Stainless Steel Stress Corrosion Cracking - Primer for Aircraft Mechanics

Stress Corrosion Cracking

What does an indoor swimming-pool-roof-collapse in Switzerland that kills 14 people, an aileron failure in a Bonanza, and rock-climbing-bolts that break when slightly tapped have in common?

• Failures occurred in parts made from 300 series (austenitic) stainless steel. The most commonly used grade. 
• Parts were exposed to salts and chlorides 
• Parts were stressed in tension. 
• Engineering standards and tests at the time said it couldn't happen.

Seven aircraft have lost flight control because the stainless control cable terminals cracked due to corrosion. The turnbuckle terminals (part number MS21250 or AN669) are made from 303 stainless - a common grade of stainless. This type of terminal is used on most general aviation aircraft and helicopters. Piper reportedly manufactured 51,600 airplanes containing these terminals. One Navy aircraft suffered a failure. There is also a long history of turnbuckle breakage in sailboats.

When chloride-salts get into crevices where there is a lack of oxygen, pits form in the stainless and the part eventually breaks from the inside out. This is called Chloride stress-corrosion cracking. Since the corrosion forms pits inside crevices, the part may look perfectly good from the outside. In the case of the turnbuckle terminals, general corrosion pits were found on the surface of "most" of the broken terminals. Also, in the AN669 series, the safety wire wrapped over the terminal hid the corrosion pits.

In the case of the rock climbing bolts, they looked fine until lightly tapped and broke flush with the rock face. Pretty scary if you are dangling from one of those bolts. In the roof collapse, the stainless hangars were above the ceiling panels hidden from view. 300 series stainless is now banned in the European Union, Switzerland, and Australia for use in indoor swimming pools when used in safety critical applications. It is still being used on aircraft flight controls!

Stainless steel (especially the common 300 series) does not like chlorides. Chlorides are found in salt water, road salt, and some cleaning solutions such as trichloroethane, and methylene chloride. (trichloroethane is often found in the cleaner portion of dye penetrate cleaners that are used in the aircraft industry to find cracks.) Some insulation material contain chlorides. The worst corrosion combination for stainless steel is low-oxygen and high chlorides as might be found in crevices.

Salt-deposits are hygroscopic, they absorb moisture from the air. When the relative humidity is over 50%, the surface becomes wet and corrosion starts. Wash off any salts that may have been deposited on you're equipment.

Failure Characteristics:

• SCC failures can occur rapidly or very slowly. Inspections or replacement based on time-in-service may not be a useful criteria.
• SCC failures are rapid, complete break of the part. There is no tell-tale bending or sagging.
• Visual inspection has not been helpful in identifying suspect parts before failure.

Best Maintenance Practices:

• It appears that the best maintenance practice is to keep the parts clean so that chlorides don't concentrate on the surfaces. Structural strength without adequate corrosion protection is a temporary condition.

Best Engineering Practices:

• Use better stainless grades, such as the Superaustenitic or Duplex grades.
• Use shot-peening to improve the SCC resistance. Shot peening is a proven method of improving the SCC resistance in austenitic stainless steel parts. 

Other areas to be concerned about:

• Load bearing stainless parts exposed to chlorides where the failure could result in a safety hazard. Some examples might include: 
• Aircraft structural parts, such as bolts, turnbuckles, etc. where aircraft are based or operated next to the ocean 
• Swimming pool ladders and bolts used on swimming pool slides and diving boards. 
• Bolts used on trailer hitches on vehicles next to the ocean or where salt is used on the roads to melt ice. 
• Bolts used on road signs next to the ocean or where salt is used on the roads to melt ice.