Hydrogen Induced Cracking (HIC) is a form of wet H2S cracking caused by high hydrogen concentrations in metals. It is characterized by parallel cracks that form along the surface of the metal in the hoop stress direction. HIC is more common in sour service environments due to the presence of wet H2S, as well as other elements like arsenic, antimony, selenium, and cyanides. It is more likely to occur in ferrous alloys due to their restricted slip capabilities in their BCC structure, and can cause blistering damage to metals and alloys with a Rockwell C hardness of 22 or more at relatively low temperatures.
Contributing Factors to Hydrogen Induced Cracking
HIC can occur during various elevated temperature processes, such as electroplating, pickling, phosphating, cathodic protection, and arc welding. The API Nelson curve provides a basis to understand the temperature zone over which the possibility of HIC increases. This typically indicates a zone that is either hydrogen-induced cracking or non-hydrogen induced cracking.
Preventing HIC Damage
To prevent hydrogen-induced cracking, it is important to monitor the hydrogen concentration in metals and alloys, as well as the temperature of the metal during processing. It is also essential to ensure that the metal has been adequately treated to minimize the risk of hydrogen embrittlement. Additionally, it is important to avoid any processes that could increase the hydrogen concentration in metals and alloys, such as welding and galvanizing.
Hydrogen Induced Cracking Mechanism: Causes and Effects
Hydrogen-induced cracking (HIC) is a type of material failure caused by the ingress of hydrogen atoms into the metal structure. The mechanism of HIC begins with the formation of atomic hydrogen in a wet H2S environment. These atoms diffuse throughout the metal and accumulate at voids or impurities in the metal structure, where they combine to form hydrogen molecules. The resulting high pressure within the cavity reduces the ductility and tensile strength of the metal, leading to the formation of stepwise internal cracks known as HIC.
Detection and Prevention of Hydrogen Induced Cracking
HIC is often visible on the metal surface as horseshoe-shaped cracks. Regular inspection and testing are necessary to eliminate the possibility of HIC. Wet Fluorescent Magnetic Particle Inspection is commonly used to detect HIC damage. For components with cracks, Phased Array Ultrasonic Testing is the most widely used and reliable non-destructive method for detecting HIC.
Materials Susceptible to Hydrogen Induced Cracking
Low alloy steels and high-strength titanium and nickel steels are more prone to HIC. On the other hand, low-strength steels with tensile strength below 1000 MPa are usually not susceptible to HIC. Copper, aluminum, and their alloys are the most resistant to HIC.
By understanding the mechanism of HIC and identifying materials that are susceptible to HIC, it is possible to take preventative measures to reduce the likelihood of material failure due to HIC.
Requirements of HIC Resistant Materials for Different Steel Types To prevent hydrogen-induced cracking, certain requirements must be met for the material used in different steel types.
Requirements for Carbon and Low Alloy Steels To ensure HIC resistance in carbon and low alloy steels, the following requirements must be met:
- The hardness of the parent material should be less than 22HRC (237 BHN).
- The steel should be fully killed, using silicon/aluminum.
- The material should be heat treated by normalizing, annealing, or Q and T.
- The carbon content should be less than 0.23%, and the carbon equivalent (CE) should be less than 0.43.
- Sulfur and phosphorus should be less than 0.002%.
- Additional requirements like calcium treatment and inclusion control are also necessary.
Requirements for Austenitic Stainless Steel To ensure HIC resistance in austenitic stainless steel, the following requirements must be met:
- The hardness of the parent material should be less than 22HRC (237 BHN).
- The material should be heat treated by solution annealing.
Requirements for Duplex and Super Duplex Stainless Steel To ensure HIC resistance in duplex and super duplex stainless steel, the following requirements must be met:
- The hardness of the parent material should be less than 25 HRC.
- The material should be heat treated by solution annealing.
- The ferrite content should be between 35% and 65%.
Hydrogen-Induced Cracking Test To test the HIC resistance of materials, the HIC test is performed according to NACE TM0284. The test specimen is exposed to the specified process environment saturated with hydrogen sulfide gas at 1 bar pressure for a duration of 96 hours (4 days) for the standard test. Fitness for purpose of testing is performed for durations of up to 30 days using reduced partial pressures of hydrogen sulfide.
The crack sensitivity ratio, crack length ratio, and crack thickness ratio are the usual ratios used for hydrogen-induced cracking tests. Another HIC test method is the stress oriented hydrogen-induced cracking (SOHIC) test method, which is performed according to NACE MR0175 or ISO 15156 using the full ring test method, tensile test method, or four-point bend method.