Oxide layer on aluminium: structure, thickness and effects on welding

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

What is the oxide layer on aluminium?

In air, aluminium is immediately covered by a layer of aluminium oxide (Al₂O₃). It is only a few nanometres thick, yet it reliably protects the metal against further corrosion. In welding it is the most important source of trouble: it melts only at around 2,050 °C, it is heavier than the molten metal and it binds moisture.

Every bare aluminium surface, whether brushed, milled or freshly drawn, is covered by this layer again after a short time. It cannot be removed permanently. What matters for welding is to reduce it to a thin, uniform layer shortly beforehand.

How is the oxide layer built up and how thick is it?

The natural oxide layer consists of two parts: directly on the metal, an almost pore-free barrier layer of amorphous Al₂O₃ about 1 to 2 nm thick, and above it a porous, hydrated top layer about 5 to 10 nm thick containing aluminium hydroxide and bayerite. It grows thicker with time, temperature, humidity and oxygen supply.

Structure of the natural oxide layer on aluminium: barrier layer and top layer (schematic)

The top layer also contains mixed oxides of the alloying elements. With aluminium-magnesium alloys, magnesium oxide and spinel (MgAl₂O₄) also form at high temperatures, for example in the weld pool. These oxides are thicker and less protective than pure Al₂O₃.

For comparison: an anodic layer is artificially produced oxide. At several micrometres it is around a thousand times thicker than the natural layer, see Anodising.

In production, the natural layer can hardly be measured. Where welding has to be highly reproducible, for example in mechanised welding, the layer is therefore removed completely by pickling and the parts are then stored under defined conditions. This gives a known, uniform layer at the time of welding.

Is aluminium oxide dense?

The lower barrier layer is: it is almost pore-free and protects the metal against corrosion as long as it remains intact. The top layer, by contrast, is porous and absorbs moisture. It is this hydrated part that matters for welding, because it brings hydrogen into the weld.

If the layer is damaged, for example by a scratch, it forms again immediately on the bare spot. That is why aluminium is resistant in air without needing a coating. In strongly acidic or alkaline media, however, the layer dissolves and aluminium corrodes.

Why does the oxide layer cause weld defects?

Aluminium oxide melts only at around 2,050 °C, aluminium already at 660 °C. The oxide skin therefore stays solid in the weld pool, breaks up and ends up in the weld. With a density of 3.9 g/cm³ it is heavier than the melt, so it does not float to the top like oxides in steel but sinks. The results are oxide inclusions, lack of fusion and brittle areas.

In steel, the melting points of oxide and metal are close together, and iron oxides are lighter than steel. They rise to the surface as slag. With aluminium, by contrast, the oxide layer must be thin and clean before welding and has to be broken up by the arc during welding.

Video (in German): how oxide inclusions form and how to prevent them.

How does the oxide layer affect the arc?

When welding aluminium, the arc needs oxide: its cathode spots preferentially attach to the oxide and break up the oxide skin. This is the cleaning action with the electrode connected to the positive pole. If the thickness and condition of the oxide layer change, the arc and the weld change too.

In MIG and TIG welding, the surface condition affects

the stability of the arc,
the geometry of the cathode spot,
the voltage drop in the arc and thus the arc length,
the weld geometry and weld quality,
reproducibility, especially in mechanised and robotic welding.

In MIG welding the wire electrode is therefore connected to the positive pole. In AC TIG welding the positive half-wave provides the cleaning, while the negative half-wave relieves the tungsten electrode. The cleaning is visible as a bright band beside the weld. More on starting the arc under Arc ignition.

Why does oxide lead to porosity?

For two reasons. The porous top layer binds water, which breaks down into hydrogen in the arc. And oxide films in the melt are the places where dissolved hydrogen preferentially collects into pores during solidification. More oxide therefore means more porosity, even at the same hydrogen content.

Research from foundry technology shows that hydrogen pores can practically not form on their own in liquid aluminium. They grow on folded-in oxide films, so-called bifilms, which contain a fine gap. Hydrogen supplies the gas, oxide supplies the site. In practice this means: keep oxide and moisture low together, on the workpiece and on the wire.

Background under Solubility of hydrogen and Condensation.

How do you remove the oxide layer before welding?

Degrease first, then remove the oxide layer mechanically or chemically, and weld soon afterwards. Brushing first rubs grease and dirt into the surface. Suitable mechanical tools are stainless steel brushes, scrapers and milling cutters used only for aluminium; the chemical method is pickling.

Degrease with a cleaner that evaporates without residue.
Mechanically brush, scrape or mill. Use the tools only for aluminium, otherwise iron and steel dust are carried in. Grinding discs can press abrasive grit and binder into the soft surface. For edges and roots, milling instead of grinding is the clean alternative.
Chemically pickle: this removes the layer completely and evenly. Then rinse and dry thoroughly.
Afterwards weld promptly and protect the cleaned parts from moisture and dirt.

Detailed guidelines on storage, cleaning and joint preparation: Surface preparation before welding and Joint preparation.

What role does the oxide layer on the welding wire play?

The wire carries its surface straight into the arc. A thin wire has a large surface area relative to its mass, so oxide and moisture on the wire have a strong effect on porosity and arc stability. What counts is a clean, low-oxide wire surface and dry storage until use.

That is why MIGAL.CO shaves its aluminium welding wires. Shaving removes the original surface together with its oxide layer; the new oxide layer then forms under defined conditions. This gives every wire a uniform, thin oxide layer.

Unpack spools only immediately before use; store opened spools in their original packaging or in a bag.
Store the wire in the production environment for at least 24 hours before use so that it reaches the ambient temperature and no condensation forms.
With long feeding distances, watch out for abrasion, because shavings carry oxide into the torch. More under Wire feeding.

All MIGAL.CO aluminium welding wires and rods with data sheets: Aluminium welding wire.

Frequently asked questions

What is the melting point of the oxide layer on aluminium?

Aluminium oxide (Al₂O₃) melts at around 2,050 °C, more than three times higher than aluminium at 660 °C. The oxide skin therefore stays solid in the weld pool.

How quickly does the oxide layer form again?

Immediately. In air, a bare aluminium surface is covered again by a thin barrier layer practically at once. Over time the layer keeps growing, especially in warm and humid conditions. So remove the oxide only shortly before welding.

Can you weld over the oxide layer?

The arc breaks up a thin, uniform oxide layer through its cleaning action. It cannot cope with thick, hydrated or uneven layers, for example after long storage, after heat treatment or on anodised parts. These must be removed beforehand.

Why is there a bright band beside the weld?

That is the cleaning zone, where the arc has broken up the oxide layer. If it is missing or uneven, the polarity, gas shielding or surface condition is often not right.