Dr E. Ramanathan PhD

Surface Coating Consultant

Validation & quick checks

• Your 100 kg concentrate sums to 100 kg and scales linearly to a 500 kg batch (×5):
  H₂SO₄ 125 kg · Formic 50 kg · HF 50 kg · H₃PO₄ 25 kg · IPA 15 kg · DM Water 235 kg → 500 kg total.
• If the working bath is made at 25% w/v concentrate, then every 1 L bath contains 250 g concentrate. With 500 kg concentrate, you can make ≈ 2,000 L of working bath (500,000 g ÷ 250 g L⁻¹ = 2,000 L).
  (If your practice is 25% v/v, treat it as 1:3 concentrate:water; results will be slightly stronger—pick one convention and stick to it in your SOP.)

Why welding “burn/black marks” persist on CRCA

1. Scale chemistry & morphology: Weld heat forms adherent FeO/Fe₃O₄ (sometimes Si/Mn-rich scale) that is less reactive to your acid blend than mill rust.
2. Fluoride placement: HF helps dissolve silica/silicates and breaks tenacious films, but on plain carbon steel the benefit is local and the risk of base-metal attack/fuming rises; un-buffered HF can undercut without lifting whole scale.
3. Insufficient wetting/penetration: Limited surfactants (IPA is volatile, poor wetting under strong acid) → slow capillary access under heat-tint edges.
4. Bath loading & iron build-up: High Fe²⁺/Fe³⁺ suppresses acid activity; without accelerators or bleed-and-feed, time drifts up.
5. Surface oils & weld soot: Carbonized residues block acid; needs alkaline degrease + mechanical pre-brush.
6. Temperature control: <25 °C slows scale lift; >40 °C with your current mix increases odor/fuming and metal loss.

Root-cause-aligned remedies

• Process sequence (mandatory):
  1. Alkaline cleaner (50–60 °C, 3–5 min) → rinse
  2. Mechanical wipe/SS-wire brush on weld halos → blow off dust
  3. Pickle (revised acid below) with forced agitation → rinse
  4. De-smut/activation (brief oxidizing or fluoride-buffer dip) → rinse
  5. Phosphating/zirconium → rinse → DI rinse → dry → paint
• Chemistry tuning:
  • Replace free HF with ammonium bifluoride (ABF) to buffer fluoride and reduce fuming while keeping weld-halo attack.
  • Introduce HCl fraction (faster on Fe oxides at ambient) while reducing H₂SO₄ to control base-metal loss.
  • Add a proper inhibitor package (thiourea/acetylene-alcohol derivative + phosphate ester) to protect steel but leave oxides exposed.
  • Add low-foam nonionic wetting agent and a micro-defoamer; drop IPA (major odor/flash contributor).
  • Add a mild accelerator/oxidant gate (controlled H₂O₂ in make-up or nitrite in de-smut stage) to keep Fe²⁺ low.
  • Odor control via no-VOC surfactants, masking trace, and covered tank + exhaust/scrubber.
• Operating window (target):
  Temp 30–35 °C, time 4–6 min, agitation (air sparge or pump eductor), working bath 1:3 concentrate:water by w/v, Fe (total) <40 g/L, F⁻ 0.8–1.2 g/L, free acidity in control band (see QC below).

Revised concentrate (for CRCA weld-scale) — 100 kg basis

Focus: faster weld halo removal, lower odor, controlled fluoride, better wetting, inhibitor protection.

• Hydrochloric acid (31–33%) …… 20.0 kg
• Sulfuric acid (98%) …………………… 15.0 kg
• Phosphoric acid (85%) ……………… 10.0 kg
• Formic acid (85–90%) ………………… 5.0 kg
• Ammonium bifluoride (ABF) …… 8.0 kg (controlled F⁻ source)
• Nonionic wetting agent (EO/PO, low-foam, acid-stable) …… 1.2 kg
• Phosphate-ester surfactant (acid-stable) …………………………… 0.6 kg
• Pickling inhibitor blend (thiourea + propargyl alcohol derivative) … 1.5 kg
• Silicone-free defoamer, acid-stable ……………………………………… 0.3 kg
• Odor-mask (acid-stable, optional) ………………………………………… 0.2 kg
• DM water …………………………………………………………………………… 38.2 kg
  Total = 100.0 kg

Scale-up to 500 kg: multiply each line ×5 (e.g., HCl 100 kg, ABF 40 kg, etc.).

