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Corrosion

04-Feb-2010
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On August 23, Brad Chum Carter and Boyd Fackler walked up to climb on the Sport Wall at Index's Upper Town Wall near Seattle. Carter chose the techy, vertical Calling Wolfgang (5.12b), bolted in 1990 by Greg Child.

Carter climbed to the first bolt and paused to check it out. I knew the route was old and had not been inspected for a while, he said. The bolt seemed OK so I figured they were all the same.

He reached the second bolt, asked Fackler to Take so he could brush dirt off the wall and continued upward. After clipping the third bolt, Carter again hung to dust off the holds.

He heard the ping of breaking metal, then plunged. Another ping sounded when the second hanger also broke, the metal shards bouncing off a ledge to his right.Luckily, the high first bolt held and Carter stopped inches short of the ground, with Fackler holding down tight.

Carter was bruised and scraped but otherwise unharmed.

 

ANALYSIS

Accidents involving broken bolts are thankfully rare, but far from unheard-of. Bolt failures most often occur at climbing areas in marine environments, such as Thailand or Sardinia, and involve a process known as Chloride Stress Corrosion Cracking (SCC). The chloride ion present in salt water instigates corrosion that follows lines of stress in a tightened steel bolt, much like grass grows in small cracks in concrete and forces the pieces apart, according to a report posted by the American Safe Climbing Association. SCC is a well-documented phenomenon, and can cause bolt failure in as little as 18 months.

The night after his accident, Carter described it on cascadeclimbers.com. Several climbers suggested SCC or galvanic corrosion, an electrochemical process in which one metal corrodes when in contact with a different type of metal, e.g., an aluminum hanger on a steel bolt.

But two days later, when the shards of the busted hanger were recovered and examined, and a photo showing the flaky interior edges at the break was posted, another, equally insidious process seemed to be at work.

The hanger was a circa 1990 Kong-Bonatti aluminum bolt hanger, which showed signs of a type of deterioration known as exfoliation corrosion, possibly exacerbated by a galvanic action brought on by pairing dissimilar metals in the wet environment of the Pacific Northwest. In a post on cascadeclimbers.com a metallurgist described the process: In this particular form of intergranular corrosion the expansive force of insoluble corrosion products tends to force the grains apart and leads to exfoliation corrosion, sometimes known as lamellar or layer corrosion. In extreme cases, the edges of the affected area are leaf-like, and resemble the separated pages of a wetted book that has become swollen and begun to open up.

According to a report by metalimprovement.com, a metal-treatment company specializing in aircraft maintenance: Where fasteners are involved, exfoliation corrosion extends outward from the fastener hole, either from the entire circumference of the hole, or in one direction from a segment of the hole. In severe cases, the surface bulges outward, but in less severe cases, there may be no telltale blisters.

 

PREVENTION

Just like you would with natural protection, always visually inspect bolts and hangers on older rock climbs. Hand-tighten loose nuts and look for corrosion.

All aluminum hangers are suspect and should be replaced. Unfortunately, there is no practical test for whether a hanger is aluminum. Several manufacturers still make aluminum hangers to be used as emergency anchors, mainly in caving applications.

This particular incident illustrates the value of guidebooks listing the first ascentionist and the year of the ascent, since similar routes can be easily flagged and maintained. Greg Child told Rock and Ice that he and Greg Collum used these hangers in the 1980s. According to Child, the hangers were stamped as full-strength. He says. There are a lot of them sprinkled around Index.

Finally, always use the highest quality equipment when placing permanent anchors. Match your hardware to your environment. If you're bolting in a marine environment, use a titanium glue-in bolt such as the Ushba Tortuga.

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