Procedure for analyzing stress corrosion cracking

Standard test for analyzing sccStandard tests are generally designed to test a material for itsweakness to scc in an environment that induces corrosion. Thefollowing are the procedures generally used to analyze thestress corrosion cracking.

• Constant stress or constant displacement test• Fracture mechanics test• Slow strain rate test or constant extension rate test

Constant stress or constant displacement test

• In this method the specimen is subjected to a constant load while exposed to the solution to induce corrosion.

• The susceptibility to scc is than concluded by the time taken for failure of the specimen or the development of cracks on its surface.

Fracture mechanics test• For this test a specimen with pre existing crack

(produced by fatigue cycling) is used.

• The procedure involves recording the time taken to failure, change in length of crack with time and plotting the graph of crack growth rate to the stress intensity factor.

The slow strain rate test or constant extension test

• This test involves the application of the slow extension rate to the specimen in order to keep a continuing plastic strain at the surface of the Specimen.

• This will induce the initiation and growth of stress corrosion cracking on the surface of Specimen.

• The results are evaluated by calculating the time taken for failure to occur or the appearance of the fracture surface.

Hydrogen damage & its types

Hydrogen damage refers to the mechanical damage of metal caused by the presence of or interaction with, hydrogen. It may be classified into:

• Hydrogen blistering• Hydrogen embrittlement• Decarburization• Hydrogen attack

Here a cross section of a wall of a tank is shown in fig. The interior contains the acid electrolyte, the exterior is exposed to the atmosphere. Hydrogen evolution occurs on the inner surface as a result of a corrosion reaction on cathodic protection. The hydrogen atom diffuse through the metal and forms hydrogen molecule on the exterior. Some hydrogen atoms get trapped in side the void and accumulate as hydrogen molecule. As the concentration of H2 increases the pressure increases which is enough to rupture any engineering materials which is called blistering.

Hydrogen blistering

Hydrogen embrittlement

• It is the process by which various metals, most importantly high strength steel become brittle and fracture following exposure to hydrogen. The mechanism is almost similar to hydrogen blistering. The H2 inside the voids have increased pressure which is enough to reduce the ductility and tensile strength to the point of cracking.

Decarburization

• Decarburization is the process of reducing the carbon content in a metal.

• Decarburization occurs when carbon in the metal reacts during heating with oxygen or hydrogen in the atmosphere. Which results in the mechanical damage to the metal.

C + O2 CO2

Hydrogen attack

• Hydrogen present in metals can produce several kinds of internal defects like blistering, fracture ,porosity.

• Carbon steel exposed to hydrogen at high temperature experience hydrogen attack which leads to internal decarburization and weakening.

Prevention• The best method of controlling hydrogen damage is to control

contact between the metal and hydrogen by using coatings such as metallic coating, rubber and plastic coating etc.

THE END