MYSTERIOUS 'MELTED' steel fragments recovered from the debris of the World Trade Center disaster are still baffling experts 12 months on, NCEI has learned.
Evidence of 'severe high temperature corrosion attack' on several beam sections from the WTC7 building was documented in the Federal Emergency Management Agency's initial report into the catastrophe.
Signs of a similar phenomenon were also apparent in a column fragment from one of the Twin Towers.
These samples were among more than 150 recovered from the four scrapyards processing WTC debris by volunteer engineers.
Preliminary metallurgical analysis indicated that temperatures in excess of 1,000infinityC in conjunction with the presence of sulphur had caused surface melting and severe thinning of the steel section.
But no source for the sulphur could be identified - and there was no way of knowing whether the reaction with sulphur had actually begun long before 11 September.
Such a reaction might have reduced the steel's resistance to fire, FEMA suggested.
This could help to explain the catastrophic collapse of WTC7 which burned unchecked for seven hours, but fire experts are yet to be convinced.
'The most likely explanation is that the affected steel was literally part of a chemical stew that 'cooked' for weeks in the red hot debris pile at Ground Zero, ' said Corus fire engineer John Dowling.
'In full scale fire tests we've seen steel resist temperatures well above 1,000infinityC without any signs of melting.
But this is a very rare event and more needs to be known.'
Research into the apparent melting - now dubbed 'extreme erosion' - will be carried out by the US National Institute of Standards & Technology as part of a two year, $15M investigation of the disaster.