HCl in Steel Pickling — Industrial Rust Removal Process & Technical Notes (CAS 7647-01-0)

--- ## 1. What is Steel Pickling? Steel pickling is a chemical process that removes the iron oxide layer — rust, mill scale and thermal residue — from carbon steel surfaces before subsequent processing stages such as cold rolling, hot-dip galvanizing, electrostatic painting or metal plating. This process is present in most modern steel production lines and metal processing plants. Steel after hot rolling or heat treatment forms an oxide scale layer 10-100 µm thick on the surface. This scale has a three-layer structure: outermost is Fe₂O₃ (hematite — reddish brown), middle is Fe₃O₄ (magnetite — black), and adjacent to the steel base is FeO (wustite — gray). If not removed, the oxide layer will: - Cause surface defects during cold rolling (pitting, dimpling) - Prevent zinc/paint coating from adhering to the steel base - Reduce product life due to localized corrosion under the paint HCl is currently the most common pickling agent in the steel industry, accounting for approximately 70% of the global pickling market, thanks to fast reaction rate, good by-product solubility and low cost. CAS of HCl is **7647-01-0**. ### Chemical reaction mechanism When carbon steel with a rust layer is immersed in HCl solution, the following reactions occur simultaneously: **Dissolution of iron oxides:** Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O Fe₃O₄ + 8HCl → FeCl₂ + 2FeCl₃ + 4H₂O FeO + 2HCl → FeCl₂ + H₂O **Reaction with the steel base (undesirable, requires control):** Fe + 2HCl → FeCl₂ + H₂↑ The reaction with the steel base generates hydrogen gas — this is the cause of over-pickling and hydrogen embrittlement if immersion time is not controlled and inhibitor is not added. FeCl₂ and FeCl₃ products are both highly soluble in water, easily rinsed away in the subsequent rinse bath. This is an important difference from H₂SO₄: FeSO₄ has limited solubility (~26 g/100 mL at 20°C) and tends to crystallize on the part surface when acid concentration drops — causing surface defects in subsequent stages. --- ## 2. Why Use HCl for Steel Pickling? Comparison with H₂SO₄ Although both HCl and H₂SO₄ can be used for steel pickling, HCl is preferred in most modern steel mills and processing workshops. The table below compares the two acids on key operational criteria: | Criteria | HCl (hydrochloric acid) | H₂SO₄ (sulfuric acid) | |---|---|---| | **Pickling speed** | Fast — 10-25 min at 25°C for carbon steel | Slow — requires heating to 50-80°C for equivalent speed | | **Operating temperature** | 20-40°C (room temperature — no heating needed) | 50-80°C (energy intensive heating) | | **By-product solubility** | FeCl₂/FeCl₃ fully soluble | FeSO₄ limited solubility, easily crystallizes | | **Steel base corrosion** | Yes — controlled by 0.1-0.5% inhibitor | Also present — harder to control at high temperature | | **Active surface creation** | Excellent — Cl⁻ effectively breaks down passive film | Medium — SO₄²⁻ less effective | | **Wastewater treatment** | Neutralize with NaOH → NaCl (less sludge) | Neutralize with NaOH/Ca(OH)₂ → Na₂SO₄/CaSO₄ (more sludge) | | **Evaporation** | HCl 32% slightly fuming, needs local exhaust | H₂SO₄ 98% almost non-volatile | | **Acid consumption/kg steel** | ~15-25 kg HCl 32%/ton steel | ~20-35 kg H₂SO₄ 98%/ton steel | | **Overall operating cost** | Lower — no heating + less sludge | Higher — energy + waste treatment costs | **Practical conclusion:** For most steel mills and plating workshops in Vietnam, HCl is the optimal choice — especially when the pickling line operates at room temperature and has no acid recovery system. H₂SO₄ is only suitable for large-capacity continuous pickling lines (≥500,000 tons/year) with integrated acid regeneration systems. --- ## 3. 4-Step Steel Pickling Process A complete industrial steel pickling process consists of 4 consecutive steps. Errors in any step affect final product quality. ### Step 1: Degreasing Before acid immersion, the steel surface needs to be free of grease, oil, dirt and organic contaminants. If grease residue remains, it will form a barrier preventing HCl from contacting the oxide layer — reducing pickling effectiveness and causing uneven pickling. **Operating parameters:** - Solution: NaOH 5-10% or specialized alkaline degreaser (Loc Thien supplies NaOH 32%-50%) - Temperature: 60-80°C - Time: 5-15 minutes - Check: Surface after degreasing must achieve "water break-free" — water spreads evenly, no droplets ### Step 2: HCl pickling immersion This is the main step of the process. The steel workpiece is immersed in an HCl bath at controlled concentration and time to completely remove the oxide layer. **Recommended operating parameters (for carbon steel):** | Parameter | Value | Notes | |---|---|---| | HCl concentration | 15-30% (diluted from HCl 32%) | Optimal ~18% — surveyed and confirmed on actual