Stress-corrosion-cracking

can cause Sudden Failures.

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Corrosion And Stress

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Stress-corrosion-cracking (SCC) is the unexpected sudden brittle failure

It can occur even in metals which are known to fail generally by ductile fracture.

That is true especially at elevated temperatures, if subjected to a tensile stress in a corrosive environment.

SCC is highly chemically specific.

Certain alloys are likely to undergo this kind of failure only when exposed to a small number of chemical environments.

The chemical environment that causes SCC for a given alloy is often one which is otherwise only mildly corrosive to the metal, in the absence of tensile stress.

Stress corrosion cracking is described as a mechanical-environmental failure process in which sustained tensile stress and chemical attack combine to initiate and propagate fracture in a metal part.

Stress-corrosion-cracking can be a real danger

There is in fact a synergistic action of the presence together of both factors (environment and stress) that promotes the catastrophic failure.

In the absence of one of the causes the material would not suffer destruction.

Since Stress-corrosion-cracking develops tiny cracks mostly invisible to the naked eye, metal parts can appear bright and shiny, although already endangered by the presence of microscopic cracks.

Therefore it is not uncommon that SCC is not detected prior to failure.

Stresses may also originate from service loads. Cyclic stresses can cause corrosion fatigue failures.

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Chlorides presence undermines stainless steels and aluminum alloys, alkali and nitrates attack mild steel, ammoniacal solutions crack copper alloys.

Besides metals, also plastic polymers can be attacked by specific agents like ozone for rubbers (natural, nitrile and styrene-butadiene rubber).

Chlorine attacks acetal resin and polybuthylene.

Other polymers are subject to attacks from strong acids, alkalies or UV (ultra-violet) radiation.

Back to metals, the sources of high local stresses are identified as

  • thermal processes including welding,
  • the presence of stress raisers,
  • abusive surface finishing and
  • fabrication procedures like bending, straightening or assembling.

The initiation of Stress corrosion cracking may be submicroscopic and start from surface irregularities acting as stress raisers.

Tensile stresses may rupture superficial protective films permitting crack propagation to continue.

Stress-corrosion cracks may or may not exhibit (in metallographic examinations) extensive branching.

Their general direction of propagation is mainly perpendicular to the tensile stresses contributing to their formation.

At least two theories were proposed to explain Stress-corrosion-cracking: the electrochemical theory and the stress sorption theory.

However successful may the theories be to explain part of the features, there is still no unified theory capable of explaining all characteristics.

Under some conditions Stress-corrosion-cracking can be produced in most metals. Small grain size structures are more resistant to SCC than large grain size in the same metal.

Grain orientation has a definite importance in determining the path of Stress-corrosion-cracking.

However the formation of transgranular or intergranular cracks is sometimes dependent on the pH of the aqueous solutions in contact with the alloys.

Furthermore it appears that there is a threshold of stress intensity below which Stress-corrosion-cracking does not occur.

In conclusion there is no better way to avoid Stress-corrosion-cracking than screening materials and all the conditions, from design details, fabrication procedures, residual stresses and chemical compatibility.

Once the structures are in operation, Acoustic Emission analysis may help in obtaining advance notice of intervening destructive processes.

Online resources on Corrosion and on SCC can be found in The Middle Month Bulletin of Practical Welding Letter for November 2006.
Click on PWL#039B to see it.

Watch the following Video

Stress Cracking Corrosion pull test

http://www.youtube.com/watch?v=z0bmZGcQOu8

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