Base Metal Preparation and Cleaning
Why cleanliness decides quality
Oil, grease, paint, mill scale, rust and moisture are the main sources of porosity and hydrogen-assisted cracking. Clean at least 25 mm on each side of the joint, including the root face and, on open-root work, the inside of the pipe or plate.
Correct sequence
- Degrease first with solvent, alkaline cleaner or an aqueous wash. Grinding before degreasing smears oil into the surface rather than removing it.
- Grind or wire brush to bright metal where the code requires it, and remove mill scale from the weld zone and the area the arc will touch.
- Remove the oxide and nitride layer from thermally cut edges - typically 1.5-2 mm of grinding - before welding on a bevel produced by oxy-fuel or plasma cutting.
- Dry the joint. Preheating, where the WPS requires it, also drives off surface moisture.
- Use dedicated consumables: stainless-only grinding discs and stainless wire brushes for stainless, aluminium-only for aluminium. A carbon steel brush on stainless leaves iron that rusts and can initiate corrosion.
Solvent safety
Chlorinated solvents (trichloroethylene, perchloroethylene) decompose under arc UV to phosgene. Never weld or gouge near a degreasing bath or freshly cleaned parts. Acetone, isopropyl alcohol and aqueous cleaners are safer alternatives with adequate ventilation and drying time.
Worked example
A 304 stainless V-groove bevel, oil-contaminated and mill-scaled. Degrease with acetone and allow to dry; grind with a fresh alumina disc kept only for stainless; brush with a dedicated stainless brush; check for embedded iron with a ferroxyl or copper-sulphate test; then weld with matched E308L-16.
Traps
- Grinding before degreasing.
- Using the same grinder disc for carbon steel and stainless.
- Ignoring contamination on the root side of an open-root joint.