pickling lines | | Temperature | 25-40°C | No heating needed; above 45°C HCl evaporates heavily | | Immersion time | 10-30 minutes | Heavy-scale hot-rolled steel: 20-30 min; clean cold-rolled steel: 5-10 min | | Inhibitor | 0.1-0.5% by volume | Hexamine or commercial inhibitor — reduces steel base corrosion by up to 95% | | Iron (Fe) content in bath | < 100 g/L | Above 100 g/L requires bath change | | Agitation | Light air sparge or mechanical | Maintains uniform concentration | ### Step 3: Water rinsing After pickling, the steel surface carries HCl solution and iron salts. If not thoroughly rinsed, residual acid will continue to corrode the steel in subsequent stages, and iron salts will cause surface defects during plating or painting. **Standard rinse configuration:** - 2-3 counter-current rinse tanks - Tank 1: regular water, mechanical agitation - Tank 2: clean water, may add NaOH 0.1% to neutralize residual acid - Tank 3: deionized (DI) water for high requirements - Time per tank: 1-3 minutes - Check: pH of final rinse water ≥ 5.5 ### Step 4: Neutralization & drying The final step aims to stabilize the steel surface before entering the next processing stage. - **Neutralization:** Quick dip through NaOH 1-2% or mild alkaline solution to neutralize residual acid on the surface - **Drying:** Dry with hot air at 80-120°C or infrared, ensuring completely dry surface before plating/painting > **See related process:** HCl in metal plating — rust removal, activation and pH control in plating baths --- ## 4. Optimal HCl Concentration for Steel Pickling — Why 18%? One of the most common questions from operations engineers is: "What concentration should I dilute HCl 32% to for the steel pickling bath?" The answer is based on the balance between pickling speed, acid cost and base metal corrosion control. ### Survey data on carbon steel Experimental studies show: | HCl concentration (% vol) | Pickling speed (g/m²/min) | Base steel corrosion level | Notes | |---|---|---|---| | 10% | 1.2-1.5 | Low | Slow — only for clean steel or activation | | 15% | 2.0-2.5 | Medium | Acceptable for light steel | | **18%** | **3.0-3.5** | **Medium** | **Optimal concentration — best efficiency balance** | | 20% | 3.5-4.0 | Medium-High | High speed but needs good inhibitor | | 25% | 4.5-5.0 | High | Risk of over-pickling, heavy evaporation | | 32% | 5.5-6.5 | Very high | Only for fast pickling with tight control | Conclusion: **18% concentration** is the optimal point for carbon steel pickling because: - Pickling speed reaches 3.0-3.5 g/m²/min — fast enough for production lines - Base steel corrosion at controllable level with standard inhibitor - HCl evaporation significantly lower than at ≥25% - Acid cost per ton of steel is lowest when calculated on overall efficiency ### Dilution formula: HCl 32% to 18% To prepare 1000 L of 18% pickling solution from HCl 32% and water: - HCl 32% needed: V₁ = (18 × 1000) / 32 = 562.5 L - Water needed: 1000 - 562.5 = 437.5 L Rule: **add acid to water, never water to acid** — always. Pour slowly, stir continuously. HCl 32% has density d = 1.150-1.155 (at 20°C). Exact formula by mass: m(HCl 32% needed) = (C%_target × m_solution) / 32 For V = 1000 L, 18% solution mass is approximately 1080 kg (density ~1.08), requiring: m(HCl 32%) = (18 × 1080) / 32 = 607.5 kg Equivalent to ~528 L HCl 32% (density 1.15). > **Reference:** HCl 32%-35% handbook — choosing concentration, reading COA and legal compliance ### Signs that the pickling bath needs replacement When the HCl pickling bath turns dark brown/black and the pickling time is more than double that of a fresh bath, FeCl₂/FeCl₃ has saturated (iron concentration > 100 g/L). Continued use causes: - Significantly reduced pickling speed - Iron salts redepositing on the steel surface - Cross-contamination to rinse baths and subsequent stages Operating rule: replace bath when [Fe] > 100 g/L or pickling time > 2× design time. --- ## 5. HCl Recycling from Pickling Baths — Recovery Process and Benefits Spent pickle liquor contains FeCl₂ and FeCl₃ at high concentrations — previously considered hazardous waste requiring expensive treatment. However, with the oxidation recycling process, HCl can be recovered and reused. ### HCl regeneration reaction When FeCl₂/FeCl₃ solution is heated and sparged with air (O₂) at high temperature, the following reaction occurs: **4FeCl₂ + 4H₂O + O₂ → 8HCl + 2Fe₂O₃** This reaction — also called the pyrohydrolysis process — allows: - Recovery of HCl gas, reabsorbed into water to create fresh HCl - Production of fine Fe₂O₃ powder — raw material for pigment, ferrite or ceramic tile production - Reduction of >90% of solid waste compared to alkaline neutralization ### Economic benefits | Item | No recycling (neutralize + disposal) | With recycling (pyrohydrolysis) | |---|---|---| | Waste treatment cost/ton steel | ~$15-25 | ~$3-5 | | Fresh HCl needed/ton steel | ~20-25 kg | ~3-5 