What You Need To Know About Stress Corrosion Cracking

The tragic collapse of the roof of an indoor swimming pool in Uster, Switzerland where 12 people died, has been well-documented. Engineers explored the reason for this terrible calamity and came to the conclusion that the steel rods which supported the 4-inch thick concrete roof failed due to stress corrosion cracking (SCC).

There have been similar incidents around the world where structures have collapsed long before their life expectancy was reached, and with little or no warning.

What is Stress Corrosion Cracking?

We place a great deal of trust in steel, notably to form a strong and stable backbone of large and heavy structures. However, almost all alloys are susceptible to stress corrosion cracking, given the right circumstances.

Simply put, stress corrosion cracking is the result of the presence of a corrosive environment combined with a degree of tensile stress. It’s a complicated problem which can have a multitude of influencing factors.

Possible Corrosive Environments

The collapse of the swimming pool roof mentioned above came after only 13 years of use, well before the expected failure of any of the components. Corrosion Doctors has this to say: “The Federal Materials Testing Institute, based in Duebendorf, Switzerland, and the Federal Materials Research and Testing Institute of Berlin concluded that the collapse was the result of chloride-induced stress corrosion cracking. The steel rods had been pitted, causing the roof to cave in. The roof collapsed in a zipper-like fashion, starting with the corroded rods. The collapse continued as the remaining rods were unable to bear the increased load. Chloride is a major factor in corrosion of reinforced concrete, as in the case at Uster. The chloride was either already present in the concrete or came from the pool via water vapor.”

Besides chloride, the presence of alkali, nitrates or ammoniacal solutions can have a rapid degenerative effect on various alloys, as well as extreme temperatures.

Possible Stress Elements

The tensile stress that results in SCC can come from directly applied stress in the environment i.e: working stress, or can stem from the actual fabrication process. Welding, heat treatment, or cold deformation can all contribute to the ultimate failure of a component.

Some materials are more susceptible to SCC than others and this needs to be factored in when selecting materials for use in a high-risk environment.

Preventing Stress Corrosion Cracking

SCC is possible under many circumstances, but requires at least the following:

  • The use of a susceptible steel in a given environment
  • Residual or structural tensile stress
  • The presence of a compound which creates a corrosive film on the material

Indeed, the responsibility of a steel manufacturer or fabricator is a serious one. If you require skills and expertise of a qualified team of specialists, then we’d love to chat with you. Call Steelmor today on 011 747 5700.

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