these pieces comes from one of the towers, the other from Building 7
in Appendix C of the FEMA WTC report noted: "The severe corrosion and
subsequent erosion of samples 1 and 2 are a very unusual event. No clear
explanation for the source of the sulfur has been identified… A detailed
study into the mechanisms of this phenomenon is needed."
A New York Times article noted that pieces of steel were found that
were "apparently melted and vaporized not solely because of the heat of
fires, but also because of a corrosive contaminant that was somehow released
in the conflagrations."
York Times called these findings
the deepest mystery uncovered in the investigation." 
one-inch column of steel has been reduced to half-inch thickness. Its
edges--which are curled like a paper scroll—have been thinned to almost razor
sharpness…Gaping holes--some larger than a silver dollar--let light shine through a formerly solid steel
flange. This Swiss cheese appearance shocked all of the fire-wise professors,
who expected to see distortion and bending--but not holes."
Swiss cheese steel shocks professors::
unrealistic study hypothesized the sulfur could have come from acid rain