Corrosion seems like an unlikely cause. Every few years, each Shuttle is sent for a very complete overhaul. They pretty well strip out the systems and inspect everything. If memory serves, Colombia just came back from such an overhaul last year. Each orbiter is also closely inspected before each launch. While not impossible, it seems unlikely that corrosion would be a factor.

The leading edges of the wings are covered by carbon-carbon, so corrosion wouldn't be a factor there. There was a news article a few weeks ago about the possibility of damage to those surfaces due to weathering. While it seems unlikely to be a cause, it's possible it might be a contributing factor.

I saw a news article a few days ago about how aluminum will burn under certain conditions. A high percentage of the Shuttle's structure is made of aluminum. The article suggested that once the breech began, the aluminum actually ignited and doomed the vehicle. Would there be enough oxygen to support combustion at 200,000+ feet? I don't know.