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When a pass a construction site where someone is welding two pieces of metal together there is a very distinct smell that I associate with 'burning' metal, although I am not quite sure it is burning in the traditional sense. Also, when someone ignites a sparkler this same (or at least similar) smell emerges.

I would like to know what is causing the smell? Is it the smell of metaloxide particles that are dispersed from the weld site? Or is it something in the air that reacts and causes the smell?

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  • $\begingroup$ Would you happen to know if all metals have the smell when being welded, or is it just steel? I suspect the smell comes from the release of trace amounts of carbon/phosphorous/sulphur present in alloys, especially steel, which immediately react with air and each other forming odorous compounds. Very pure metals may be odorless when welded $\endgroup$
    – Nicolau Saker Neto
    Commented Feb 9, 2014 at 12:06
  • $\begingroup$ @NicolauSakerNeto I'm not sure whether it is only steel or more (all) metals, although I suspect that the thing I smell near construction sites will usually be from welding steel $\endgroup$
    – Michiel
    Commented Feb 9, 2014 at 13:42
  • $\begingroup$ I always suspected that it is a burning iron itself, but without proof and further thinking. As noted, similar smell comes from welding and disc grinding. $\endgroup$
    – ssavec
    Commented Feb 9, 2014 at 18:18
  • $\begingroup$ At least in the welding cases there are of course shield gases used to prevent immediate oxide contamination of the flame weld site - maybe those are involved, but you certainly smell something similar whenever metals are abused mechanically - overheated motors to a degree. Even metal utensils left in water too long produce a similar whiff. $\endgroup$
    – civitas
    Commented Jun 3 at 0:22

2 Answers 2

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According to the Canadian Centre for Occupational Health and Safety page Welding - Fumes And Gases, the welding fumes are

a complex mixture of metallic oxides, silicates and fluorides. Fumes are formed when a metal is heated above its boiling point and its vapours condense into very fine, particles (solid particulates).

This composition varies dependent on the composition of metals being welded and fluxes. The website lists several main types and their associated health risks. Often associated with metallic oxides are some pungent/noticeable gases:

  • Gases produced from the thermal breakdown of coatings, from welding and cutting processes, stated in the article linked as being examples of "welding gases" (examples from the linked website include ozone)
  • And depending on the coating that may be on the metal, organic vapours (including aldehydes, diisocyanates, phosgene and phosphine)
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The smell is often noticed at construction sites, when reinforcing bars are welded or cut. Constructional steel contains about 0.1% carbon and phosphorus.

Phosphorus normally isn't welcome since it renders steels more brittle. On the other hand, it increases the corrosion stability. This is is a desired property for rebars embedded in concrete.

Is it conceivable that noticable (alkyl)phosphanes are formed when this steel is welded or cut with a disc grinder?

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    $\begingroup$ My first impression was also to say phosphines from the P in the metal. Often, just by holding steel up to your nose you can detect a faint aroma $\endgroup$
    – Stian
    Commented Dec 7, 2016 at 11:33

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