Hot-Dip Galvanizing Chemistry: Pickling Bath Control with HCl & NH₄Cl Fluxing Technology
Hot-Dip Galvanizing (HDG) technology compliant with international standards ASTM A123/A123M and ISO 1461 requires stringent chemical harmony between hydrochloric acid pickling—supplied as technical 32% Hydrochloric Acid (HCl) via dedicated 5–30-ton road tankers—and pre-flux surface activation using high-purity Ammonium Chloride (NH₄Cl 99.5% in 25kg double PE-lined moisture-proof bags, road tankers strictly prohibited) distributed by Loc Thien Chemical. Maintaining precise free acid titration, dissolved iron constraints (Fe²⁺ ≤ 120 g/L), and the optimum eutectic ratio of ZnCl₂:NH₄Cl inhibits secondary surface re-oxidation, decreases zinc bottom dross and surface skimming generation by 55–65%, and ensures metallurgical bonding of smooth, corrosion-resistant Fe-Zn alloy phases. Technical proposals delivered within 15 minutes via Hotline: +84 979 891 929.
1. Standard Hot-Dip Galvanizing Chemical Processing Architecture
Modern HDG facilities utilize a 7-stage surface preparation sequence where chemical purity directly dictates coating adhesion and zinc consumption:
| Process Stage | Chemical Reagents | Supply Mode (SSOT) | Key Operating Parameters |
|---|---|---|---|
| 1. Alkaline Degreasing | Liquid NaOH 32% / Solid NaOH 99% | 5–30-ton Tanker / 25kg Bag | Total saponification of cold-rolling lubricants |
| 2. Acid Pickling | Industrial HCl 32% Technical Grade | 5–30-ton Tanker (FRP Composite) | Free HCl 12–16%, Fe²⁺ ≤ 120 g/L, 15–30 min |
| 3. Double Water Rinse | Counter-current Industrial Water | Continuous Overflow | Rinse pH ≥ 4.5 to prevent iron transfer to flux |
| 4. Pre-Fluxing | NH₄Cl 99.5% + Zinc Chloride (ZnCl₂) | 25kg Bag (Road tanker prohibited) | Density 1.15–1.20, Temp 60–70°C, pH 4.0–4.8 |
| 5. Pre-Drying | Hot Air Recirculation (120–140°C) | Kiln Waste Heat Recovery | Moisture-free preheating, zero zinc splash |
| 6. Zinc Immersion | Special High Grade (SHG) Zinc 99.995% | Palletized Ingots | Kettle temperature 445–455°C, withdrawal rate 1.2–1.5 m/min |
2. Advanced Pickling Chemistry: Minimizing Iron Drag-In and Hydrogen Embrittlement
Hydrochloric acid dissolves iron oxide scales via rapid chemical reactions:
If dissolved iron concentrations exceed 120 g/L in the pickling tanks, rinse efficiency collapses. The carried-over Fe²⁺ reacts immediately in the molten zinc bath (Fe + 13Zn → FeZn₁₃), where every 1 kg of iron drags down and consumes 25 to 30 kg of prime zinc into bottom dross. Loc Thien Chemical guarantees consistent 32% HCl delivered by 5–30-ton tankers with tight quality control to protect structural integrity.
3. Ammonium Chloride (NH₄Cl) Fluxing Salt Specifications & Alloy Formation
Upon dipping preheated steel into molten zinc at 450°C, Ammonium Chloride decomposes thermally:
Nascent gaseous HCl cleans trace iron oxides instantly, while escaped ammonia creates a localized inert blanket allowing pure molten zinc to wet the substrate, forming sequential iron-zinc alloy layers: Gamma (Γ, 21–28% Fe), Delta (δ₁, 7–11.5% Fe), Zeta (ζ, 5.8–6.7% Fe), and pure outer Eta (η, 100% Zn).
4. Technical Comparison: Loc Thien NH₄Cl 99.5% vs Generic Commercial Grade
| Quality Parameter | Loc Thien NH₄Cl 99.5% | Generic Commercial Grade | Impact on Galvanizing Quality |
|---|---|---|---|
| Assay (NH₄Cl) | ≥ 99.5% | 97.0 - 98.5% | Maximum fluxing efficiency, minimum surface ash |
| Moisture | ≤ 0.4% | 1.2 - 2.5% | Free-flowing crystals, instant dissolution |
| Iron Content (Fe) | ≤ 10 ppm | 50 - 150 ppm | Zero iron buildup in zinc kettle dross |
| Packaging (SSOT) | 25kg Double PE-lined Bag (No Tanker) | Single bag, moisture prone | 12+ months shelf life without clumping |
5. B2B Case Study: 1,800 Tons/Month Structural Steel Galvanizing Plant in Dinh Vu IZ (Hai Phong)
Project Summary: Quality Optimization for Power Transmission Towers & Fasteners
- Initial Operational Challenge: The facility experienced 8.5% surface defect rates (pimples and bare spots) and generated 14 tons of zinc dross monthly due to manual pickling bath management and contaminated fluxing salts.
- Loc Thien Chemical Solution:
- Direct supply of technical 32% HCl delivered via 5–30-ton tankers with inline specific gravity monitoring (Fe²⁺ ≤ 110 g/L).
- Full conversion to high-purity 99.5% Ammonium Chloride (25kg bags) maintaining ZnCl₂:NH₄Cl = 1:2.8 at 65°C.
- Measured Results After 60 Days:
- Surface defects plummeted from 8.5% to under 0.8%, fully passing ASTM A123 inspections.
- Zinc bottom dross generation decreased from 14 tons to 4.8 tons/month (65.7% reduction).
- Net Financial Savings: Exceeded 420 million VND/month in prime zinc consumption and rework labor.
6. Frequently Asked Questions by HDG Plant Engineers
When should the HCl pickling bath be renewed?
Standard engineering protocols dictate bath replenishment when dissolved iron (Fe²⁺) exceeds 120 g/L and free HCl drops below 5%. Operating an iron-saturated bath triples pickling time and drastically inflates zinc dross production.
Can Ammonium Chloride be delivered by road tankers?
STRICTLY PROHIBITED. Ammonium chloride is a solid crystalline salt prone to rapid caking when exposed to ambient moisture. Loc Thien Chemical exclusively delivers in 25kg double PE-lined palletized bags to guarantee dry crystal integrity.
6. Thermal Kinetics of Flux Bath & Liquid Zinc Splash Prevention
Maintaining the preflux solution at 65°C – 75°C optimizes wetting kinetics on structural steel geometries before dipping into 450°C molten zinc. Thermal activation facilitates rapid moisture evaporation during tunnel drying (100–120°C), eliminating dangerous molten metal splashing hazards.
Upon immersion, Ammonium Chloride NH₄Cl instantly thermally sublimates into reactive HCl and NH₃ vapors, stripping micro-oxides in milliseconds to achieve flawless, pinhole-free Fe-Zn alloy bonding. Calculate chemical replenishment rates using the Loc Thien Online Chemical Dosage Calculator. Explore our full surface finishing line in the electroplating and metal finishing chemical catalog.
⚡ Request Technical Proposal for HDG Chemicals (15-Minute Response)
Loc Thien Chemical delivers bulk 32% HCl via 5–30-ton road tankers and high-purity 25kg NH₄Cl bags nationwide across Vietnam.
- Direct B2B Technical Hotline: +84 979 891 929
- Official Corporate Email: [email protected]
- Contact Loc Thien Technical Support: Contact Loc Thien Technical Support