Hydrogen induced crack (HIC), also known as hydrogen stress crack or static fatigue, is a typical form of hydrogen embrittlement.It is characterized by brittle fracture of normal ductile alloys under sustained loading in the presence of hydrogen.The mechanism of HIC begins with the diffusion of hydrogen atoms through materials.At high temperatures the solubility of hydrogen increases, causing it to disperse into the material.When these hydrogen atoms recombine in very small metal gaps to form hydrogen molecules, they create pressure in the cavity.The pressure from the accumulation will increase further, causing the metal to lose its tensile strength and ductility to the point of cracking, known as HIC.
The hydrogen induced cracking failure of an API 5L X60 line pipe
More commonly, fracture occurs under sustained loads below the yield strength of the material.The crack mechanism depends on hydrogen fugacity, material strength level, heat treatment/microstructure, applied stress, and temperature.Many ferrous and non-ferrous metals may be sensitive to hydrogen damage in water or gas containing hydrogen.These metals include carbon steel, alloy steel, nickel and nickel alloys, etc.
The materials most likely to undergo HIC reactions are low-alloy and high-strength steels such as titanium and nickel.In general, steel with hardness below 30 HRC and tensile strength below 145,000 psi or 1000 MPa will not be affected by HIC.
Some of the best metals that can withstand HIC include copper and aluminum, as well as its alloys. HIC can take place throughout different manufacturing operations. This can be anywhere that metal comes into contact with molecular or atomic hydrogen. Various processes that can result in this include:
- Electroplating
- Phosphating
- Pickling
- Cathodic protection
The microstructure at the HIC fracture of a mild carbon steel The white hairlines in the photo are the HIC fracture zone
HIC typically occurs during arc welding, when hydrogen is released from moisture and other organic compounds.The HIC can occur 72 hours after welding with a diffusion rate of 1 inch per hour at 450 degrees Fahrenheit.This problem can be prevented by making sure the weld surface is clean and moisture free, and keeping the temperature at 400 to 450 degrees Fahrenheit for an hour.
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