Why welding marks appear
- Heat-Affected Zone (HAZ): local temper colors (straw → blue/black) from 200–600 °C exposure.
- Oxide scale: thick FeO/Fe₃O₄/Fe₂O₃ on carbon steel; Cr-depleted oxides on stainless; Al₂O₃ smut on aluminium.
- Spatter & slag: fused droplets and flux residues that trap contaminants and hinder coating adhesion.
Chemistry of discoloration (at a glance)
Mild/Carbon Steel
- Scale = FeO/Fe₃O₄/Fe₂O₃
- Promotes underfilm corrosion if left beneath paint.
Stainless Steel
- Heat tint consumes Cr at the surface → Cr-depletion.
- Must be repassivated to restore corrosion resistance.
Aluminium
- Weld smut (Mg/Si/Al oxides) and embedded silica.
- Needs desmutting then conversion coating.
How to remove — Mechanical methods
- Blend weld and HAZ using 120–180 grit flap wheel → refine to 240–400 grit or Scotch-Brite for uniform finish.
- Use stainless-only wire brushes on stainless (avoid cross-contamination).
- For best coating anchor profile on MS, abrasive blast to Sa 2.5 with 25–50 µm profile.
- Deburr spatter with carbide burrs; avoid overheating (no blue burns).
How to remove — Chemical methods
- Stainless steel pickling: commercial HNO₃ + HF pickling paste/gel. Apply 10–20 min, scrub, rinse copiously, neutralize with 5% NaHCO₃, then passivate (20–25% HNO₃ or proprietary citric formulations).
- Carbon steel descaling: mechanical is preferred. Where needed, hot alkaline clean → rinse → inhibited mineral acid pickling (HCl/H₂SO₄) → rinse → flash rust inhibitor → immediate priming.
- Aluminium: alkaline clean → desmut (nitric-fluoride or sulphuric-ferric) → rinse → conversion coat (chromate-free zirconium/titanium) before paint/powder.
Coating performance note: leaving heat tint/scale or chemical residues causes poor adhesion, blistering and rust creep. Proper removal and neutralization improves primer adhesion and durability.
Safety & environmental aspects
- Wear PPE: chemical-resistant gloves, face shield, apron, and respiratory protection for grinding dust and pickling fumes.
- HF-bearing pickling pastes are highly hazardous. Keep calcium gluconate gel available; train staff on first-aid; restrict to trained personnel only.
- Provide local exhaust ventilation; collect and neutralize effluents (target pH 6–9) before ETP discharge.
- Segregate stainless tools from carbon-steel tools to prevent contamination.
After-treatment & painting
- Water-break-free surface → immediate priming (zinc phosphate epoxy/epoxy primers).
- Verify DFT and adhesion (ASTM D3359) before top coat.