Nitric Acid HNO3 68% in Stainless Steel Pickling, ASTM A380 / ASTM A967 Passivation & Electroplating Pretreatment

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Industrial Nitric Acid (HNO₃ 68%) is the cornerstone oxidizing reagent in stainless steel surface engineering and precision electroplating. Unlike non-oxidizing mineral acids such as Hydrochloric Acid HCl 32% or Sulfuric Acid H₂SO₄ 98%—which actively attack stainless steel and leave chloride pitting—the nitrate anion (NO₃⁻, standard reduction potential E° = +0.96 V) dissolves exogenous free iron (Fe⁰) from the metal surface while simultaneously oxidizing alloyed Chromium into a dense, self-healing Chromium(III) Oxide (Cr₂O₃) passive film (2–5 nm thick). By operating mixed-acid pickling baths (10–15% HNO₃ + 1.5–3.0% HF) followed by ASTM A967 passivation baths (20–25% v/v HNO₃ 68% at 49–60 °C), fabrication plants elevate the surface Cr/Fe atomic ratio from ≤ 0.4 to ≥ 1.8, guaranteeing 500–1,000+ hours of neutral salt spray resistance (ASTM B117).

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1. Electrochemistry of the Cr₂O₃ Passive Film & Why HCl Is Strictly Prohibited on Stainless Steel

Austenitic (SUS304, SUS304L, SUS316, SUS316L), duplex (SAF 2205), and martensitic (SUS410, SUS420) stainless steels contain 10.5% to 26% Chromium (Cr). However, after mechanical cutting, stamping, grinding, or TIG/MIG welding, the surface corrosion barrier is compromised by two mechanisms:

Exogenous Free Iron Contamination: Carbon-steel tooling, wire brushes, and forming dies embed microscopic metallic iron particles (Fe⁰) into the stainless steel surface, which rapidly rust into reddish-brown Fe₂O₃·nH₂O upon exposure to ambient humidity.
Chromium-Depleted Heat-Tint Zone: Welding temperatures (450–850 °C) cause Chromium to diffuse rapidly to the outer surface to form dark blue-black iron-chromium oxides (FeCr₂O₄ / Cr₂O₃), leaving a subsurface layer (10–30 µm deep) depleted of Chromium (Cr ≤ 8–9%), well below the 10.5% threshold required for self-passivation.

Chemical Reactions During Nitric Acid Passivation & Pickling

Anodic Dissolution of Free Iron (Fe⁰): Fe + 4HNO₃ (dilute) → Fe(NO₃)₃ (soluble) + NO↑ + 2H₂O
Direct Oxidation of Alloyed Chromium into Dense Cr₂O₃ Barrier Film: 2Cr + 2HNO₃ + H₂O → Cr₂O₃ (solid passive film 2–5 nm) + 2NO↑ + 2H₂O
Dissolution of Weld Scale (FeCr₂O₄) in Mixed HNO₃ + HF Pickling Bath: FeCr₂O₄ + 8HF + 4HNO₃ → Fe(NO₃)₃ + CrF₃ + H₃FeF₆ + NO↑ + 6H₂O

2. Standard Operating Parameters: ASTM A967 & ASTM A380 Passivation Profiles

Table 1: ASTM A967 / AMS 2700 Nitric Acid Passivation Bath Formulations (Using Commercial HNO₃ 68%)

Standard Procedure	Target Stainless Steel Grades	Volume % of Commercial `HNO₃ 68%` (`v/v`)	Mass % of `HNO₃ 100%` (`w/w`)	Sodium Dichromate (`Na₂Cr₂O₇·2H₂O`)	Bath Temperature (`°C`)	Minimum Immersion Time (`minutes`)
ASTM A967 — Nitric 1	Austenitic `SUS304, SUS316L`, Duplex `2205`, Martensitic `400-series`	`20 – 25% v/v`	`15.5 – 19.8%`	`2.0 – 3.0% w/w`	`49 – 54 °C`	`20 min`
ASTM A967 — Nitric 2 (Industry Standard)	Austenitic `SUS304 / 304L / 316 / 316L` (Chromate-free)	`20 – 45% v/v`	`15.5 – 37.5%`	None (`0%`)	`49 – 60 °C`	`20 – 30 min`
ASTM A967 — Nitric 3	Large tanks & piping passivated at ambient room temperature	`20 – 25% v/v`	`15.5 – 19.8%`	`2.0 – 3.0% w/w`	`21 – 32 °C`	`30 min`
ASTM A967 — Nitric 4	Ambient-temperature chromate-free passivation (`SUS304 / 316`)	`25 – 45% v/v`	`19.8 – 37.5%`	None (`0%`)	`21 – 32 °C`	`30 min`
High-Concentration Nitric	Free-machining stainless steels (`SUS303` high-S/Se) & `SUS410`	`45 – 55% v/v`	`37.5 – 46.8%`	Optional	`49 – 54 °C`	`30 min`

