Clean to the eye doesn’t always mean ready for coating or for welding!
Even brand-new steel can have mill scale, oxidation, contaminants, or other surface material that needs to come off before the next step.
Every aluminium part has always an oxide cover layer that can be tricky!
That’s where proper surface prep comes in.
We’re not just making it look clean.
We’re creating the surface the coating needs to do its job.
Sometimes the biggest difference is the one you didn’t realize was there, the one you can't see!
Oxide Removal by laser before welding
Before welding and several other surface treatments, metal surfaces must be clean and without oxides.
Laser oxide removal can be used to remove those contaminants without introducing impurities in the base metal.
Using the laser’s micron precision, oxides can be removed in controlled layers and from predefined areas.
Photonclean Trident Fiber laser cleaning systems can remove oxides from ferrous and non-ferrous metals alike.
Our systems stand out compared to chemical treatments, mechanical brushes, and other common methods.
Rust/oxide Removal Before Welding with laser abblation: Why Surface Prep Matters
Surface contamination on metal before welding is not an aesthetic problem — it is a metallurgical one.
Rust, mill scale, aluminium oxides, zinc coating residue, oil, and paint introduce specific elements into the weld fusion zone that cause porosity, hydrogen embrittlement, inclusions, and cracking through mechanisms that are well-documented in welding metallurgy.
Removing these contaminants before welding is not optional for code-critical structural welds; the method of removal affects how thoroughly the contamination is removed and whether the cleaning process itself introduces problems.
Laser cleaning immediately before welding offers specific advantages over grinding, wire brushing, and solvent cleaning for certain weld joint configurations.
Why Surface Contamination Causes Weld Defects
Rust and moisture: the hydrogen mechanism
Rust — iron oxides and oxyhydroxides, including goethite (FeOOH) and haematite (Fe₂O₃) — contains bound water in its crystalline structure. When the welding arc heats a rusted surface to arc temperatures (above 6.000°C at the laser beam core), the bound water in the rust decomposes:
2 FeOOH → Fe₂O₃ + H₂O (dehydration of goethite)
The water vapour at arc temperature immediately dissociates:
H₂O → 2H + O (atomic hydrogen generation)
Atomic hydrogen has a much higher solubility in molten steel than in solid steel. As the weld pool solidifies, hydrogen that was dissolved in the melt must diffuse out — but if the cooling rate is fast enough, hydrogen is trapped in the solidified weld metal or the heat-affected zone. Trapped hydrogen is the primary driver of hydrogen-induced cracking (also called cold cracking or delayed cracking), which can develop hours or days after welding at stresses below the material’s yield strength.
This mechanism is why welding codes specify dry surfaces before welding. AWS D1.1 (Structural Welding Code — Steel - EN 1090-2 (Execution of steel structures), for instance, address surface condition requirements for structural steel welds. The specific provisions in the current edition should be confirmed before citing them as compliance requirements.
The practical significance: rust that appears light and superficial still contains bound water that dissociates under the welding arc. Cleaning off visible rust is not sufficient if the steel surface directly in the joint has FeOOH present.
Mill scale: porosity and arc instability
Mill scale is the blue-grey oxide layer (predominantly magnetite, Fe₃O₄, with iron wüstite, FeO, on the inner surface) that forms on hot-rolled steel during cooling after rolling. It is harder than the steel substrate, brittle, and has different electrical conductivity properties.
In laser and in MIG (GMAW) welding, mill scale under the weld arc or laser beam causes the keyhole to wander and become unstable as the conductivity changes across the scale. This keyhole instability produces irregular fusion and spatter, and — more significantly — the reduction of iron oxides by carbon in the weld pool generates CO and CO₂ gas:
FeO + C → Fe + CO (carbon-oxygen reaction in the melting pool)
Carbon monoxide gas that does not escape the solidifying weld pool is trapped as porosity. The result is a weld with distributed porosity that reduces tensile strength and fatigue life, and that may fail radiographic or ultrasonic inspection.
In TIG (GTAW) welding, mill scale causes oxide inclusion problems and requires more aggressive mechanical preparation to achieve clean base metal.
Zinc coatings: the porosity and fume problem
Galvanised steel (zinc-coated) is used extensively in structural, automotive, and fabrication applications for corrosion resistance. Zinc melts at 420°C and boils at 907°C. The laser welding beam temperature far exceeds zinc’s boiling point, rapidly vaporising the zinc coating in the weld zone. This rapid vaporisation creates zinc vapour bubbles in the weld pool. If zinc vapour is generated faster than it can escape through the weld pool surface, it is trapped as porosity in the solidified weld. In GMA (MIG) welding on galvanised steel, this is one of the most common sources of weld porosity in fabrication practice.
Zinc fume is also a recognised health hazard — metal fume fever. Welding over zinc coatings without adequate ventilation exposes the welder to zinc oxide fume. As covered in the laser welding galvanized steel article, the approach of leaving a controlled gap in the joint near the weld to allow zinc vapour to escape is one technique used in production welding; removing the zinc from the immediate weld zone is another.
Oil and organic contamination: carbon pickup and porosity
Cutting fluids, forming lubricants, storage oils, and fingerprints on steel surfaces introduce organic carbon-containing compounds into the weld fusion zone. The organic matter decomposes under the heat, releasing carbon (which picks up into the weld metal if above the base steel’s carbon content), hydrogen (from CH compounds), and combustion gases that create porosity if trapped.
Carbon pickup in low-carbon structural steel welds can shift the weld metal into a higher-carbon range, potentially reducing toughness and increasing hardenability of the weld and heat-affected zone. For low-hydrogen welding processes (where hydrogen control is a primary objective), oil contamination on the joint surface partially defeats the hydrogen control that the low-hydrogen consumable is intended to provide.