Example of exfoliationexfoliation of an aircraft component

Dealloying (selective leaching) Dealloying or selective leaching is a selective

removal of one or more elements from an alloy by the corrosion processes.

Examples: dezincification of unstabilized brass graphitization of gray cast iron

Environmental cracking

Stresses that cause environmental cracking arise from:

Residual cold work Welding Grinding Thermal treatment Service conditions … To be “effective” the stresses must be tensile.

Types of environmental cracking

Stress Corrosion Cracking (SCC)– Sulfide stress cracking

– Chloride induced SCC

– Caustic induced SCC

– … Hydrogen embrittlement Corrosion fatigue

Cracks

The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment.

As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload.

Stress Corrosion Cracking

• Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment.

intergranular SCC of an Inconel heat exchanger tube

Stress Corrosion Cracking

Usually, most of the surface remains unattacked, with fine cracks penetrating into the metal.

In the microstructure, these cracks can have an intergranular or a transgranular morphology.

Macroscopically, SCC fractures have a brittle appearance.

SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the failure not easily predicted.

Example of SCC

• SCC in a 316 stainless steel chemical processing piping system containing chlorides.

Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt“ transgranular crack pattern.

A catastrophic case history• On October 4, 1992 an EL AL

Boeing 747 freighter crashed in Amsterdam, killing all four people on board and over 50 people on the ground.

The cause of the crash was the number 3 and 4 engines separated from the wing, causing a loss of control.

The reason for the number 3 engine separation was a breakage of the fuse pin weakened by a crack.

The pin was designed to break when an engine seizes in flight, producing a large amount of torque.

Hydrogen embrittlement

Also called: hydrogen induced cracking Involves the ingress of hydrogen into the

metal causing:– Reduced ductility and load-bearing capacity,– Subsequent cracking and– Catastrophic brittle failures at stresses below the

yield stress of susceptible materials Most vulnerable are high-strength steels,

titanium alloys and aluminum alloys

Sources of hydrogen

In the metal making process In meatl processing (phosphating, pickling) From welding In storage or containment of hydrogen gas As a by-product of general corrosion From cathodic protection In electroplating

Hydrogen embrittlement ofstainless steel Hydrogen (atoms) present at the surface enters

the steel Hydrogen diffuses along the grain boundaries

and combines with the carbon, which is alloyed with the iron, to form methane gas

Methane gas is not mobile and collects in small voids along the grain boundaries and reduces ductility

The gas can build up enormous pressures that can initiate cracks

Corrosion Fatigue Corrosion-fatigue is the result of

the combined action of an alternating stress and a corrosive environment.

The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated.

The introduction of a corrosive environment often eliminates the normal fatigue limit of a ferrous alloy, thereby creating a finite life regardless of stress level.