Corrosion suggested in shuttle crash

MSNBC ^ | Feb. 9, 2003 | James Oberg

[No Truce With Kings]

Corrosion suggested in shuttle crash

  Weakened wing may have been vulnerable to impact of debris

By James Oberg
SPECIAL TO MSNBC.COM

       IN RECENT DAYS, NASA officials have expressed their frustrated bafflement over the observed debris impact on Columbia’s left wing. They have repeated studies made during the flight and still come up with results that show the worst-case damage is still far short of a mortal wound that could have prompted the catastrophic failure of the wing.
       If the falling insulating foam were the triggering event, some additional factor or factors must have been present, they say, to multiply its harmfulness. Accident investigators continue to seek such factors and evaluate them.
       
‘ONE POSSIBLE EXPLANATION’
       In a two-page memo sent to the NASA space engineering office in Houston, obtained exclusively by MSNBC.com, veteran space shuttle engineer Ray Erikson offers “one possible explanation”: “corrosion of the leading edge spars” on the left wing had already so weakened that structure that the small additional damage from the debris was then enough to cause the later failure.
       In this suggested scenario, the loss of several tiles just behind the left wing leading edge led to significant heating (“not catastrophic by itself”) that is conducted into the support structure for the leading edge panels. Aluminum is notorious for weakening rapidly at even relatively small temperature increases, and combined with pre-existing corrosion this could have caused a bolt to break free and one leading edge panel to separate. From that point, vehicle loss was inevitable.

       

James Oberg, a former shuttle flight controller, is an author and NBC analyst. Among his books is a history of the U.S.-Russian space alliance titled "Star-Crossed Orbits.”
       

[No Truce With Kings]

According to this article, if this proves true, it is a Columbia-unique problem. A different technique was used for building the wings of the other Orbiters.

[StriperSniper]

if this proves true, it is a Columbia-unique problem

That would be good news. Then again, corrosion could prove to be a big problem anyway if it is from the salt air at the Cape. Who knows where else it could be.

[FReepaholic]

Well, they've found part of one wing so they should be able to inspect it for any corrosion.

[You Dirty Rats]

I thought that the media had this all figured out a week ago.

Sure looks like it will be a long time before we know what actually happened.

[No Truce With Kings]

"Then again, corrosion could prove to be a big problem anyway if it is from the salt air at the Cape."

You have to read the article (which I could only excerpt). They have been extensively checking the Orbiters for corrosion. Deal is that Columbia had monal bolts screwed into aluminum. Only one where they did that. Theory is that galvanic corrosion rotted the attach points. It would have been a very slow process, but when combined with a hard whack, allowed the wing glove to slip.

Columbia had been away from the Cape for several years prior to returning from a refit. My bet is that some of the institutional memory about the differences in Orbiters disappeared during that time, so that people still there failed to realize how different the vehicle was from its sisters. Not the stuff that hits you in the face, but the little things, like monal bolts.

[WFTR]

By unhappy coincidence, wrote Erikson, the bolts were composed of a material that reacted badly with the aluminum mounting structure. When exposed to salt water, the metal in the bolts and the metal in the mounting structure acted like a weak chemical battery. This led to a gradual eating away of the aluminum structure.

The bolts were made of Inconel 718 alloy and of A286 corrosion-resistent steel (CRES), and they were not affected. But the structure, fabricated from 2024 aluminum honeycomb, did corrode when salt spray from the nearby ocean was dissolved in rainwater and seeped into the structure while Columbia was on the launch pad.

The phenomenon that they are describing where the two metals interact in solution to cause enhanced corrosion is called galvanic corrosion. In order to have this mechanism, there must be a continuous electrolyte (salty water) layer touching both metals. Maybe some part of the wing structure would allow salt spray to condense and run into this area to form a continuous pool, but I don't usually think of these condensation conditions as being ideal for causing galvanic corrosion.

A common problem around bolting is crevice corrosion. This mechanism occurs because a small crevice holds electrolyte and conditions in the crevice become more severe than they are outside the crevice. Salt spray condensing into a crevice can be very aggressive. Galvanic corrosion could be operating within the crevice as well, but the bigger factor might be the crevice.

A problem with inspecting for this problem is that they have to figure out where the salt spray settled on the wings and seeped to the bolts. They may inspect one part of the wing and find no evidence of corrosion at all, but the part of the wing that failed may be full of galvanic or crevice corrosion because that's where the electrolyte solution was. I've seen columns where only a few places suffered cracking but those few places were where the salt spray settled.

In any case, this article is particularly interesting.

WFTR
Bill