Sunday, April 01, 2012

Chloride Stress Corrosion Cracking

The American Safe Climbing Association has some good information about bolting and retro-bolting. I copied/pasted an article off their site. It has to do with the issue of using expansion bolts on sea cliffs and dis-similar metals. Please visit their site for more good information on how to inspect bolts for safety. There are a good number around I would not trust.

The Devil and the Deep Blue Sea
New Warnings about Sea Cliff Bolts
John Byrnes, Skip Harper and Mike Shelton

"My last month was spent climbing in Thailand on some very questionable bolts... About a week ago while I was waiting for a guided party to finish a second pitch on Thaiwand wall, a leader broke a bolt by pulling on the quickdraw. This caused him to fall and break the next bolt. He was lowered off with a badly injured leg. Both bolts fractured in the shaft at the hanger."

"Just returned last night from Cayman Brac and I wanted to drop you a quick line. Really great trip but... when Gene weighted the second bolt on 'Throwin' The Hula Girl' it broke! He came down on his butt and foot and suffered contusions, luckily missing his spine. It could have been ugly. The bolt was stainless steel..."

"Andy... reached down, grabbed the draw and said "Take". As he lowered his weight (the hanger broke). He landed on his back on some big boulders (breaking ribs and puncturing a lung). I tried to help him as my girlfriend ran for help, but he kept insisting that I not touch him and he just wanted to try to keep breathing."

Sardina, Thailand, Cayman Brac, Calanques. Warm weather, beaches, exotic food and incredible sea-cliff sport climbing, but all is not well in paradise. With the growth of sea-side climbing around the world there has been an ever increasing number of bolt failures. The marine environment is tough on metals. The constant exposure to salt water promotes a specific powerful corrosion mechanism that is not seen at inland climbing areas. High quality stainless steel bolts and hangers have failed in as little as 18 months when placed near the sea. Most of the time there is no visible indication they are unsafe. They may look fine but not hold body weight, much less a fall. Affected cliffs don't even need to be right over the water. Breaking waves create a fine mist which can be transported inland for miles by the wind.

The stainless steels used today in almost all climbing bolts are susceptible to a failure mechanism called Chloride Stress Corrosion Cracking or SCC. Just like it sounds, the chlorine ion, which results from dissolving salt in water, and stress must both be present. A typical multi-piece expansion bolt has the shaft in tension, and the hanger has complex stresses placed on it when it is clamped against the rock as you tighten the nut. This type of bolt placed in a sea cliff is a bomb with a short fuse. Once started, SCC spreads like a disease following the stress lines in the steel, much like grass grows in small cracks in concrete and forces the pieces apart. The cracks get larger over time, and soon the microscopic grains of the metal are no longer in contact.

Although SCC can occur on any part of a bolt exposed to salt water, it usually occurs under the hanger where salt water wicks into the crevice between the hanger and the rock surface. Bolts corroded by SCC typically break flush with the surface of the rock. Hangers typically break at the ninety-degree bend, and nuts can crack just about anywhere. At least one bolt vendor sells stainless steel "clad" bolts and nuts. These have a thin outer cladding of stainless steels urrounding a core of mild steel. This type of bolt seems to be worse then others, since once SCC cracks the cladding,oxidation (rust) finishes the job in short order. If you see a stainless bolt with a rust "beard" on the rock under it, beware!


A few years ago the Thailand locals started rebolting with one-piece glue-in bolts made of stainless steel for the simple reason that they knew regular expansion bolts weren't working. These should be an major improvement since they don't have the high levels of stress that expansion bolts have. However, they are not impervious to SCC.

Steel gains much of its strength from "work hardening" which is residual stress created by the manufacturing process, and glue-in bolts have these stresses designed into them. No one knows how long stainless steel glue-ins will last.