Such corrosion over the years could have weakened the carbon panels along the edge enough to break when struck by a chunk of foam during liftoff, the accident investigators said Tuesday.
"We're studying the effects of aging," said the board's chairman, Harold Gehman Jr., a retired Navy admiral.
Air Force Maj. Gen. John Barry said pinholes in the quarter-inch-thick protective carbon lining the edge may have allowed air pockets to form. The air could have gnawed away at the carbon, a process called oxidation.
The pinholes have appeared on all four space shuttles. NASA patched the larger holes, but did not check thoroughly for underlying air pockets, investigators said.
The accident investigation board is trying to determine what caused Columbia's destruction over Texas two months ago Tuesday. Barry and other members have focused from the start on the left wing, which was hit by debris barely a minute after liftoff.
Pinholes were first discovered on Columbia in 1992, after 12 flights, and later appeared on the other shuttles, Barry said. As many as 20 to 40 pinholes formed on each carbon panel over the years, quite possibly the result of zinc leaching out of the paint primer on the metal tower that swings around the shuttle and protects it on each of the two seaside launch pads.
Because the structure was not repainted and refurbished, more primer was exposed and every time it rained, corrosive zinc oxide washed onto the shuttles, Barry said.
"This dates back to Apollo days, and a lot of that infrastructure has not been replaced because they had made a decision they were going to go" with next-generation spaceships that have yet to materialize, Barry said. "Infrastructure is definitely going to be part of our investigation."
Barry said NASA was aware of the leaching problem, but noted: "I don't think it quite got the attention maybe that it has now, obviously, with the mishap."
NASA addressed the problem of zinc oxide sprays by slathering the leading edges with sealant and other material, Gehman said.
"The question is, once again, just like everything at NASA: Who knew it high up? Was it a serious consideration? Did they have anybody looking at aging as a weakening? We don't know that yet," Gehman said.
The board stressed that the leaching paint primer is a leading potential explanation for the pinholes. But it's also possible salt spray from the Atlantic formed the holes.
Columbia, NASA's oldest shuttle, spent an accumulated 2½ years on the launch pad.
At their weekly news conference, the investigators also said they had found additional defects in a spare shuttle fuel tank, including a bonding problem with the insulating foam.
The fuel tank being analyzed at the Michoud Assembly Facility in New Orleans had 32 air pockets in the foam on the so-called bipod area, an attach point between the tank and shuttle, Barry said. This is where a 2-pound piece of foam - estimated to be 25 inches long, 15 inches wide and 5 inches thick - broke off Columbia during liftoff.
Air pockets like these, mostly 2 inches to 3 inches in size, could cause the foam to pop off. Some foam in this location also was not bonded properly to the tank, Barry said.
As for the mystery object that floated from Columbia just one day into its flight, it almost certainly was a so-called carrier panel from the underside of the wing's leading edge, the board said. The 2- to 3-foot-long strips of metal connect the carbon on the leading edges to the thermal tiles that cover the rest of the wings.
The Air Force Space Command detected the object via radar, but did not know it was there until after the Feb. 1 accident.
Right before the object appeared out of nowhere, the shuttle performed a maneuver in orbit that probably shook it loose.
A single missing carrier panel would be enough to allow the hot deadly gases of atmospheric re-entry to penetrate and burn through the wing, said board member Roger Tetrault, a retired corporate executive who worked with nuclear submarines. But he said the breach in the leading edge of the left wing also could have involved the carbon panels, seals, stainless steel support structures or even bolts.
By Marcia Dunn