Table 2: Mixed-Acid Weld Scale Pickling Bath Formulations (`HNO₃ 68% + HF 49%`) Under ASTM A380

Operation Stage	Commercial `HNO₃ 68%` (`L / 1,000 L` bath)	Commercial `HF 49%` (`L / 1,000 L` bath)	Bath Temperature (`°C`)	Immersion Time (`minutes`)	Maximum Dissolved Iron Limit (`Fe³⁺`, g/L)
Standard Austenitic Immersion (`SUS304 / 316L` at `25–35 °C`)	`150 – 220 L` (`10–15% HNO₃`)	`20 – 45 L` (`1–2.5% HF`)	`21 – 38 °C`	`15 – 40 min`	Dump or regenerate when `Fe ≥ 35–45 g/L`
Heated Rapid Pickling (`SUS316L / Duplex 2205` at `45–55 °C`)	`120 – 180 L` (`8–12% HNO₃`)	`15 – 30 L` (`0.8–1.6% HF`)	`45 – 55 °C`	`5 – 15 min`	Dump or regenerate when `Fe ≥ 30 g/L`
Spray / Circulation Pipe Pickling (CIP Loop)	`180 – 250 L` (`12–17% HNO₃`)	`30 – 50 L` (`1.5–2.8% HF`)	`20 – 35 °C`	`30 – 90 min`	Maintain `Fe³⁺ / F⁻` molar ratio properly

3. Electroplating Applications: Copper/Nickel Rack Stripping & Zinc Chromate Bright Dip

Beyond stainless steel passivation, commercial HNO₃ 68% is heavily utilized in electroplating lines across electronics, automotive, and hardware industries:

Selective Stripping of Copper & Nickel from Stainless Steel / Titanium Plating Racks: After repeated plating cycles, SUS304 / SUS316 or Titanium contact hooks accumulate thick nodules of Copper (Cu) and Nickel (Ni). Immersing the racks in 40–55% v/v HNO₃ 68% (with corrosion inhibitors) rapidly dissolves the Cu and Ni buildup (Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂↑ + 2H₂O) while automatically passivating and protecting the underlying stainless steel or titanium rack frame.
Bright Dip & De-smutting Before Zinc Passivation: Adding 0.3–0.8% v/v HNO₃ 68% as an oxidizing acid dip immediately prior to Trivalent Chromium (Cr³⁺) passivation removes organic brightener films from newly plated zinc surfaces and yields a mirror-like metallic luster.

Engineering Calculation Example: 3,000-Liter ASTM A967 Nitric 2 Passivation Tank

Technical Illustration (Reference Engineering Model): A precision food-machinery fabrication plant constructs a 3,000-liter passivation tank (PVDF/PP lined) operating under ASTM A967 Nitric 2 (25% v/v commercial HNO₃ 68% at 52 °C, specific gravity d = 1.405 kg/L).

Initial Bath Makeup:

Volume of DI water (EC ≤ 10 µS/cm, Cl⁻ ≤ 15 ppm): 3,000 × (1 - 0.25) = 2,250 L.

Volume of commercial HNO₃ 68%: 3,000 × 0.25 = 750 L (Mass: 750 × 1.405 = 1,053.75 kg HNO₃ 68%).

Active HNO₃ (100%) concentration: 18.6% w/w (209.6 g HNO₃/L), resulting bath specific gravity d ≈ 1.108 g/cm³.

Quality Verification (ASTM A967 Acceptance Tests):

Copper Sulfate Test (Practice E): Apply CuSO₄·5H₂O + H₂SO₄ test solution for 6 minutes—zero metallic copper deposition (Cu⁰) indicates zero free iron on SUS304/316L.

