OH)₂ and Fe(OH)₃.

   FeCl₂ + 2NaOH → Fe(OH)₂↓ + 2NaCl
   FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl

2. Oxidation of Fe²⁺ → Fe³⁺ (optional): Aeration or addition of H₂O₂ is used to convert Fe(OH)₂ (difficult to filter) to Fe(OH)₃ (easier to filter).
3. Coagulation-flocculation: More PAC (Poly Aluminum Chloride) Dose 10–50 mg/L, stir quickly for 2 minutes → Add anionic polymer 0,5–1 mg/L, stir slowly for 15 minutes to form large flocs.
4. Be still: Let it settle for 30–60 minutes. The sludge will settle. Fe(OH)₂/Fe(OH)₃ subside
5. Filter: Use a filter press or sand filter to thoroughly separate the sludge.
6. Check before flushing: Measure pH (5,5–9,0), total Fe (<5 mg/L column B QCVN 40:2011), SS (<100 mg/L)

Required Standards for QCVN 40:2011

Buying 32% HCl Acid for Industrial Rust Removal — Criteria for Choosing a Supplier

The quality of 32% HCl directly affects rust removal efficiency and production safety. These are the technical criteria that procurement engineers need to consider when selecting a supplier:

1. Actual Concentration — Check COA

Request the supplier to provide Certificate of Analysis (COA) For each shipment, please confirm:

• HCl concentration ≥32% (usually 31,5–33,5%)
• Testing method: titration with NaOH
• Specific gravity: 1,155–1,165 g/mL corresponding to 32% HCl

Warning: Many smaller suppliers sell diluted HCl (25–28%) labeled as 32%, resulting in less effective rust removal and higher costs.

2. Impurities — Important for Special Applications

• Fe < 5 ppm: Important if used in the food, pharmaceutical, or electronics industries.
• As (arsenic) < 1 ppm: Mandatory requirement according to pharmaceutical standards.
• Hg < 0,1 ppm: Heavy metal control for sensitive applications
• Standard industrial HCl for rust removal: Fe < 50 ppm, total impurities < 200 ppm are acceptable.

3. Packaging Suitable for the Scale of Use

• HDPE cans, 20–35 kg: Suitable for routine maintenance, small workshops (<500 liters/month)
• 1000-liter IBC: Suitable for medium-sized workshops (500–5.000 liters/month), saving on packaging costs.
• 5-10 ton tanker trucks: Suitable for continuous acid pickling lines or large plants.

4. Complete Legal Documentation

• Vietnamese MSDS/SDS according to GHS standards
• Hazardous goods transport license (according to Decree 34/2024/ND-CP)
• Red VAT invoice (important for businesses)
• CO/CQ (Certificate of Origin/Quality) if required.

Loc Thien - A Reliable Supplier of 32% Industrial HCl

Loc Thien Company provides 32% industrial HCl acid with the following advantages:

• Warehouse at Binh Tan District, Ho Chi Minh City , and Di An, Binh Duong — Fast delivery to VSIP Industrial Park, Amata, Long Hau, DEEP C and nationwide.
• Concentration guaranteed ≥32%, with a Certificate of Analysis (COA) included with each batch.
• Packaging: 35kg can, 1000L IBC, 5-10 ton tanker truck
• Complete VAT invoice, MSDS, and hazardous goods transport permit.
• Our technical team provides free consultation on dosage and procedures.

Hotline: (+84) 979 891 929 | See more products at Industrial chemicals in Ho Chi Minh City.

Frequently Asked Questions