Why this works

• HCl accelerates black-oxide removal at ≤35 °C.
• ABF supplies F⁻ to break tenacious weld films with less fume than free HF.
• H₃PO₄ conditions the surface and improves paint adhesion downstream.
• Inhibitors limit base-metal loss → brighter, uniform surface; surfactants ensure penetration under heat-tint edges; defoamer keeps rinseability.
• No IPA → reduced odor/flash.

Working bath make-up & targets

• Make 25% w/v: 1 L bath = 250 g concentrate in DI/DM water.
• Example: 2,000 L tank → 500 kg concentrate + water to level.
• Start at 30 °C; try 4 min on mild parts, up to 6 min on heavy welds.
• Agitate continuously; brush-assist first runs if legacy parts are heavily tinted.

Optional weld-spot booster (shop-floor paste)

For stubborn halos on CRCA only (not stainless):

• H₃PO₄ (85%) 45 wt% + ABF 10 wt% + water 43.5 wt% + nonionic 1.0 wt% + inhibitor 0.5 wt%.
  Apply locally 1–2 min, scrub nylon, rinse → then run the normal pickle. Safer and lower-odor than HNO₃/HF gels.

QC & bath control (simple, robust)

• Free acidity (as HCl): Titrate 10.0 mL bath with 1.0 N NaOH to phenolphthalein; maintain setpoint (establish during first week; typical 18–24 points for this chemistry at 25% w/v).
• Total iron: Photometric or permanganate; keep <40 g/L; bleed-and-feed 5–10% when high.
• Fluoride (F⁻): ISE or SPADNS; aim 0.8–1.2 g/L. If low, add ABF predissolved; if high, extend bleed.
• Foam check: <5 mm stable foam; if higher, adjust defoamer 10–20 ppm increments.
• Rinse pH after pickle: 5.0–6.0 before phosphating.

Safety & compatibility

• Never add water to acid; always acid into water.
• ABF/HF require PPE, local exhaust, scrubber; segregate from strong alkalis, oxidizers, and glass/silicates.
• Avoid chlorinated solvents carry-in (reactive in HCl media).
• Use 316L or HDPE-lined tanks; avoid plain SS near F⁻.

If you must stay with the current formula

• Replace HF 10 kg → ABF 8–10 kg;
• Drop IPA 3 kg → 0;
• Add nonionic 1.0–1.2 kg, inhibitor 1.5 kg, defoamer 0.3 kg;
• Reduce H₂SO₄ 25 kg → 18–20 kg; shift H₃PO₄ 5 kg → 8–10 kg.
  These changes typically cut time by 30–40% and reduce smell while improving weld-mark lift.

Acceptance test (prove 6-minute target)

Run a DOE on scrap welded CRCA:

• Factors: time (3–6 min), temp (30–40 °C), ABF level (0.8–1.2 g/L F⁻ in bath), inhibitor (0.8–1.2× setpoint).
• Responses: weld-halo Lab* delta vs base metal, underfilm rust rating after phosphating, metal loss (g m⁻²).
  Adopt the lowest temp/time meeting visual and adhesion criteria.

Downstream paint-defect linkage (what you’ll prevent)

• Black specks/poor wetting → from residual weld scale/soot: solved by sequence + wetting.
• Blistering/undercut → from chloride-rich or over-etched steel: controlled via inhibitor & F⁻ buffering + Fe bleed.
• Odor/complaints → via removal of IPA and scrubbed exhaust.