Ferroxyl Test for Free Iron (ASTM A380): Zero potassium ferricyanide (K₃[Fe(CN)₆]) blue spots within 15 seconds.

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4. NOₓ Fume Suppression, Nitrate Wastewater Treatment & Chemical Safety

Liquid-Phase NOₓ Fume Suppression with Hydrogen Peroxide (H₂O₂ 50%): Dosing small increments of H₂O₂ 50% into the HNO₃ pickling bath oxidizes nascent nitrous acid (HNO₂) and nitric oxide (NO) back into nitric acid inside the liquid phase (2NO + 3H₂O₂ → 2HNO₃ + 2H₂O and HNO₂ + H₂O₂ → HNO₃ + H₂O), cutting reddish-brown NO₂ fume emissions by 75–85% and recovering 15–20% of nitric acid consumption.
Two-Stage NOₓ Wet Scrubber: Exhaust gases are treated in a two-stage packed bed scrubber using Liquid Caustic Soda NaOH 32% combined with a reducing agent (Na₂S₂O₃ or Na₂SO₃ at pH 9.0–10.5) to convert NO₂ into harmless nitrogen gas (N₂) and soluble sulfate/chloride salts, meeting QCVN 19:2024/BTNMT (NOₓ ≤ 400 mg/Nm³).
Storage & Material Compatibility: Store HNO₃ 68% in high-density virgin HDPE, PVDF, PTFE, or passivated SUS304L / SUS316L tanks. Never store in standard uPVC or unlined carbon steel; keep strictly segregated from organic solvents, alcohols, sawdust, and reducing agents.

5. Industrial Supply of Nitric Acid HNO₃ 68% by Loc Thien Chemical

Loc Thien Investment Development Co., Ltd. (LOC THIEN INVESTMENT DEVELOPMENT COMPANY LIMITED — Tax ID: 0313650856) is a premier commercial supplier and distributor of industrial chemicals in Vietnam (while serving as the direct manufacturer of FeCl₂ 30% solution and wastewater-grade FeCl₃):

Product Quality & Origin: Imported HNO₃ 68% (South Korea / Thailand / Taiwan) with HNO₃ ≥ 68.0%, ultra-low Chloride (Cl⁻ ≤ 5–10 ppm) and heavy metals, complete with batch-specific COA and 16-section GHS MSDS.
6 Strategic Warehouses Across Vietnam (3,000+ tons/month capacity, 5–30-ton specialized tanker fleet):
1. Ba Ria - Vung Tau Warehouse: Phu My 1 IP, Phu My Town, Ba Ria - Vung Tau.
2. Dong Nai Warehouse: Long Thanh IP, Long Thanh District, Dong Nai.
3. Binh Duong Warehouse: Tan Dong Hiep Ward, Di An City, Binh Duong.
4. Da Nang Warehouse: Hoa Khanh IP, Lien Chieu District, Da Nang City.
5. Can Tho Warehouse: Tra Noc IP, O Mon District, Can Tho City.
6. Bac Ninh Warehouse: Tien Son IP, Tien Du District, Bac Ninh.
- Legal Headquarters: 452/6B Tinh Lo 10, Binh Tri Dong Ward, Binh Tan District, Ho Chi Minh City.
Quotation & Delivery SLA: Technical quotation within 15 minutes (unit price structured around 5 transparent factors: packaging in 35 kg cans / 280 kg drums / 1,000 L IBC totes / 5–30-ton bulk tankers, order volume, delivery distance, technical documentation requirements, and contract schedule). Delivery within 4 hours in inner HCMC, 24–48 hours across Southern & Mekong Delta industrial zones, and 2–3 days in Northern & Central Vietnam. Contact hotline 0979 891 929 for detailed delivery procedures.

Product Pages: Industrial Nitric Acid HNO₃ 68% | Liquid Caustic Soda NaOH 32%–50% | Hydrogen Peroxide H₂O₂ 50%
GHS Safety Data Sheets (MSDS): MSDS Nitric Acid HNO₃ 68% | MSDS Hydrofluoric Acid HF 49%
Technical Inquiry & 15-Minute Quotation: Hotline 0979 891 929 (tel:0979891929) — Email: [email protected].