In Appendix C of the FEMA report we can find this statement:

"The severe corrosion and subsequent erosion of Samples 1 (from WTC 7) and 2 (from WTC 1 or 2) are a very unusual event. No explanation for the source of the Sulphur has been identified. The rate of corrosion is also unknown. It is possible that this is the result of long-term heating in the ground following the collapse of the buildings. It is also possible that the phenomenon started prior to the collapse and accelerated the weaking of the steel structure."
FEMA - Appendix C - Limited Metallurgical Examination

What we notice is, the interpretation has not restricted the length of time 'prior to the collapse' that this corrosion took place. Thus, FEMA has identified that this corrosion either represented 'pre-existing' damage to the towers or post-collapse damage. As we will observe later in the analysis, when we examine some basic corrosion equations, it is not feasible for this to have developed between the time of the impact and the resulting collapse.

As we noted earlier, the WTC was subject to secondary fires from a basic oxygen deficient office fire:

"The jet fuel, which ignited the fires, was mostly consumed within the first few minutes after impact. The fires that burned for almost the entire time that the buildings remained standing were due mainly to burning building contents and, to a lesser extent, aircraft contents, not jet fuel."
Key Findings of NIST’s June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster

We also note the key findings leading to the collapse:

"The NIST investigation team has formulated the following chronological sequence of major events leading to the eventual collapse of the towers:

1. Aircraft impact damaged the perimeter columns, causing redistribution of column loads to adjacent perimeter columns and to the core columns via the hat truss (the steel structure that supported the antenna atop the towers and was connected to the core and perimeter columns).

2. After breaching the building’s exterior, the aircraft continued to penetrate into the buildings, damaging core columns with redistribution of column loads to other intact core and perimeter columns via the hat truss and floor systems.

3. The subsequent fires, influenced by the post-impact condition of the fireproofing, weakened columns and floor systems (including those that had been damaged by aircraft impact), triggered additional local failures that ultimately led to column instability.

4. Final column instability resulted when redistributing loads could not be accommodated any further."
Key Findings of NIST’s June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster

Without pre-existing structural damage we are driven to make this statement:

1. No high rise building has ever collapsed from fire.

Thus, the following facts become clear:

1. It must shown that fires, of the temperatures indicated by NIST, can induce such effects
2. The burden of proof exists with those making this claim. That is, they must demonstrate conclusively that the fires were the sole cause. It is not acceptable just to indicate that it is within the realms of possibility.

In Reference to WTC 7 Specifically:

1. No high rise building has ever collapsed from fire.
2. The collapse was visually consistant with a controlled demolition.
3. FEMA's analysis said a fire induced collapse had "minimal probability".

Until such material is provided, the collapse of WTC 7 is scientifically more consistant with controlled demolition. There is no scientific evidence to support a near-uniform structural failure of a highrise building solely due to fire, neither past nor present.

Occam's Razor:

Occam's razor suggests the collapse theory to be incorrect and thus NIST's basic working hypothesis for investigation was and is inappropriate for WTC 1, 2 & 7. The conclusion that fires were responsible for the collapse, is really a guess based upon assuming a wide range of factors that may or may not have occured. It is also opposed to all evidence from high rise structural fires within the US.

The burden of proof exists with NIST to conclusively prove that fires were the sole cause of the collapse. To date, this has yet to be achieved and thus, scientifically, it cannot be held as the primary theory behind the collapses.