What Does Hydrogen-Induced Cracking (HIC) Mean?
Hydrogen-induced cracking (HIC) refers to the internal cracks brought about by material trapped in budding hydrogen atoms. It involves atomic hydrogen, which is the smallest atom, that diffuses into a metallic structure. In the case of a crystal lattice becoming saturated or coming into contact with atomic hydrogen, many alloys and metals may lose their mechanical properties.
In case the buildup of molecular H is repressed, the emerging atomic H can disperse into the metal rather than forming a gaseous reaction. This, in turn, produces a crack in the metal or material. Certain chemical elements may contribute to this like selenium, antimony, arsenic and cyanides. However, the topmost species is H2S, or hydrogen sulfide.
Corrosionpedia Explains Hydrogen-Induced Cracking (HIC)
The mechanism of HIC begins with atoms of hydrogen diffusing throughout the material. At elevated temperatures, hydrogen tends to have increased solubility, allowing it to disperse into the material. When such hydrogen atoms combine again in very small metal voids to build molecules of hydrogen, they produce pressure within the cavity. The pressure created from the buildup can further elevate, which makes metal lose its tensile strength and ductility, reaching the point of cracking, or HIC.
The materials most prone to HIC are low alloys and high-strength steels like titanium and nickel. Generally, steels with a hardness below 30 HRC and tensile strength below 145,000 psi or 1000 MPa are not susceptible to 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:
- Cathodic protection
HIC often occurs in arc welding, where hydrogen is discharged from moisture and other organic compounds. HIC can occur 72 hours after welding and can diffuse at a rate of one inch per hour at 450°F. This can be alarming, but it can be prevented by ensuring that weld surfaces are clean and moisture free, and maintaining a temperature of 400-450°F for one hour.