kg | | By-product | Fe(OH)₃ sludge — no value | Fe₂O₃ — sellable at $200-400/ton | | Energy consumption | Low | High (needs ≥300°C) | **Conclusion:** HCl recycling is economically viable for pickling lines with capacity ≥100,000 tons steel/year. For smaller capacity, the investment cost of a pyrohydrolysis furnace (typically $3-10 million) is not yet attractive. ### Alternative process for small and medium scale For small and medium steel mills in Vietnam, instead of investing in a pyrohydrolysis furnace, more common solutions include: - **Hire a treatment unit** — collect spent pickle liquor, transport to centralized treatment station - **On-site neutralization + Fe(OH)₃ precipitation** — use NaOH or Ca(OH)₂, lower cost but generates more sludge - **Sell spent liquor to FeCl₂/FeCl₃ manufacturers** — some facilities can utilize FeCl₂ to produce coagulants for water treatment --- ## 6. Case Study — Steel Mill in Phu My Industrial Park Switches from H₂SO₄ to HCl Pickling *The case study below has been anonymized per customer request. Data shared with the consent of the plant's technical department.* **Context:** A steel mill in Phu My Industrial Park, Ba Ria — Vung Tau — specializing in cold-rolled coil production with a capacity of 120,000 tons/year — used H₂SO₄ for pickling since 2018. The old system consisted of 2 H₂SO₄ 15% baths operating at 70°C, heated by an oil-fired boiler. **Problems:** - Oil heating cost: ~400 million VND/month - Pickling line speed: 35 m/min — limiting overall productivity - FeSO₄ crystallization in rinse bath and on coil surface → subsequent galvanizing defect rate up to 2.3% - Wastewater treatment sludge: ~15 tons/month — hazardous waste treatment cost ~50 million VND/month **Solution — switch to HCl pickling (Q1/2025):** - Replace 2 H₂SO₄ baths with 2 HCl 18% baths, operating at 30-35°C - Add hexamine inhibitor 0.3% - Install local exhaust system and HCl absorption tower **Results after 6 months of operation:** - Line speed increased from 35 → 52 m/min (+48%) - Acid consumption: from 28 kg H₂SO₄ 98%/ton steel → 18 kg HCl 32%/ton steel - Heating energy cost: from 400 → 0 million VND/month (room temperature operation) - Galvanizing defect rate: from 2.3% → 0.7% - Waste treatment sludge: from 15 → 4 tons/month - Investment payback period: 8 months **Lessons learned:** For medium-scale steel mills, switching from H₂SO₄ to HCl pickling delivers clear ROI through energy savings, increased productivity and reduced defect rates. The key factor is controlling HCl evaporation with an exhaust system and absorption tower. --- ## 7. Loc Thien Supplies HCl for Industrial Steel Pickling Loc Thien is a direct importer of HCl 32%-35%, serving steel mills, pickling workshops and hot-dip galvanizing lines nationwide. **Technical specifications — HCl 32%:** | Specification | Value | |---|---| | Formula | HCl | | CAS | 7647-01-0 | | Concentration | 32% ± 1% or 35% ± 1% | | Density (20°C) | 1.150-1.155 (32%) | | HS Code | 28061000 | | Packaging | 30L Carboys · 200L Drums · 1000L IBC · 5-30 ton tankers | **Why steel mills choose Loc Thien for HCl pickling?** - **Direct import, official channel** — COA and MSDS per batch, available for regulatory inspection - **6 warehouses nationwide:** HCMC, Dong Nai, BR-VT, Can Tho, Da Nang, Bac Ninh — delivery to any industrial park within 24 hours (Southern region) and 3-5 days (Northern and Central regions) - **5-30 ton tanker trucks** — suitable for all pickling bath scales, from small mills to large capacity plants - **Experience serving steel mills** — understands dilution concentration requirements, inhibitors and pickling safety procedures - **MST 0313650856** — established since 2016, framework contracts, full VAT invoices > 🛒 **Need HCl for steel pickling?** Hydrochloric Acid HCl 32%-35% — batch COA/MSDS, 5-30 ton tanker delivery nationwide. Hotline 0979 891 929. --- ## 8. Frequently Asked Questions (FAQ) --- ## Request a Quote Loc Thien is a direct importer of HCl 32%-35%, serving steel mills, galvanizing workshops and pickling lines nationwide. With 6 warehouses in HCMC, Dong Nai, BR-VT, Can Tho, Da Nang and Bac Ninh — we deliver to any industrial park within 24 hours (Southern region) and 3-5 days (Northern and Central regions). Packaging: 30L carboys, 200L drums, 1000L IBC, 5-30 ton tankers. COA and MSDS provided per batch. Call **0979 891 929** for HCl pickling consultation — quote within 15 minutes. --- > **See also:** > - Hydrochloric Acid HCl 32%-35% — technical specifications and ordering > - HCl 32%-35% handbook — choosing concentration, reading COA and legal compliance > - HCl in metal plating — rust removal, activation and pH control in plating baths > **LOC THIEN INVESTMENT DEVELOPMENT CO., LTD — MST 0313650856** · 452/6B Tinh Lo 10, P. Binh Tri Dong, HCMC · ☎ 0979 891 929 · hoachatlocthien.com **Learn more:** What is Hydrochloric Acid (HCl)? Properties, production & applications