Discontinuity
Oxide inclusion
Surface oxide folded into the weld pool rather than broken up by the arc, most troublesome on aluminium where the oxide skin melts far above the metal it covers.
TitleOxide inclusion
ClassInclusion
ViewIndicative
What it looks like
- Grey or black films and stringers running through the weld metal in the direction of travel
- On aluminium, a dull grey smudged appearance in the bead rather than a bright clean face
- On film, faint irregular dark indications that can be harder to see than slag
- On a fracture face, thin dark films lying along the fusion boundary
Why it happens, and how it is stopped
Causes
- Aluminium oxide left on the joint, which melts at a temperature far above the aluminium beneath it
- Inadequate mechanical or chemical cleaning of the joint immediately before welding
- Loss or contamination of shielding gas allowing fresh oxide to form on the molten pool
- Welding stainless or aluminium over heavy heat tint or mill oxide without removing it first
- Wire brushing with a brush previously used on carbon steel, dragging contaminants into the joint
Prevention
- Clean aluminium and stainless joints with a dedicated stainless brush kept for that material only
- Weld promptly after cleaning, since oxide reforms on aluminium within a short time
- Use alternating current or the appropriate polarity so the arc's cleaning action breaks up the oxide film
- Protect the pool and the cooling bead with adequate shielding gas coverage and trailing shields where needed
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- RTOxide films have a small density difference from the parent metal, so they show as faint indications and can be missed without careful technique.
- UTThe film is a planar acoustic discontinuity, so a beam striking it near perpendicular returns a clearer signal than radiography provides.
- VTIn-process observation of a dull, smudged, grey bead face is often the first practical warning that oxide is being folded in.
How inspectors think about it
- Oxide films behave more like planar discontinuities than like rounded pores, so their extent along the joint matters more than their thickness.
- Because they are hard to see on film, an inspector places more weight on process control and cleaning practice than on post-weld examination alone.
- A pattern of oxide indications is usually read as a cleanliness or shielding failure across the job rather than an isolated defect in one weld.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Slag inclusionNon-metallic slag trapped inside the weld instead of floating to the surface, most often left in a corner or undercut of a previous pass and then buried by the next one.Tungsten inclusionA fragment of tungsten electrode broken off into the weld pool during gas tungsten arc welding, leaving a hard, brittle, very dense particle embedded in the deposit.Uniformly scattered porosityRounded gas pockets frozen into the weld metal and spread fairly evenly through the deposit, rather than bunched together or lined up, showing that shielding or cleanliness was marginal for the whole pass.Hot crack (solidification cracking)A crack that forms while the weld is still partly liquid, when low-melting films between growing grains are pulled apart by shrinkage before the metal has any strength to resist it.